How to stop one cat from using another cats box

Question: I have 4 cats and 4 boxes. They're all good animals but one has anxiety and temperment. 3 cats will use any box, don't care , she will only use her box and once another cat uses it she will no longer use her  box; she will do her business wherevever . I was really sad for a moment thinking that she would never use a box again from how she was acting for a long time but I fixed that by putting her in a room by herself with a new/cleaned box. This works well because she started using it again and No one else has access to the box. Problem is... I don't want to leave her in a room all day by herself
Comment: Welcome to pets.se liz! An extra tip to a complicated issue - it is better to have # of boxes X # of cats + 1 extra box!
Answer: To be honest, your one cat, being anxious, sounds like it'd be happier as an only cat, but hopefully you can make it as happy as possible in the situation by continuing to give it privacy with its box.
So continue to keep your cat's box in that room, and the other cats out of that room. But give your one cat outings from the room. If you're concerned the cat might try to eliminate during one of its outings, try to only let the cat out right after it has used its box, and keep an eye on it. If you catch it about to eliminate, try to distract it by either making a loud noise (clapping is good) or throwing a toy in its direction, whatever gets its attention, or whatever you find that works, as long as you don't do something that is rewarding it.  Then quickly put the cat back into its room until it has used the box.  Obviously don't shut the cat out of its room for very long periods of time, so it has plenty of opportunity to relieve itself.


Missing volume app from notification area in Xubuntu panel!

Question: The volume control icon is missing from the panel. The n/w and battery icons are shown. When I try to edit the Notification Area from the Panel preferences I see the volume is not available as a known app. I need help. Am a newbie :)
Comment: I meant "known applications" when I said recent apps.
Answer: The volume control icon is part of the xfce4-indicator-plugin, which can contain various icons. You can accidentally remove the volume control icon, which will close the plugin; when it is running it is listed in system monitor as xfce4-indicator-plugin.
To return to your old panel setup as displayed here:       , please follow these instructions.
(Only the volume and mail icons are actually part of the indicator plugin; the networking and power-management icons are separate plugins).
1) Just right-click the panel and go to panel preferences, as in the screenshot below:

2) Then, just make sure the appropriate panel is selected, move to the items tab and click the green + in the items tab (not the one at the top of the screen) to add an item.

3) Now, scroll down the list and select the indicator-plugin, click add, and close the menus. Your volume control icon and mail icons will be restored.

This should sort out your problem unless you have uninstalled some xfce4 items or icons.
Comment: Thanks a lot. That did it :) I'm still new to this community, in the future I shall check the database thoroughly before popping in a question.
Answer: I ran into this in an Ubuntu installation that I had installed the xfce4 pacakage on, not a proper Xubuntu installation. But in case if anyone else has the same problem, I had to install the indicator-sound-gtk2 package:
sudo apt-get install indicator-sound-gtk2

and then log out/log back in.
Comment: Using your answer and [this](http://askubuntu.com/questions/360806/volume-indicator-issue-after-xubuntu-13-10-upgrade) one, i finally got my volume control back! :)
Answer: That's not intuitive.
You need an icon separate from the notification area called "sound icon" or "volume control."
This is a common problem.
(EDIT: answer in 16.04 be sure to install & add to panel:
'xfce4-pulseaudio-plugin' current version is 0.2.4-1 as of 7/2017
Seems to work fine, ugly icon but you can also just hover & mousewheel it up and down. )
I was using an indicator that I like better than the standard one,that shipped with 13.04 to 16.04 (I think) using the indicator-plugin, it is called "Volti" and it's available for Precise and Oneric. (EDIT... AND the rest up to ZESTY. Current version 0.2.3-7 from Zesty works on 16.04 as well.) Simply add it using Synaptic (or, I imagine, the Ubuntu Software Center.) This uses ALSA without having to use Pulse Audio, as well.
Unfortunately there is a problem where it does not work after resume from suspend! :( Have to kill it then restart it.
Info: code.google.com/p/volti/
Answer: sudo apt-get install xfce4-mixer

This installs a plugin that you can add straight to any panel.
Comment: This didn't work on a new install of xUbuntu 16.04LTS where I'm using the notificatin area instead of the buggy "indicator plugin" but `sudo apt-get install xfce4-pulseaudio-plugin` did.
Comment: @jtlindsey You should add your comment as a separate solution. It will be easier for others to find and will be more helpful that way.
Answer: I am missing the volume icon in Xubuntu 14, and the solution here didn't work for me (I already had the Indicator plugin added to the panel). The only way that worked (and I also use this when the panel has other issues too), is to restart the panel, using:
xfce4-panel -r

I actually have this setup as an alias in ~/.bashrc since I run into panel issues as lot.
alias panelbug='xfce4-panel -r'

Answer: Similar to @Ulad answer, if you want to install a sound plugin that will work with your panel, try
sudo apt-get install xfce4-pulseaudio-plugin

That worked for a new install of xUbuntu 16.04LTS where I'm using the notification area instead of the buggy "indicator plugin".


Does a legal compilation of all laws included in the New Testament exist?

Question: Jews compiled Old Testament laws and found 613 commandments. I wonder if the same kind of compilation exists for the New Testament laws.
Comment: I think most Christians would say that aren't any laws in the NT. The NT doesn't have anything intended for setting up a legal system.
Comment: Of whom are you asking this question ? And what is your definition of 'law'. _The rule of the Spirit is that Life is in Christ Jesus_ is how I read the original of Romans 8:2. There are but two commandments which are primary : Believe in the Lord Jesus Christ . . .  and . . .  love one another. But it all depends who you ask.
Comment: @curiousdannii: See for example: https://famguardian.org/Subjects/LawAndGovt/ChurchVState/BibleLawIndex/8.htm I am looking for something like this.
Comment: The Christian New Testament is, more or less, a (re)interpretation of Jewish Law (as started by Christ, and continued by the Apostles, and their disciples), rather than a new and original set of laws and teachings in and of itself.
Answer: When a Pharisee asked Jesus which is the greatest commandment in the Law, Jesus said this:

Hear, O Israel, the Lord our God, the Lord is one.  Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.  The second is this: Love your neighbour as yourself.  There is no commandment greater than these (Mark 12:29-31).

The first quotation is known as the Shema, named after the first word of Deuteronomy 6:4.  To the Shema Jesus joined the commandment from Leviticus 19:18 to show that love for neighbour is a natural and logical development of love for God.
As recorded in Matthew 22:40, all the Law and the Prophets hang on these two commandments.
Jesus went a bit further with regard to the law to love your neighbour as yourself.  He said:

But I tell you who hear me: Love your enemies, do good to those who hate you, bless those who curse you, pray for those who ill-treat you...  Do to others as you would have them do to you...  Be merciful, just as your Father is merciful (Luke 7:27-36).

The Ten Commandments were essentially a summary of the entire Old Testament law.  Nine of the Ten Commandments are clearly repeated in the New Testament (all except the command to observe the Sabbath day). Obviously, if we love God, we will not be worshiping false gods or bowing down before idols.  If we love our neighbours, we will not be murdering them, lying to them, committing adultery against them, or coveting what belongs to them.  The Old Testament law was never intended by God to be the universal law for all people for all of time. We are to love God and love our neighbours. If we obey those two commands faithfully, we will be upholding all that God requires of us.
The answer to your question is that there is no list of New Testament laws.
Note: This view is from a Protestant Christian.
Comment: +1 but it is interesting to note that in Matthew 28:20 those (in context) who are baptised are to receive : _teaching them to observe all things that I have commanded you;_ They are to 'observe' such things as loving one another, remembering the Lord in his death until he come, and all else that Jesus 'began to both to do and to teach'. This is all conveyed in the Gospel, but not the Law.
Answer: There is no consolidated list of "commandments" in the NT; however, all of the 10 commandments in the OT are repeated (some several times) in the NT.  Leaving these aside, here is a very incomplete list of moral requirements set out in the NT.

1 Peter 1, 2 - holiness (1 Peter 1:15), Purity (v22), Obey the truth (v22), love (v22), "rid yourselves of all malice and all deceit, hypocrisy, envy, and slander of every kind" (1 Peter 2:1), abstain from sinful desires (1 Peter 2:11), submit to civil authorities (v13-17), see also Rom 13:1-7, etc.
Regular and frequent prayer, 1 Thess 5:17, 1 John 5:16, 1 Chron 16:11, 2 Chron 7:14, Psalm 5:3, Mark 1:35, Eph 6:18, James 5:13, Matt 5:44, 26:41, Prov 15:8, Ps 17:6, 102:17, Rom 12:12, 8:26, Luke 18:1, Phil 4:6, Col 4:2, 1 Tim 2:1, 2, 8, 4:5, 1 Peter 4:7, etc, etc.
Discipling of new people, Mat 28:19, Acts 1:7, 8.  See "Discipling".
Celebration of the New Covenant via the Lord's Supper/communion, etc.  1 Cor 11:23-29, Matt 26:26-30, Mark 14:22-26, Luke 22:15-23, John 13:2-17.
Baptism following conversion: Matt 3:6, 13-17, Mark 1:9-11, Luke 3:21, 22; Acts 8:36-38, John 3:22, 23, 4:1, Acts 8:36, 38, 1 Cor 1:14, 16.
Being generous to the poor, Ex 23:11, Prov 3:27, 28, 11:24, 25, 14:31, 17:5, 19:17, 21:13, 22:2, 9, 16, 22, 23, 28:3, 8, 27, 29:7, 13, 31:9, 20, Isa 10:1, 2, 58:1-21, Jer 7:3-6, Amos 4:10, Micah 6:8, Matt 23:23, Acts 4:32-35, Gal 2:10, James 1:27.  More specifically, feed the hungry and thirsty, be hospitable to strangers, clothe the naked, care for the sick, visit prisoners, Matt 25:31-46.
Hospitality, 1 Tim 3:2, Titus 1:8, Heb 13:2, 1 Peter 4:9.
Providing the means, voluntarily, for the support of church ministry and outreach, Matt 10:10, Luke 10:7, 8, 1 Cor 16:2, 2 Cor 9:3-14 (including a believing wife, v5)
Being just and fair, 2 Chron 19:6-10, Micah 6:8, Matt 23:23.
Doing good to others, especially believers, Prov 3:27, 28, Prov 11:25, Luke 3:10, 11, 6:27-45, Gal 6:2, 9, 10, Col 3:23, Heb 13:16, 1 Peter 4:10, 11.
Obey civil legal laws, including paying government taxes, Matt 22:21, Rom 13:1-7, 1 Peter 2:13-17.
Keeping in good physical health, Rom 12:1, 1 Cor 6:19, 20, 3 John 2.
Prohibition against eating blood, Acts 15:29 (See Noahide laws Gen 9:4-6, 17).
Regular Christian gatherings of believers, Heb 10:24, 25.
Being kind to, and providing for the needs of animals.  Luke 14:5.  (See also Prov 12:10, 27:23, Deut 25:4, etc)
Caring and protecting the environment. Luke 12:22-48, Matt 24:45-51, 25:14-30, Luke 19:12-28, Rev 11:18.

There are many more but this illustrates how broad-ranging and encompassing is the Christian life who imitates Jesus.
Answer: Yes.
In the upper half verses of James Chapter 2, James covers the OT law of "respect of persons" (which is to show favoritism of one man over another and originates from OT Leviticus 19:15 and Deuteronomy 1:17). This is NOT one of the 10 commandments, but James goes on to quote two of the 10 commandments.  He states that if you violated this first command of "having respect of persons" you violated the "whole law" which includes the 10 commandments and become guilty of all of them.
Though being guilty isn't the same as suffering judgement from that guilt.  That's why Jesus was sent from God!  1 John 2:1-2

And if any man sin, we have an advocate with the Father, Jesus Christ
the righteous: 2 And he is the propitiation for our sins: and not for
ours only, but also for the sins of the whole world.
Comment: That's a clear ethical principle. But is it a "law"?
Comment: Well, James continues in chapter 2:

9 "But if ye have respect to persons, ye commit sin, and are convinced of the law as transgressors."

Yea, I would say that's pretty seriously bad if you have "respect to persons"
Comment: And when you read the very next verse, it gets even more serious!
James 2:10 For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.
Comment: Is that referring to the Jewish Law in the OT, or to a new NT law? I thought most people said the first.
Comment: Well... just keep reading... James 2:11 "For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law."  This is obviously OT law.  But then James throws a curveball and continues in verse 12 "So speak ye, and so do, as they that shall be judged by the law of liberty."  Jesus gives us liberty to those of us who believe.
Comment: All of this explanation needs to be put forward in your post. Make you full case for there being a new law in the NT. Don't wait for people to drag it out of you.
Comment: But it's not a "new law", this command came from OT Leviticus 19:15 and Deuteronomy 1:17.
The original question is "Does a legal compilation of all laws included in the New Testament exist?"  Yes, James Chapter 2.  If you violate 1 law you are guilty of all of the law.  And since James 2 quoted OT law, this means all OT laws.
Comment: Well then is it really a "New Testament" law? You need to explain all of this, **in your answer**.
Comment: In this format, the comments are not content. Only what is in your answer is content.  Please take the points you make in further support of your answer, based on the dialogue you are having there with curiousdannii, and fold it into your answer. SE is very happy with longer answers if that's what is  needed for support/clarification.
Comment: Yes, I'm new here and have to get used to this new style.


"I'm teaching at university VS I teach at university"

Question: What is the difference between "I'm teaching at university" and "I teach at university"?
another example: "I work in the library every day" VS "I am working in the library every day"
Answer: Your two examples are not equivalent.

"I'm teaching at university"
  "I teach at university"

My current job is as a teacher. I do this at the university. There is the slight connotation in the first that you are doing that right now whereas the second example is habitual - you do it Monday to Friday, but not necessarily at the time you are speaking.

"I work in the library every day"

Again, habitual. That's what you do for your job, even if you aren't there right now.

"I am working in the library every day"

You are trying to push the 'habitual' behaviour aspect of the first usage onto this sentence, where it doesn't function.
Better would be to re-use the teaching examples from earlier

"I'm working in the library."  or
  "I work in the library."

You could use the first as an answer to both "Where are you? What are you doing?" & also "What do you do for a living?"
You can imply habitual behaviour in common speech by using the present tense.
The second example is habitual only & can't be really used to describe what you are doing right now.
Answer: We might as well add an AmE answer, to the BrE and InE answers, since we don't say "at university".
What are you doing at the moment?
-- I'm teaching at a university.
(But you may be a former diplomat or high-ranking public official who plans to go back into politics.)
What is your profession?
-- I teach at a university.
You are a teacher by profession.  If you have tenure, you are a professor by profession.
Answer: The first sentence is present continuous tense, and the second is present simple tense.  Present continuous often implies that the action is happening in the present, right now, while present simple tense usually implies that the action is not happening right now.  The tense has implications [in brackets] which are unspoken:

Present Continuous Tense
"I am walking [right now]."
Present Simple Tense
"I walk [but I am not walking right now]."

These are not hard and fast rules though, and the description of employment is especially ambiguous because while one may not be engaged in one's work at the moment he is speaking, if he is still employed at that moment, then present continuous tense is acceptable.
If you initiate a statement regarding employment, it's generally correct to use the present simple tense.  However, if you're answering a question, it's common to match the tense of the questioner.  The following questions are common ways to ask about someone's employment status:

Present Continuous Tense
"What are you doing these days?"
"I'm teaching at university."
Present Simple Tense
"I'm an accountant.  What do you do?"
"I teach at university."

However, it is common to include a determiner, such as a, the, this, or that, when discussing universities in American English.  The proper use of determiners is outside the scope of this question, but for example:
I teach at a university/I'm teaching at a university.
I teach at the university/I'm teaching at the university.
I teach at this university/I'm teaching at this university.
I teach at that university/I'm teaching at that university.

The proper use of determiners goes a long way toward mastering English and speaking like a native.
Answer: It seems that the main guidelines have been well discussed here already, but I feel that some examples may help clarify. These are in the form of conversational snippets for context:
Q: What do you do for a living?
A: I teach at a university... I'm teaching at...
Note: Both are acceptable and widely used, but they invoke slightly different moods. While former implies more immediacy (i.e. I'm doing it right now) people regularly speak like this conveying a sense of connection to and intimacy with the activity. One more thing——it would be quite uncharacteristic to add everyday as in your examples. You'd hardly ever say "I teach at a university everyday."
Q: Can you come and help me with this for a moment?
A: I can't. I'm washing my hair.
Note: Never I wash my hair. This would be too broad and irrelevant to the current situation.
Q: Are you speaking at the conference on Saturday?
A: Yes. I'm speaking.
Note: A sense of immediacy and connection to near-future events is depicted in this way. It would be quite uncharacteristic (though not technically wrong) to reply "I will speak," unless, for some reason, a sense of determination were intended.
Q: I'm having trouble handing in all my essays on time. What would you do in my situation?
A: I make sure I do one paragraph a day.
Note: In this case the respondent is giving advice directly from his/her current ongoing experience, so this style of answer is very common. Note also that the question began "what would you do?" and not "what do you do?"
One more:
Q: Would you like a burger?
A: I'm eating a sausage.
or
A: I don't eat meat.
This last example illustrates how the two different tenses, present continuous and simple present, relate to different contexts. The first answer is about an immediate activity while the second one implies ongoing conditions or activities.
I hope these are helpful. I suppose it depends on what type of learner you are. In my case, when I'm learning stuff, it's the examples that usually solidify my understanding, and in most cases the more examples I get the better. Good luck and happy learning!
Answer: As a profession, you generally 'do' things at places.

I work at Microsoft  I teach at university

As an answer to what you are doing (probably now), you are generally 'doing' things. This talks more about the current status/position of yours. For instance, if someone calls you and asks "What are you doing?/Where are you?"

I'm teaching at university

So, in this context, to tell someone your profession, general verb, and for current the status/position, -ing verb.

In InE, it's absolutely fine to use -ing verb to show the profession. Everyone here understands that.

Meet my friend Vivek. He is working in Infosys.
Comment: Hmm. What is "InE"?
Comment: @Anixx Indian English
Answer: In both examples, neither present simple nor progressive simple refers to what one is doing at the exact moment of utterance. Both sentences refer to action over time.

What is the difference between "I'm teaching at university" and "I teach at university"?

The verb is 'teach', which is an event verb (as opposed to a 'state' verb such as 'be'.) As such, the simple present refers to an undefined duration that includes the past and future and, of course, the present moment of speaking. In this sentence, it is referring to a series of repeated events, that is, habitual action over time. The duration is not limited.
The progressive present, by contrast, refers to an ongoing action that includes the present. But it refers to an action ('event') that is limited, that is, it is subject to change; it is not permanent.
"I am teaching at the university [nowadays, but who knows how long it will last; it is subject to change]"
"I teach at university [and there is no implication that the habitual action will end]"

another example: "I work in the library every day" VS "I am working in the library every day"

The basic meanings are the same. 'Work' ia also an event verb. For the simple present, it just reiterates that we are talking about a series of repeated events, I work, such that it includes "every day."
I work in the library every day [and there is no implication that this will change].
For the progressive present, the meaning is not altered from the first example:
"I am working in the library every day [nowadays, but who knows how long this will last; it is not permanent. ].


Inconsistent translation on a WP site

Question: I have a WP site that I installed with the French translation, as provided by Wordpress France, and which is quite accurate. I wanted to apply an English-only theme, which is not using the PO files, but rather has all text hard-coded in the PHP code. I translated all files, but I noticed that when I display the home page, some of the words remain in English, but whenever I display something other than the home page, those are all in French. Is there something either in Wordpress or the theme files that actually make a distinction between the home page and others to display different pieces of text ?
Answer: When every message is hard coded in the theme files there can be no problem with gettext anymore. So, yes, depending on the theme WordPress can use different templates for home and other pages. But no, this has nothing to do with automatic translations. You have simply forgot to translate some strings, possibly in front-page.php or home.php.

Inconsistent translation on a WP site


Sketch App with label colors on Sidebar

Question: In the Sketch App I have at home, he displayed a few different items on siderar than I use at work. Basically you have the option of selecting label colors for artboards and layers and also a line next to the layers. I would like to enable this at work, would anyone know how to enable this? Thanks for the thought.
Answer: Answering my own question. I did some tests activating / deactivating the plugins, and this feature is activated by the https://midnightsketch.com plugin.

Sketch App with label colors on Sidebar


Eosio account create problem

Question: cleos create account eosio mark prikey1 prikey2
if i run this it will show error
Whether I must import key to default wallet or to eosio etc?
Thanks
Comment: What error is it showing?
Answer: To create an account, you must first have imported the eosio key.  This key comes by default to all new installs and it's the same for everyone (at least at the time of this posting..).  To import the key you must have created a wallet.
cleos wallet create --to-console

Copy password here, you will need it to unlock wallet.
cleos wallet unlock

cleos wallet import --private-key 5KQwrPbwdL6PhXujxW37FSSQZ1JiwsST4cqQzDeyXtP79zkvFD3

Then you need to import the private key for your account you want to create:
cleos create key --to-console will give you a new key for your mark account
Then cleos wallet import --private-key [THAT NEW KEY]
Then you should be good to:
cleos create account eosio mark [OWNER] [ACTIVE] -p eosio
Link to docs covering all the steps: https://developers.eos.io/eosio-home/docs/wallets
Comment: thanks
i do not need to import the private key for eosio manually?

Eosio account create problem


Why is the moment of forces on a rigid body in static equilibrium must be zero about any point, even if the position of that point is not on the body

Question: Why is it necessary for the moment of the forces on a rigid body in static equilibrium to be zero about any point, even if the position of that point is not on the body itself.
Comment: Because what do you think would happen if there were a point at which the net force is not zero?
Comment: Succinctly put, of the body is in equilibrium then any point of reference can be used for calculating that equilibrium including being on the surface of the sun due to laws of motion #1... that body will remain at rest until its equilibrium its upset by another force acting upon it.
Answer: Consider a body to be in equilibrium under the action of forces $F_i$, $i=1,\ldots,n$ which act at locations $r_i$, $i=1,\ldots,n$.
Equilibrium of the body implies
$$\sum _{i=1}^n r_i \times F_i=0 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \sum _{i=1}^n F_i=0$$
What about the moment about an arbitrary point p (that may or may not be on the body).

That can be computed as
$$\sum _{i=1}^n (r_i-p) \times F_i = \sum _{i=1}^n r_i \times F_i-p\times\sum _{i=1}^n F_i = 0-p\times 0 = 0$$
Hence the result. The moment is zero about any point, whether it is on the body or not.


From where electrons flow to make a bulb light?

Question: Suppose we have the "basic" stuff like a battery 2 piece of wire and a bulb. Battery has a potential difference. But from where electrons flow to make the bulb light? from wire or from battery or from both? also if electrons flow from battery and they go through the wire (conductor) then why in insulators this doesnt happen? insulators dont give electrons but why they dont let electrons flow?
Answer: Every piece of the circuit has the molecular structure in which the electrons can either be bound to its atoms nuclei or they have enough energy to detach from their atom and roam in the field of metallic bond. Conducting metals have low energy threshold that electrons need in order to detach. A force caused by a potential difference can direct their collective movement and therefore we have a stream of electrons or an electrical current. Now, insulators are structured in a way that their electrons must have a greater energy in order to be detached from their own atom so, in standard conditions, force by a potential difference won't be able to move electrons bound to their atoms and therefore there is no electrical current through the insulator material.
Comment: Many electrons (in particular the few ones that participate in electric conduction) are actually free. They are moving at speeds ranging from zero up to roughly the Fermi speed. And that's without any electric field applied
Comment: Well not because of the electric field but because they are already at energy level of conducting band. So because they're in the conducting band they can be moved through the conductor by the electrical force.
Answer: Electrons from the material in the entire circuit flow. Conductors are different to insulators because their atomic structure is made of a "sea of electrons" around the positive nuclei. These electrons are free to move from atom to atom in the conductors and not in the insulators. This may be better explained why by a chemist, but I believe it's a property metals near the "center" of the periodic table have (I'm probably not technically right about this).
Comment: yeah but the metal some time must be run out of electrons ...so how the bulb lights when the valence electrons of atoms have lost?


Username to Logon to Vigor Draytek 2950

Question: I have a dual-wan DrayTek Vigor 2950 router and it's working really really good! The only thing that bothers me, without being a huge problem, is that I can't configure a user name to connect to the router's configuration page. It's very easy to set a password, but after looking everywhere in the user's guide and on the web, I still stay without answers. Does anyone have an idea, how I could resolve that case? Thanks.
Answer: You can't configure the username on the Vigor 2950. It's always "admin". In effect there is no username, and in fact if you telnet to it rather than using the web interface you'll find it doesn't even ask you for a username.
Good routers, aren't they :-)
JR
Comment: So it has to stay this way, and there's nothing I can do about it? Ok, it's actually better without having to type a user name! Thanks.
P.S.: Excellent Router!
Answer: I heard the new firmware for the Vigor 2950 series should be integrating the option of user setup. Different kind of rights, etc...
But that kind of Firmware update will require a reconfiguration of the system I assume.
Anyway, I would leave it the way it is if it works! The best routers in my opinion!
Comment: I just received an update from Draytek containing the possibility to change the username. Thank you for your advice Ron.

Username to Logon to Vigor Draytek 2950


geolocalization of tweets

Question: Do you know a technique to find spatial indicators using the package TwitteR in R?
For instance if I have to find the word "dog" using the package twitteR, but I miss the coordinates, there is the possibility to find the localization of the tweets thanks to some indications included in the  time-zone, websites, UTC24offset,text message. How can I say to R to detect my list of Tweets contains the word "dog" using the spatial indicators just mentioned?
Comment: There was a recent post on twitter's blog that the USGS is using twitter to verify earthquakes : https://blog.twitter.com/2015/usgs-twitter-data-earthquake-detection  ... you might try checking if the people/projects mentioned have any presentations, or try contacting them.
Answer: You could use the utc_offset or time_zone keys of the user section of a tweet.json (I trimmed the user dict) - full tweet description:
"user":{
   "utc_offset":-28800,
   "time_zone":"Pacific Time (US & Canada)",
   "created_at":"Wed May 23 06:01:13 +0000 2007"
       }

But this is based on the user account and not on a specific tweet. Still, it gives you a probabilistic geolocation for non-geotagged tweets.

geolocalization of tweets


How would the Airbus A320 PFD respond to failure of all pitot and AOA sensors?

Question: In theory, if all 3 pitot tubes and all 3 AOA-sensors would fail simultaneously for a period of time, could the PFD still deliver correct roll angle and horizontal displays? How would the plane calculate the actual airspeed?
Answer: The primary flight display¹:

is a single display, but it shows five separate instruments:

On the left, the speed tape. This shows the value from the pitot probe.
In the middle the attitude indicator. This shows attitude from gyroscopes. All gyroscopes are grouped in the "inertial reference unit".
On the right the big tape is altitude. This shows the value from the static port.
The smaller tape at very right is vertical speed. It is normally taken from the inertial navigation system, which is part of the inertial reference unit, but can fall back to the static port.
On the bottom is compass. This shows the value from the inertial reference unit, which has drift automatically compensated by reference to magnetometer.

The angle of attack is not displayed anywhere. It is only used for stall warning, as part of the alpha-protection logic and for checking the weight set in FMGC.
For all the instruments, each instrument shows value from one input. The three pitot probes, static ports and inertial reference units each feed one of the three displays in the cockpit: captain's PFD, copilot's PFD and the standby instrument (which in the modern version looks like smaller PFD and is placed between captain's (left) displays and the centre display.
When any of the input fails, the computer will generate a warning (accompanied with the annoying cricket sound) stating that the values do not match, but it leaves it up to the pilots to determine which one is wrong. The pilots can then switch the displays to show the one they determined as trustworthy.
So:

If all pitot probes failed, the speed would show nonsensical values, but the attitude, altitude and heading would remain operational. The system would fall back to alternate law. There is an unreliable airspeed procedure where the aircraft is kept in safe speed range by reference to attitude and vertical speed. Failure to execute that procedure properly was an important part in the accident sequence of Air France flight 447.
If the angle-of-attack vanes fail, the stall warning and alpha floor protection will not work, but all displays would work. However, the system may actually not notice that they failed as evidenced by the accident of XL Airways Germany flight 888T. This was a test flight where they intentionally tried to activate the alpha-floor and it did not activate without any prior indication because the AoA-vanes froze due to improper wash.

¹ The image shown clearly is supposed to be Airbus PFD, but some elements like the protection indicators and the Mach number are missing. I just didn't quickly find better as this one is very readable.
Answer: AoA (Angle of Attack) vanes, as the name says, only measure the angle of attack, i.e. the angle between the velocity vector and the x-axis of the aircraft reference system.
This is used neither for roll angle measurements (the inertial units do that) nor for airspeed measurements (it is taken care of by the pitot tubes).
So if all 3 AoA sensors should fail simultaneously, the PDF could still display the artificial horizon and the airspeed correctly.
Comment: But what if theoretically all 6 probes would fail for a certain peroid of time?
Comment: Would there be any way the flight computers could calculate the actuall airspeed?
Comment: make enough sensors fail and it will not be able, but that's a different question.


How to run full length cable housing to front mech on a frame without cable stops?

Question: I have an All-City Space Horse frame that has cable guides for full length housing where you use zip ties to secure the housing. But there are no traditional cable stops. The housing runs from the shifter all the way to the derailleur, uninterrupted. On the rear this is no problem since rear derailleurs have a cable stop built right into it. On the front derailleur (bottom pull) however I need to cut the housing somewhere in order to clamp the exposed cable to the derailleur.
What is the recommended method to install full length cable housing on such a frame?
Answer: It looks to me like you may be able to use a clamp-on cable stop. All you  need is a round steel-alloy frame tube that is strong enough to accept a clamp. If the clamp is located close to the point where the tube joins the bottom bracket, the frame tubing should be plenty thick enough. (I am not an engineer, and if you have doubts, especially where a frame warranty is involved, consult with a good bike mechanic.)
I looked up the Space Horse, and that looks like a sweet bike. Enjoy!
Answer: That is surprising. I wondered for a moment if they only intend you to run it 1×, but I can see on the All-City website that they show it set up 2×.
My first thought was to get a clamp-on stop, as Ichabod suggests. However, there are some front mechs that have integrated cable stops, so see if one of those would work for you.
Comment: I think that's that reason the frame is like that. Many newer FDs have it built in.
Answer: I believe the part you require is shown here:
https://www.ison-distribution.com/english/product.php?part=BBPSG7600
It is available from all Surly/All-City/Problem-Solvers dealers.
Single-sided, bottom bracket mounted cable guide, with integrated cable housing stop.

Intended to fit 40mm diameter bottom bracket shells
Includes M5 mounting bolt
Direct replacement for:  All-City Cosmic Stallion, All-City Gorilla Monsoon, All-City Macho King, All-City Space Horse Disc

I used one of these to install a front derialleur on a Velo Orange Neutrino where it was also necessary to drill the bottom bracket shell and tap the threads. It's easy to do this but I think your bike will already be drilled.
You could also use a Shimano Side Swing front derailleur that takes its pull fron the front (which makes more sense than you'd think) as long as your chainrings are small enough.
Comment: Yes, this cable guide with integrated cable stop is exactly what I need. They really should've included this small part with the frameset
Comment: Yes, I'm surprised it wasn't included with a new frame.
Answer: Not quite your situation, but I 3D printed some cable retainers with a curved back-side to match my tube profile, and epoxied these on.
https://www.tinkercad.com/things/56fo0JH2oJF-epoxy-on-zip-tie-mount-for-53mm-tube-holding-7mm-cable

This example would NOT work as an end stop, but it may be possible to make something that does that task.

Separately, there exist derailleurs that have a built-in cable stop, though those were mostly Bottom-Pull format like for a road bike whereas yours is likely top-pull, meaning the cable comes down from above.
  From https://www.bikeforums.net/classic-vintage/914874-front-derailleur-built-cable-stop-how-upgrade.html


What is the difference between exponential and geometric distribution?

Question: I don't really understand the difference between exponential and geometric distribution.
Answer: Did you try looking at Wikipedia?

The exponential distribution may be viewed as a continuous counterpart of the geometric distribution, which describes the number of Bernoulli trials necessary for a discrete process to change state. In contrast, the exponential distribution describes the time for a continuous process to change state.
Answer: The geometric distribution belongs to the exponential family and so does "the exponential distribution". They only differ in the parameters and sufficient statistics used in factored expression for conditional distributions from the exponential family.
Comment: Welcome to the site! Saying they "only differ" sounds a little strong, particularly since the sets of values they can take on are quite different. :)
Answer: Exponential distributions involve raising numbers to a certain power whereas geometric distributions are more general in nature and involve performing various operations on numbers such as multiplying a certain number by two continuously. Exponential distributions are more specific types of geometric distributions.
Exponential distributions: 2, 4, 16, 256 or 3, 9, 81, 6561.
Geometric distribution: 2, 4, 8, 16, 32, 64.
Just my two cents anyway.
Comment: +1 to whuber's comment above. For what it's worth, people typically call the geometric distribution a special case of the exponential because the latter is defined only on integers. For example, if $X$ is an exponentially distributed r.v., with parameter $\lambda$  then $\lfloor X\rfloor$ is a geometrically distributed r.v. with parameter $p=1-e^{-\lambda}$
Comment: Because you use "exponential distribution" in a non-standard way here, your answer is likely to be misunderstood until you edit it to explain what *you* mean by [exponential distribution](http://en.wikipedia.org/wiki/Exponential_distribution).
Comment: I think this post is very misleading, the given examples appear to make no sense. (-1)


Comparing a career in medicine vs. medical research?

Question: My friend recently graduated with a bachelor degree in biology. He is thinking that he can help more people by studying a biomedical engineering Ph.D., rather than becoming a medical doctor.  He is wondering, however, about the differences between these two paths from a career perspective.  What the advantages and disadvantages of an academic career vs. a medical career?
Comment: I have changed the question to be less situation-specific and less opinion-driven, so that it better fits with the pretty good answers it received.
Answer: As a person who faced a similar dilemma a few years ago, I think there are major practical differences:

The working environment. Every doctor will spend his early career in a hospital, whether he plans on practising in the community later or not. A hospital environment is really different than the research one. In the former you are constantly on your feet, working long hours, nights, etc. However, you are constantly working with the people to whom you're helping. In a "lucky" week (depends on the POV), you might save a life every day.
Taking work home. While a M.D works long and hard hours at his/er practice, when she goes home - she can rest. Her patients doesn't follow her home. As a researcher I can tell you that I'm even dreaming on my research.
Independence. The medical environment is a very hierarchical one. Starting out as a M.D, you can't even sign off your own orders (in first world countries, at least :) ), contrary to a graduate student who might be given an independent project very early in his or her study course.
Goals. As a researcher, you end up working for the sake of science. You might find yourself pursuing a direction which might take decades before it translates into helping people. As a practitioner - you're working for the people in front of you.
Demand for your skills. In general - there are hardly any M.Ds who go unemployed. Most healthcare systems are understaffed and there is always a demand for M.Ds. I wish we could say the same about positions in the academy, and the industry is never as stable (and here it's field specific and I can't say much about the specific field in question).
Comment: I think that stands as a general statement regarding the private sector. The industry, that means - research done outside of the academy, is not a stable job market in many fields, esp. those related to biology.
Comment: "and the industry is never as stable". By "the industry" you mean biomedical research? The last sentence is not so clear.
Answer: One thing that comes to mind: for doctors, the move abroad is far more difficult than for the researchers in biomed, because the job market in healthcare is usually heavily protected against international workforce by licensing barriers, see e.g. http://immigrationimpact.com/2013/12/17/licensing-barriers-leave-immigrant-doctors-driving-cabs-instead-of-practicing-medicine/ for the US.
Comment: In Europe titles are more homogeneous, the main barrier is the language. Researchers work in English, but a doctor has to speak the language of their patients. Also note that Canada and the US do seek out specialists in areas where they have shortage, like pathology.
Comment: This really depends on where you're moving abroad to. Australia has great programs for immigrant M.Ds
Answer: For medicine: "A doctor is a medical professional who examines the sick and tries to find a way to help them" [1].
For biomedical engineering: "This field seeks to close the gap between engineering and medicine: It combines the design and problem solving skills of engineering with medical and biological sciences to advance healthcare treatment" [2].
So if your friend want to help patients intimately he should be a doctor but if he wants to help the doctors and advance healthcare and thus indirectly helping the patients he should be a biomedical engineer.


Fermentation Just finished up, does this look right?

Question: This is probably my 10th batch and I've never had an issue with drinkability or infections, so i just want to make sure this is normal. I assume it's just Co2 bubbles, but i am not 100% sure. What else could it be? The Krausen falling maybe? I'd assume it's just bits of yeast and wort proteins or Co2 bubbles. Can anyone confirm this? Does it look insidious?
Answer: looks good, nothing to worry about
On the sides, that's just the yeast and other particles falling back in and getting hung up. I've noticed it on that style of carboy more than smooth wall styles. Also if using starsan with water with high minerals and it air drys leaving a surface that things can cling to.
Comment: cool. not sure how high the mineral content is. I use Spring water that i buy at a grocery store. I do use Starsan, but I use it correctly, so shouldn't be much of a surface left behind.
Comment: @PatrickTyler if your starsan dries and leaves a white haze, use distilled water.  The haze is harmless but can be the start of beerstone which can be a sanitaion issue. I've seen yeast rest like in your carboy even in the best of conditions. It's just the shape of the glass, those ribs are not smooth on the inside.


Shuttle mode alternative in audio editor?

Question: After reading this thread: What audio editor are you using?, I was dooing a little research to choose the best audio editor for mac and pc (I need a cross-platform editor). But I would love to have the Pro Tools shuttle mode workflow in an audio editor. There is a scrub function in Adobe Audition, but it's not that powerfull like shuttle mode. Maybe somebody found that function in an audio editor?
Answer: Reaper, hands down the most flexible editor and also with fully configurable scrub modes.
Cross-platform is just the beginning of the benefits.
www.reaper.fm
Cheers.


What is the relation between airspeed and altitude at fixed throttle setting in a private jet?

Question: In the world of private jets, say a G-450, at a fixed throttle setting, does it simply go faster the higher it climbs/flies?
Answer: The basic principles needed for an answer are:

Thrust varies linearly with air density.
Lift at constant angle of attack (AoA) equally varies linearly with air density.
Increasing the angle of attack from a low value will improve the lift to drag ratio (L/D), while increasing it from a higher value than the AoA at optimum L/D will decrease it.
The aircraft will settle at the point where speed equals drag and lift equals weight. If the altitude change improves L/D, the aircraft has excess thrust to fly faster.
Due to the atmospheric lapse rate, the speed of sound decreases with altitude.
Above a certain Mach number ($M_{dd}$ = drag divergence Mach number), drag increases nonlinearly over dynamic pressure.

Now the answer depends on where you start:

Low altitude, moderate throttle setting: The aircraft will speed up, because it was flying at a low AoA in dense air. Climbing brings it closer to the polar point of optimum L/D.
Low altitude, high throttle setting: If the aircraft flew at close to its $M_{dd}$, climbing will put it in colder air where it needs to fly slower to maintain the same Mach number. Depending on the starting speed, the aircraft might slow down.
High altitude, moderate throttle setting: Now the thrust decrease with altitude will dominate the equation, because the aircraft will fly close to its optimum polar point. Once it flies above the AoA of minimum drag, climbing will slow it down.
High altitude, high throttle setting: If it flies above the tropopause and close to the polar point of minimum drag, the air temperature does not change with altitude, and climbing will not change flight speed. It will stay close to its $M_{dd}$. Once it flies above the polar point of minimum drag, however, climbing will again slow it down.
Comment: @curious_cat: Yes, I should better have used dynamic pressure. Corrected. The drag increase is hard to put into a formula; I have seen approximations using the seventh power over speed, but in the end the behavior is only to capture completely by tabulation over Mach. Hint: This would be worth a question of its own.
Comment: Probably pedantic, but isn't drag increasing non linearly over speed, even below the divergence Mach number? Is the quadratic relation between drag & speed becoming super-quadratic at the drag divergence Mach number? Basically aren't we moving from a drag coefficient that's speed independent to a drag coefficient that increases rapidly with speed? What's the functional form of this dependence?
Comment: Hint taken! :) First I will do some reading to ask a better question.
Answer: Simple answer: within normal operating altitudes, mostly yes, due to the thinner air causing less drag while the engines are producing approximately the same amount of thrust
Other simple answer: it depends, and there will be an optimal altitude above which the aircraft travels slower due to the engines losing thrust.
There are a lot of aspects that complicate it, but assuming you're basically asking 'at full throttle, does an aircraft travel faster at higher altitude?' the simple answer is yes, up to a point. Obviously that doesn't apply above the service ceiling, and the optimal altitude may be lower than this.
The optimal altitude will be a function of the engine performance and airframe.
Answer: As altitude increases, the primary effect on aircraft is the reduced density of the air it flies through. A reduction in air density has three key effects on flight:

Power at a constant setting decreases, because the air-breathing engines we use have less air to use for combustion and to push against.
Lift also decreases for the same reason; the aerodynamic lift surfaces (wings) are pushing less air and therefore the forces pushing the wings (and the rest of the aircraft) upward are reduced at a given angle of attack.
Drag is decreased, because less air means less friction drag and weaker wake vortices. This is usually offset somewhat by the need to increase angle of attack, which increases induced drag.

So, as the air thins, your engines produce less power, but you need less power because your drag is reduced, but that's offset by the need to pitch up to regain lost lift. The net results are an overall decrease in aircraft performance (ability to climb/turn and max speed), but (to a point) increased fuel efficiency.
One last effect is a reduction in temperature. By the ideal gas laws, cooler air is denser than warm air at a constant pressure, however the equation is dominated by the reduction in pressure.


Can a tissue paper make a good air filter?

Question: I realized that my vacuum cleaner does not have a HEPA air filter, and I was wondering if I could just put a few layers of tissue paper to filter the air. I looked at the surface of the existing filter and tissue paper with a 120x magnifying microscope, and the tissue paper seems to have a denser surface/less gaps than the original vacuum cleaner filter. Is that a good idea? Will it actually filter air? Will it ruin my vacuum cleaner?
Answer: tissue paper can be used as a filter for particulates in air, but because the material itself is so weak, a strong flow of air through it will cause it to tear to pieces.
In addition, even if it were mechanically strong, the particles it filters out would sit right on its surface, pile up there, and quickly plug up the pores in it and stop the flow of air through it.
For these reasons, air filters are often made with some metal mesh structure embedded in them to furnish mechanical strength and with a filter material that traps particulates within its thickness and not on its surface. This "thick" filter lets the filter last longer and trap a larger quantity of particles before it gets plugged up.
Comment: Nice - so since tissue papers are cheap, I *can* supplement my vacuum cleaner filter with a couple, and _throw them away_ each time.
Comment: you can try it, but there are washable "prefilters" available for some vacuum cleaners that slip on over the existing filter to extend its life. these are common for shopvac-style machines.
Answer: First: When you say "the particles it filters out would sit right on its surface, pile up there, and quickly plug up the pores in it and stop the flow of air through it" don't forget that in reality, this is what also happens with commercial air filters.
Second: You say the tissue paper is weak, so is the paper in commercial filter before it was treated and protected. In that sense, we can do the same thing, by providing a support before installing the paper tissue.
I personally first pre-install a mosquito screen, then add the paper towel. When the paper tissue filter is no more efficient, I simply install a new one.
Simple, efficient and cost effective.


Decision Trees on training data

Question: Wouldn't any decision tree trained on a training data set have no errors in classification? In other words, wouldn't every data point be classified correctly in the training data set? How would this tie in with the misclassification rate?
Answer: A decision tree trained on a training data set would only have no errors in classification if:

You allowed your tree to have an infinite number of splits.

Theoretically then you could have a large series of branches which lead to terminal nodes that each have one observation and the correct classification on the training data.
This model however, would not generalize to new data. The model would most likely be incredibly poor when applied to new (Test) data and you have overfitted your model. Therefore when building a classification tree model pruning must be performed.
To prune your model you use the complexity parameter which balances the tradeoff between overfitting your model and the missclassification rate
See Using Tree-Based Models in R for a good explanation in R
Also Choosing The Complexity Parameter instructions
Answer: Oftentimes you stop the training process before it gets to the point where it has zero errors on the training set. One way this can be done is to not expand any leaf nodes unless more than a certain number of training examples fall on that node.
After the first pass of training, a held-out validation set can be used to prune the tree to prevent over-fitting.

Decision Trees on training data


Having A Squirrel as a pet

Question: So Inshallah I will be Getting a Squirrel but I don't know if it is prohibited in Islam should I keep it in a cage or on a bed or nest do I have to keep it complete solitare from it's Family or if I buy it from far away should I still Inshallah get becuase rabbits you can not keep them from there family,or even keep a Squirrel in general.
Answer: short answer no it is not prohibited but remember prophet Muhammed(pbuh)'s hadeeth (quote):
{A woman was punished because she imprisoned a cat until it died. On account of this, she was doomed to Hell. While she imprisoned it, she did not give the cat food or drink, nor did she free it to eat from the earth.}
also:
He also said that a man gave a very thirsty dog a drink, so God forgave his sins for this action. The Prophet was asked, "Messenger of God, are we rewarded for kindness towards animals?" He said: {There is a reward for kindness to every living animal or human.}
so if these animals can't live away from their families, you shouldn't buy them in order not to encourage the sellers to bring more of them :(
from that I prefer leaving these animals where they belong but if you insist I think it is better to buy it from far away rather than taking it from its family.
Comment: seeing how our prophet encouraged good treatment of all animals...what do u say when to say u see a dog and you know that the dog doesn't have a home to stay in or have its parents. And u know the streets isn't a good place for that dog....are you allowed to take the dog in? While keeping in mind of the classic restrictions given by scholars that keeping dogs as pet is haram?

Having A Squirrel as a pet


Why are orbits elliptical instead of circular?

Question: Why do planets rotate around a star in a specific elliptical orbit with the star at one of it's foci? Why isn't the orbit a circle?
Comment: Eduardo's answer sums up the most of it. Although you can see my answer to a similar question on Physics SE. http://physics.stackexchange.com/questions/56657/how-is-the-equation-of-motion-on-an-ellipse-derived/56709#56709
Comment: Circular orbits are a special case of elliptical orbits.
Answer: Assume the planet has a negligible mass compared to the star, that both are spherically symmetric (so Newton's law of gravitation holds, but this normally happens to a very good approximation anyway), and that there aren't any forces besides the gravity between them. If the first condition does not hold, then the acceleration of each is going to be towards the barycenter of the system, as if barycenter was attracting them a gravitational force with a certain reduced mass, so the problem is mathematically equivalent.
Take the star to be at the origin. By Newton's law of gravitation, the force is $\mathbf{F} = -\frac{m\mu}{r^3}\mathbf{r}$, where $\mathbf{r}$ is the vector to the planet, $m$ is its mass, and $\mu = GM$ is the standard gravitational parameter of the star.
Conservation Laws
Because the force is purely radial $(\mathbf{F}\parallel\mathbf{r})$, angular momentum $\mathbf{L} = \mathbf{r}\times\mathbf{p}$ is conserved:
$$\dot{\mathbf{L}} = \frac{\mathrm{d}}{\mathrm{d}t}\left(\mathbf{r}\times\mathbf{p}\right) = m(\dot{\mathbf{r}}\times \dot{\mathbf{r}}) + \mathbf{r}\times\mathbf{F} = \mathbf{0}\text{.}$$
If the initial velocity is nonzero and the star is at the origin, then in terms of the initial position and velocity, the orbit must be confined to the plane of all points with vectors $\mathbf{x}$ from the origin that satisify $\mathbf{L}\cdot\mathbf{x} = 0$. If the initial velocity is zero, then the motion is purely radial, and we can take any one of infinitely many planes that contain the barycenter and initial position.
The total orbital energy is given by
$$\mathcal{E} = \frac{p^2}{2m} - \frac{m\mu}{r}\text{,}$$
where the first term part is the kinetic energy and the second term is the gravitational potential energy of the planet. Its conservation, as well as the fact that it invokes the correct potential energy, can be proven by the fundamental theorem of calculus for line integrals.
Define the Laplace-Runge-Lenz vector to be
$$\mathbf{A} = \mathbf{p}\times\mathbf{L} - \frac{m^2\mu}{r}\mathbf{r}\text{.}$$
It is also conserved:
$$\begin{eqnarray*}
\dot{\mathbf{A}} &=& \mathbf{F}\times\mathbf{L} + \mathbf{p}\times\dot{\mathbf{L}} - \frac{m\mu}{r}\mathbf{p} + \frac{m\mu}{r^3}(\mathbf{p}\cdot\mathbf{r})\mathbf{r}\\
&=& -\frac{m\mu}{r^3}\underbrace{\left(\mathbf{r}\times(\mathbf{r}\times\mathbf{p})\right)}_{(\mathbf{r}\cdot\mathbf{p})\mathbf{r} - r^2\mathbf{p}} - \frac{m\mu}{r}\mathbf{p} + \frac{m\mu}{r^3}(\mathbf{p}\cdot\mathbf{r})\mathbf{r}\\
&=& \mathbf{0}\text{.}
\end{eqnarray*}$$
Finally, let's also take $\mathbf{f} = \mathbf{A}/(m\mathcal{E})$, which has the same units as $\mathbf{r}$, and since $\mathbf{L}\cdot\mathbf{f} = 0$, it lies along the orbital plane. As it's a conserved vector scaled by a conserved scalar, it's easy to show that $\mathbf{f}$ is conserved as well, as long as $\mathcal{E}\neq 0$.
Simplifying
By employing the vector triple product, we can write
$$\begin{eqnarray*}
\frac{1}{m}\mathbf{A} &=& \frac{1}{m}\left[p^2\mathbf{r}-(\mathbf{p}\cdot\mathbf{r})\mathbf{p}\right] -\frac{m\mu}{r}\mathbf{r}\\
&=& \left(\mathcal{E}+\frac{p^2}{2m}\right)\mathbf{r} - \frac{1}{m}\left(\mathbf{p}\cdot\mathbf{r}\right)\mathbf{p}\\
\mathcal{E}(\mathbf{f}-\mathbf{r}) &=& \left(\frac{p^2}{2m}\right)\mathbf{r} - \frac{1}{m}\left(\mathbf{p}\cdot\mathbf{r}\right)\mathbf{p}\text{,}
\end{eqnarray*}$$
the norm-squared of which is easy to crank out:
$$\mathcal{E}^2|\mathbf{f}-\mathbf{r}|^2 = \left(\mathcal{E} + \frac{m\mu}{r}\right)^2r^2\text{,}$$
where $\mathcal{E}$ was used throughout to switch between kinetic and potential terms.
Why Ellipses?
Since $\mathcal{E}$ is energy relative to infinity, to have a bound orbit we need $\mathcal{E}<0$. Thus, from the previous section, $|\mathbf{f}-\mathbf{r}| = -\mathcal{E}^{-1}\left(\mathcal{E}r + m\mu\right)$ and therefore
$$|\mathbf{f}-\mathbf{r}| + |\mathbf{r}| = -\frac{m\mu}{\mathcal{E}}\text{,}$$
which defines an ellipse with foci $\mathbf{0},\,\mathbf{f}$ and major axis $2a=-m\mu/\mathcal{E}$.
Why Not Circles?
The circle is a special case where the foci are the same point, $\mathbf{f} = \mathbf{0}$, which can be restated as
$$\mathcal{E} = -\frac{1}{2}\frac{m\mu}{r} = -\frac{p^2}{2m}\text{.}$$
In other words, circular orbits require the orbital energy to be the negative of the kinetic energy. This is possible, but almost certain not to hold exactly. Since any values of $\mathcal{E}<0$ are allowed for bound orbits, there are many more ways to have elliptic orbits. (Although some of them would actually crash because the star and planet have positive size.)
Note that hyperbolic orbits have $\mathcal{E}>0$, and we can still find the foci using the above method, though being careful with the signs. For $\mathcal{E}=0$, the second focus $\mathbf{f}$ is undefined because this is a parabolic orbit, and parabolas only have one focus within a finite distance from the center.
Additionally, the eccentricity vector $\mathbf{e} = \mathbf{A}/(m^2\mu)$ is an alternative choice for the LRL vector; as the name suggests, its magnitude is the orbital eccentricity.
Answer: It is posible for a planet to have a circular orbit, a circle, after all, is an ellipse where both foci are in the same place; this is known as having an eccentricity of 0. Eccentricity is defined in the following way:
$$ e = \frac{r_{a} - r_{p}}{r_{a} + r_{p}} $$
where $r_{a}$ is the apoapsis (farthest point in the orbit from the center of mass), and $r_{p}$ is the periapsis (the closest distance). Just to build some intuition here, if the apoapsis is twice the distance of the periapsis, the eccentricity will be $e=0.333$.
From all the planets of the solar system, Venus, with an eccentricity of 0.007 has the most circular orbit.
As to why all orbits aren't round, it comes down to kinetic energy. The kinetic energy is proportional to the square of the speed. In the orbital plane and in polar coordinates about the star, we can decompose this is into a combination of radial velocity $\dot{r}$ and angular velocity $\dot{\phi}$:
$$v^2 = \dot{r}^2 + r^2\dot{\phi}^2\text{.}$$
Since circles have constant radii, for the orbit to be circular around the star, the planet's radial velocity must be exactly zero. Additionally, the angular speed must be such that the centrifugal force in the corotating frame exactly balances the gravitational force--a little more or a little less, the imbalance will change the radial velocity, spoiling the circle.
Given the fact that velocities vary for a large number of reasons, it's no wonder that only a few orbits end up being circular, and considering that actual orbits change with time, we know they can't stay this way for long.
If you're looking for a mathematical proof, this link shares some details about it.
Here's an image showing eccentricity of some bodies in the solar system extracted from here:
Comment: @EduardoSerra - Slow down an object in a circular orbit and it will be in an elliptical orbit with the former circular orbit radius now being the apofocal distance.
Comment: This is completely wrong: "For the orbit to be round, the planet's speed must be exactly the minimun needed to be in orbit;... a little less and it would crash into the planet it's orbiting." The paragraph is also pretty confused about what orbits what. Obviously, they minimize the *radial speed*, but that's different and doesn't connect with the kinetic energy discussion. Breaking up kinetic energy into radial and angular parts, circular orbits also minimize the effective potential if angular momentum is held fixed.
Comment: @Stan you can propose an edit or give your own answer. Could you go into detail about why that statement is wrong? If a satellite is describing a circular orbit and you slow it, it'll crash into the planet; if you speed it up, it'll form and elliptical orbit.
Comment: A circular orbit has $r_a = r_p$. A small change in satellite speed is going to produce a small change in these quantities. The satellite will only crash if its new $r'_p$ is less than or equal to the planetary radius, incl. atmosphere, but since the changes are small, that can only happen if the satellite orbit was already almost hugging the planet. ... I'll suggest an edit that keeps the tie-in to kinetic energy.
Answer: I always prefer answers which try to avoid any formula and reply on argumentation instead.  Concerning the part of the question why not all orbits are circular, an argumentation would be like this:
Consider a stationary star and a moving planet.  For each impulse the planet can have, a curve for its further movement can be predicted.  If this impulse is directed exactly orthogonal to the line from the star to the planet, and if the velocity has the exact amount, then this curve of movement can be an exact circle.
But for every deviance of this one exact impulse, the resulting curve cannot be a circle:

If the speed is too low, the planet will fall towards the star (in the extreme case of an impulse of zero, this fall will be in a straight line).
If the speed is too high, the planet will gain distance from the star (similar to a slingshot).
If the impulse is not directly orthogonal to the line to the star, the first movement will move the towards or from the star, so again the curve will not be a circle.

So, one can simply argue, a circle is a very special case for the curve a planet can take around a star.
Comment: (1) The initial orthogonality argument is a good start. (2) But the "speed is too [low/high]" considerations are unjustified: how would one know that circular orbits at multiple speeds disallowed for the same distance? One can argue against the possibility of multiple speeds by balancing gravitational and centrifugal forces, but then both (1) and (2) turn into exactly what's outlined in Eduardo Serra's answer.
Comment: So you mean that one could be under the impression that the gravitational force could be like a tight rope in the sense that it will apply more force on the planet towards the star when more force is "needed" to keep the planet on a circular path?  Hmm … yes, depending on the background of the layman this could be what one expects.  Thank you for the notion; maybe I can improve my answer to address this issue as well!
Answer: This answer is addressed at the level of the question for learners not professionals.
Because our solar system planets travel in near circles people imagine that is somehow the natural condition. But it is a tricky question.
First, we should imagine gravity as an upturned trumpet shape and a planet as a ball rolling along such surface. Depending on the direction and speed that you give the ball the trajectory will fall with a tendency to curve inwards but also speed up and so will then swing around and shoot out again then later slow down and curve back in again. There is NO REASON whatsoever to get a nice circle UNLESS you choose very carefully your initial speed and direction. In general Newtons maths shows you get ellipse for an individual planet orbit and circle is one special ellipse.
However, because of how solar system originate from gravitational collapse of giant gas and dust cloud the material tends to get herded into circular swirl much like a hurricane and planets condense by accretion in near circular paths. Complex collisions can change everything but circular paths have a decent chance of not crossing into other circular orbits and so we end with roughly these and fortunately for us our system near circular orbits are luckily quite stable, and don't drift much from perturbations of other planets into eccentric orbits which would be fatal. In fact detailed calculations show the planets help each other maintain stability but there is no obvious reason why other than if it hadn't been so we wouldn't be here.


Chainline calculation

Question: I need to replace my completely worn cheap cranks (not sure how those are called, but the chainring and right crank are one piece). The chainline of my singlespeed freewheel is approx. 43 mm. I have a brand new Sugino RD2 crankset lying about (listed as 45 mm chainline with ring in outer position when used in combination with a 103 mm JIS spindle). I pulled my left crank and it revealed a Campagnolo bottom bracket (looks like centaur or veloce). I measured the width of my bottom bracket frame tube (standard 68 mm) and the  spindle protrudes for 22 mm on the left side (measured from the edge of the bottom bracket tube), so I concluded that the spindle length is probably 111 mm.
First question, is that a safe guess? Or should I really pull the right crank too?
Quick inspection gave me confidence that the combination of the ISO bb and the JIS crank won't be a problem. According to the late Sheldon Brown, this would cause the chainring to sit 4.5 mm more inboard, so the combination should give me a chainline of 44 mm with ring in outer position. Is this right?
Comment: Update: The spindle length turned out to be 111mm indeed. However, this gave me a chainline of 48.5 mm, which should correspond with a JIS spindle length of 110 mm. So, something does not add up. Probably both Sheldon Brown ánd Sugino not being one hundred percent accurate. Ended up with 41.5 mm chainline (good enough for me) by putting the outer ring in inner position.
Answer: You're taking the right approach, although 2(34+22)=112. Measuring half of the spindle as you did is fine as long as you're 100% confident it's a symmetrical spindle. In practice it's something I might do with the knowledge I have to double check later and fix things if by chance they didn't work out.
Even if Sheldon's 4.5mm number is right on, this would give you 45mm front chainline, which would work despite not being ideal. Personally I don't know if it's just 4.5mm cut and dry like that in all cases. But it's probably worth a try.
Comment: Campy race bottom brackets (for double chainset) seem to be listed as either 111mm or 115.5mm, that's why I concluded that mine must be 111mm. I'll give it a go.

Chainline calculation


Is the gravitational field of the sun uniform?

Question: I understand that rocky bodies in the solar system all exhibit gravitational anomalies. Does the sun exhibit any gravitational anomalies?  If so are they distinct enough that it effects the orbit of the Parker Solar Probe?
Answer: The Sun rotates, taking about 24.5 days to complete a revolution at the equator, a bit longer at the poles. This makes the Sun have a bit of an equatorial bulge, which in turn means that the Sun's gravity field is not quite uniform. The Sun's J2 is very small and the effect is well known.
Very large stars are asymmetric due to being close to the Eddington limit. The Sun is too small of a star to have the kind of gravitational anomalies that very large stars and rocky planets exhibit.
Comment: This is really interesting, so I've asked [How does the Eddington limit make the gravitational field of stars asymmetric?](https://astronomy.stackexchange.com/q/36812/7982)


Using the Raspberry Pi as Webserver and btsync

Question: My Raspberry Pi has an internal IP address of say 192.168.1.5 and I'm running a webserver on this IP address. If I navigate from my Macbook to this address I'm met with my current website. I'd like to host btsync as well on this machine but if I visit the default port and IP binding for btsync, 192.168.1.5:8888, on my Macbook the browser spins for a while and eventually times out. What is the correct way to setup a webserver instance and btsync instance so that I can access the btsync Web Admin page?
Answer: You can check the status with the following command:
sudo service btsync status

(re-)start it with:
sudo service btsync restart

Or maybe you misconfigured something, so you could start a reconfiguration via:
dpkg-reconfigure btsync

Answer: Try going to 192.168.1.5 or 192.168.1.5:80, it may just be that btsync is using the defalt port of all webserver which is 80.
Comment: I believe when I setup btsync from the command line it had the option of specifiying the port so I used 8888 by default so when I visit 192.168.1.5:8888 the browser just spins for a long time and timesout. Could there be some configuration I have to make with nginx?


What stands behind the desire to want things now?

Question: We've all heard the sayings that we would rather have half of a chocolate right now, than a whole chocolate in a month, because we, as wild animals, are constantly seeking for sources. But what's really the evolutionary principle behind this? Is it just that we are better at imagining a near pleasure?
Comment: somewhat related: there was an advertising campaign by *kinder surprise*, in which they conducted an "experiment", where children were presented with the surprise egg, and were told, that either they can eat it now, but if they wait for 20 minutes, they'd receive a second egg. The "study" showed that all the kids ate the thing almost immediately after being presented with it - the study concluded that the egg is just to good to wait for eating it
Comment: This is in danger of being closed as too broad given that the study of the appeal of delayed vs. immediate gratification, as well as how it varies in between people is a pretty huge field of research.
Answer: The evolutionary significance of self-regulation is that humans are more capable of this feat than apes. We can solve problems without visual feedback, by visualization. Apes are good problem solvers, but have difficulties when the task does not offer visual cues, but requires prospective and causal reasoning. This is one of the reasons monkeys fail on delay discounting tasks.


Is it safe to discharge my body's static electricity onto my MacBook Pro?

Question: Often in the winter (low humidity), getting up from and sitting down onto the seat at my desk, or taking off a jacket, and such will build up a static electric charge on my body.  Sometimes I don't remember to touch a metal object before touching my MacBook Pro, causing me to zap my laptop.  Is this likely to cause damage?  Should I be very careful to discharge before touching my laptop?
Answer: That shouldn't be a problem at all if your MacBook is connected to the power source since it's grounded and by the way I wouldn't consider harmful to touch the MacBook body after being electrically charged even if the MacBook isn't connected to the power source. You have to pay attention only if you're touching directly motherboard components.
EDIT
My answer is not completely correct.
Check the comments
Comment: What makes you think the power source is grounded? It's a double insulated device, no ground. The reason you get away with it when touching the casing is that it's a Faraday cage & the current goes round the case rather than through the components inside. I still wouldn't recommend it though.
Comment: No - leave it - it's actually better that more people know that.
Comment: Count the number of wires in the cable going from the mains to the power supply… 2, even if it's a 3-pin plug. It is not grounded.
Comment: This is why the American use of the word 'ground' confuses everybody. That's the signal ground, it is not an 'Earth' [UK terminology, much less confusing]. If the mains cable does not have a 3rd wire going to an Earth pin then it is not Earthed.
Comment: How can you say that? [This](http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html) is another evidence but I am aware that this Megsafe Connector is shipped only with retina MacBooks
Comment: Oh, ok I didn't knew that. Deleting my answer
Answer: I wouldn't recommend it. I'd try hard to remember to discharge on something more robust first.
The reason you get away with it is that the case acts as a Faraday cage, sending the 'zap' of static charge round the outside of the casing, rather than through the components inside.
This, however, doesn't include the power adapter itself, which is taking the brunt of the power-flow. It will be protected against such, but I wouldn't recommend making a habit of it, it won't survive it forever.
The standard design of any inline or 'wall wart' type power supply is that they are not in fact Earthed [US grounded, though unfortunately that term causes confusion as it appears it can apply to a return path rather than a true earth connected to physical 'dirt'/cold water mains supply etc]
The same type of supply is responsible for the buzz or hum you can sometimes feel when you touch a device powered from this type of power supply - it is leakage of very small quantities of high voltage which the PSU fails to suppress; not in itself dangerous.
In the case of you 'zapping' the computer with built-up static electricity, the charge can actually return down the same path that causes the 'buzz'.


Sourdough starter gone wild

Question: Why is my starter bubbling out of the container within minutes of mixing.  I used  2 c. bread flour, 2 c. water and 2.25 tsp active dry yeast.
Comment: I was assuming that "gone wild" referred to local strains taking over a sourdough starter culture. You might want to adjust that title to make it more clear you are talking about the vigor of a reaction.
Answer: There are a couple of (potential) issues here.
First, sourdough starter doesn't involve store bought yeast, so I'm not sure I'd call what you have a sourdough starter-- I would call it more of a poolish or a biga.  What makes it a sourdough starter is the cultivation of wild yeasts (this takes time, usually on the order of one to two weeks), but if you were to take care of and feed what you currently have for long enough, I would imagine that it would become a proper starter, although I would expect that to take more like months for the store bought yeasts to die off and be replaced with wild ones.
Second, two teaspoons of yeast for two cups of flour and two cups of water sounds like an awful lot of yeast.  If I were baking something right away that called for that much flour I don't think I'd use that much. Related, thirdly, you don't mention anything about the size of your container, but I suspect its too small. Typically expect yeast related things to double in volume, so if your container couldn't hold at least that much it is too small.
Comment: I followed a recipe I found that called for .2 ounces of yeast...the container is a quart mason jar.

Sourdough starter gone wild


How to change the Gnome panel time format?

Question: Gnome panel displays the time with 24hrs format but I want to change it to am/pm.
Answer: Go to System Settings (from the aggregate menu) and select Date & Time. Use the drop down menu on Time Format to select AM/PM:

or fire up dconf-editor, go to org/gnome/desktop/interface/clock-format and turn off Use default value then set Custom value to 12h:

Via terminal, run
gsettings set org.gnome.desktop.interface clock-format 12h

Further customization of the date/time format can be done via gnome-shell extensions, see Daniel's answer here.
Comment: Thanks. That's so stupid that one has to go command line (and remember the command) to do that.
Comment: @YaroslavNikitenko - with more recent releases (not sure since when though...) you can do it via System Settings, see my updated post.
Comment: @don_crissti - yes, you are right. I was looking for another setting, how to show date with time. That is not available through System Settings, only through command line.
Answer: November 2018, I was about to use your still-an-answer commandline method, dconf-editor, because Date and Time are not in Settings System now. I looked around a little more, and found Date and Time in Details. "Details" is burying the Date and Time format deuce in the details, IMHO. I had given up, until you threw me the bait that got me moving and looking for the setting again.
I checked out dconf-editor. It wasn't here. sudo apt-get install dconf-editor then: org, gnome, desktop, interface, clock format: click on "12 hour" or "24 hour", pick from drop down list of two choices, 12 hour and 24 hour.
dconf-editor did add something. It notified that the re-read for menus in system still needed to be made, though I had made a change before that in Settings.Details.DateAndTime.
Comment: YES!, thank you. I was going to start mentioning about Gnome being in version 3.123123123 and still having stuff like this around.


How do I select both a shape and its stroke in Photoshop CS5?

Question: I'm trying to follow a grid pattern tutorial online that uses a stroked shape for the pattern, but every time I Ctrl+click the shape to select it the stroke does not get selected with the shape and the pattern becomes a solid color with no grid when I create it. What am I doing wrong? I can't even figure out what exactly to select because the shape has a vector mask and the layer thumbnail causes the Define Pattern option to be grayed out when I select it.
Comment: it would be helpful if you linked the tutorial and the step you're on. Is the stroke on a separate layer or is it a layer effect?
Answer: I'm not too sure what you're asking about, but as far as selecting the layer and its stroke, this should help:
When you CTRL+CLICK a layer in the layers panel, you will only select the outline of the layer itself - not its styles.
One way to get around this would be to convert the layer into a smart object by either method below.
Easiest way: CTRL+CLICK (right click on PC) the layer in the layers panel and choose Convert to Smart Object

A more time-consuming way, and one I don't know why you would use, is to convert it via the Layer option on the application menu bar.

Doing either of these will mean that when you CTRL+CLICK a layer in the layers panel, you will now select the layer and its styles.  To edit the styles, simply double click on the layer's preview in the layers panel and your smart object will open.  Another bonus to working with Smart Objects is that you can transform them without losing the original image data.
Comment: There's also the Layer Panel menu... if you *really* need a third alternative :)


Block this spammer

Question: This one has answered/commented two questions, both with spam.
https://drupal.stackexchange.com/users/12373/clavier-arabe
I suggest blocking this user by admin.
Comment: Thanks; he's gone.
Answer: @MPD has been already taken care of the spammer.
For future reference about what to do when you see a spam post, see What should I do when I see spam posts?
Most importantly, don't edit a spam post to remove the spam content; that just makes harder to identify the spam.

Block this spammer


How accurate is the location detection on the iPhone's Find My iPhone app

Question: How accurate is the GPS on the iPhone's Find My iPhone app?
I used Find My iPhone app to track a friend. The first 3 stops were dead on but when it refreshed it showed them to be several streets over from actual location. What could cause that?
Answer: The generic answer you can find at Apple support website

About location precision or accuracy
Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, Bluetooth, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, your device can determine your location using crowd-sourced Wi-Fi and cell tower locations or iBeacons.

If you want to have specific numbers, in meters, approximately they would be next (according to the 2009 study):

3G iPhone w/ A-GPS ~ 8 meters
3G iPhone w/ WiFi ~ 74 meters
3G iPhone w/ Cellular positioning ~ 600 meters

In 2011 Dr. Zandbergen tested several Android smart phones. Here he found the accuracies to be slightly better than the 2009 study. They ranged from 5-8 meters. It is likely that the iPhone 4S/5 is within this range as well. It can also be assumed that iPads and other Android tablets will be comparable.
Answer: Location in phones is a combination of:

GPS (from Satellites)
Cell phone network triangulation
Often now uses Wifi assist.

Even though it might seem like Wifi should be more accurate, GPS is best. However, it is supplemented by the other two when the GPS signal is weak.
Why is GPS better? GPS satellites use an atomic clock to send timestamped signals that are insanely accurate. If your phone is in position to receive 4+ strong satellite signals, most phones GPS should be accurate within 20 feet. A weak signal can quickly reduce that to 100+ feet or even 1,000+ feet!
Now Broadcomm is said to be launching dozen of new satellites with much higher precision but it requires a new chip. The combination of those satellites and that cheap will bring precision to about a 1 foot. Source
However, most of us will be using a more rudimentary GPS chip for a while. In that case, you'll continue to see wider variation, especially when signals are weak.
Things that hurt accuracy:

Not enough strong GPS satellite signals (in a tunnel, under a storm, etc)
Not near wifi
Weak cell signal

Now your phone actually knows how accurate it is. Some apps increase the radius circle to reflect the strength/weakness of the combination of signal strengths. The Apple API tells iOS apps about signal strength using a 1-5 rating.
Important note: Wifi assisted GPS is worth looking up. My understanding is that it's not about networks you join, just networks your phone picks up on. Your phone sends data back to the maps company, reporting locations of wifi networks detected. In a city with hundreds of networks on a single street, that data starts to form a wifi map that works well when the maps app has no/weak GPS.
Your GPS accuracy can change by the second. Satellites are flying around the Earth at thousands of miles per hour, so at any given second one GPS satellite might be falling out of range and (hopefully) another may be coming into range. So you might think you want privacy, but the next time that Google Maps dot is jumping around think about how nice it would be if satellites were actually following you!
Privacy... just a GPS circle you can't live completely in, but can't live completely outside of!
Now the list of variables just keeps going here. Another example would be your phone's battery. If your battery is low, your phone might start powering down non-essential services or checking-in less to save processing power. Background threads are likely the first to be throttled down in low power mode.
Anyways, hope this helps. It's probably just scratching the surface of why your GPS location might seem a bit erratic.
Answer: It's the same accuracy as the GPS accuracy of the device. If the GPS on the iPhone can't obtain a good signal, it may use Wi-Fi triangulation which will reduce the accuracy.
GPS accuracy may be decreased depending on the phone's environment (i.e. a tunnel would not have great GPS, but standing in an open field would).
Comment: Getting the location is actually much more complicated than that, but yes, GPS is a primary source. According to Apple, "Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, Bluetooth, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, your device can determine your location using crowd-sourced Wi-Fi and cell tower locations or iBeacons."
Answer: iPhone fell off my belt riding a motorbike across an enormous field at a model airplane contest. Got a friend with a laptop with a hotspot, linked my iPad to it, and got the location of the phone with the Find My iPhone app. Once we got close, started calling the phone. I'm pretty hard of hearing so the volume was all the way up. Found it in about 15 minutes. I'm very pleased!
Answer: I don't know about the positioning between cell towers and satellites, but I was having problems with getting an accurate reading/position of my sons iPhone (1-2 mile variance).
I took off the plastic cell phone case he had on it, and what do you know: it dialed right in to his exact position (1-2 yards). So try changing phone cases to see if it makes a difference, it did in my case.
Answer: The Find my iPhone location can be off by up to 600m. In my case, this was because my phone was in a steel and concrete structure (high rise). If you look at your location on a GPS-enabled application (other than Google Maps) and zoom in, you will see your location is distorted.
Answer: GPS doesn't use Satellites.
It is all ground based antenna arrays.
Cells use mostly cell tower triangulation.
Comment: You mean the Global Positioning System satellites don't use satellites?  I learn something new every day.
Comment: This answer is about as ill-informed as it's possible to get.
Answer: Sometimes it is way, way off. By a mile or two.
Comment: We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.


Counter siege tank mass with certain build?

Question: I'm using this build from filtersc.  It's basically a MMM build (Marine, Marauder, Medivac) with +1 attack, stim, and combat shield.  I feel like I win every single game using it except those against opponents who mass tanks.  How do I counter this with the build that I have?
Comment: scout see factories then pop out more marauders than marines.
Comment: But what if they also have air attacks? I've faced enemies with battlecruisers and tanks.
Comment: you MUST scout, the BC and tank need a lot gas, at less 2 bases, and the BC will pop out at min seven with maybe 2 or 3 tank, easily raped by a drop of marauders and marines, everything it is about economy and scout
Comment: i am pretty sure the oponets who mass tanks vs you they scout you 4  barracks, I play zerg if i scout 4 barrack or more then pop out zerling ,banelings and roaches
Comment: I always scout.
Comment: then if you see the factories why you don't change a little your army?
Answer: There are two answers, assuming you can't simply outplay your opponent.

Drops
Sky Terran

Drops means you utilize your Medivacs to attack areas where he has few tanks. Tanks gain their power because they are able to eliminate large groups of units before those units have a chance to fire. When there are only two or three tanks, they can't really do this and so you're capable of taking fights that are very cost-effective for you. As soon as the game swings beyond 3-base vs. 3-base, you have a huge mobility advantage and with 2 or 3 full Medivacs you can easily deny your opponent expansions or take out under-fortified troops. At this point, the only thing you really have to fear is a single massive attack. Beating this attack can be difficult but the main reason you'll beat it is again, with drops. As you see him start to move out, drop a few full Medivacs into his production line and that way you'll force him to pull his attack back or win the game outright with no reinforcements. Then you'll just have to trade effectively and
With Sky Terran, or air play, you gain a higher Viking count and use it in conjunction with Banshees and later possibly Battlecruisers or Ravens to force unsieges and win engagements. You can do this because you should have significantly more gas than your opponent, as he's invested into Tanks and you haven't. This also gives you free reign to drop as you please. The main issue here is that transitioning to Sky Terran leaves you very open to an attack as you do so, because he'll be hitting the critical mass of tanks that he needs to devastate you with an attack just as you start to gain air control.
The only other answer is to outmicro him by picking off out of position tanks, splitting units effectively, flanking, dropping lines of units into his tank line to negate their splash power, etc.
Comment: Thank you for the detailed answer.  I have in fact used these tactics, but usually the opponent has anticipated and surrounded his base with turrets.
Answer: Countering Siege Tanks with MMM is not that easy and there is no cute timing that will help you win. You have to rely on multiple factors to secure your win.
You cannot lay back, expand get your tech and your MMM ball and a move to the win.
Rules:

The bigger the armies get the harder it is for you to win. Keep the armies small by attacking and harassing constantly if possible.
With Siege Tanks position is everything. Do not let him move his tanks freely, make him leap forward every single inch. This will demolish low APM players as they will slip in Macro.

Here are some tactics that you can incorporate into your game:

Take far away expansions, as you are mobile and he is not. Especially in a base race you will win if you have a base in every corner of the map.
Once you get a huge amount of Medivacs try to doom drop into his base while he is sieged up in the middle of the map. This means load all Marines and Marauders into the Medivacs and drop wherever he is vulnerable (e.g. main base).
Always scout where his main force is and when it unsieges. This is the only time you can safely attack.
Attack at multiple locations at once to force him to split his army (see first rule)
Try to split your army before you attack into the siege tanks.
Try to attack from multiple directions, as this way your units are more spread out.
At later stages of the game build siege tanks yourself to prevent him from leaping forward into your base.

If you drag the game into the very late game try to go air. A Battlecruiser, Banshee, Viking mix with some Marines in the back will crush Siege Tanks. He has to rely on Marines to counter this because of his huge gas investments.
Comment: I forgot to mention Ghosts. You can use Nukes to force unsieges. Cancel the nuke with ESC when he starts to unsiege and stim right into his tanks. It is even better if he stays. Use cloak and try to stay out of range of his tanks (use the huge area of effect of nukes).
Comment: Can you give an example (preferably a VOD) of how to delay and delay and delay the tank push across the map? I imagine you'd also have to use tanks or banshees, but I'm curious about the MMM tactics.
Answer: You can try using banshees.
If you go MMM, then you'll have some tech labs with your Barracks. Swap them with your Starport and make some Banshees. If you want you can research stealth too. Anyway they are pretty good against tanks ;)
If you have stealth you can hide the banshees. The best way is to attack with just one stealthed banshee so your enemy will waste his radar for just one banshee. You can then attack with all your stealthed banshees. Otherwise you can focus tanks and try to attack Marines with your Marauders.
Comment: How do you use banshees effectively when the tanks are protected by marines or turrets?
Comment: If you have stealth you can hide the banshees. The best way is to attack with just one stealthed banshee so your enemy will waste his radar for just one banshee. You can then attack with all your stealthed banshees. Otherwise you can focus tanks and try to attack Marines with your Marauders.
Comment: That's a great addition to your answer. You can click 'edit' and add that in. Or if you'd like, I can help you edit the question too.
Comment: Also, do you have an example vods of this tactic?
Comment: I couldn't find a video on this tactic and unfortunately I don't have one by myself,but I found this video http://youtu.be/6l487jq9ki8 about Banshees vs Marines!Hope you like it:) P.s.:I'm not saying you're a noob,it's just the only video I could find that looks helpful:) @BBz
Comment: Ah it's ok, I'm just helping to improve this answer. To me (I'm a BW player) banshees would make a lot of sense, since wraiths were effective at harassing tank lines if you had good control and multitasking. But I haven't seen banshees used against tank lines (or many tank lines at all in TvT for that matter) in tournament play, so I was wondering what causes this difference.