# Wikijunior:Solar System/Introduction

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## Introduction

!These planets are little bigger than dots in the night sky, but if you
call them dots, then Earth (in the marble-sized group, topleft) is
smaller than a grain of
sand. is smaller than a grain of sand."){width="250"
height="250"}

Wikijunior books welcomes you to the children\'s book *Solar System*.
Outer space is perhaps the final frontier for humanity. Even though the
rest of the solar system objects may seem like tiny dots from Earth, our
celestial neighbors are still important to learn about. If, when you
grow up, you are going to be an astronaut and travel in space, you will
need to know quite a bit about the solar system. And even if you don\'t
travel to space, the things other people do there will affect you, so
you need to know about it. Also, if you meet an astronomer or an
astronaut, you do not want to sound ignorant! The importance of learning
about the solar system has led many experts here at Wikijunior to donate
their time and talents to bring this volume together.

Wikibooks is a project of the Wikimedia Foundation, aimed at providing
free, easily available, quality reading for adults and children to
promote the global spread of knowledge. Traditional publishing houses
make the bulk of their income from re-issues of classic books, new books
by authors with long track records, or celebrities who are famous in
their own right. The chances of a truly good new work being published
solely on the basis of merit skyrocket when the traditional business
model is overturned and the wellspring of new talent out there is tapped
using the Internet.

With this project we have reached a crossroads between the books of
yesterday and the encyclopedia of everything for tomorrow. Simply by
reading this book and telling others about it, you have advanced the
cause of free access to information and of democratizing the field of
publishing. Thank you, and once again, welcome.

## Studying the Solar System

!A stamp issued by the Soviet Union, showing Sputnik\'s planned orbit
path around the Earth.
{width="250"
height="300"} Scientists are still exploring the universe. Whether
things are very tiny, like the cells of plants and animals, or very big,
like a solar system or a galaxy, there is still a lot that scientists
don't know.

Scientists who study space are called
Astronomers or
Astrophysicists. They explore the solar
system in two different ways. Astronomers do it by observing celestial
bodies through telescopes, while astrophysicists (a specialized class of
astronomers) try to explain the observed phenomena using physics, as
suggested by the name, and theorize about what is still unseen or
unknown.

Telescopes were invented in the early 1600s in Europe and allowed
curious scientists like Galileo Galilei to look at very distant things
in close-up and see details of the solar system and the universe that
nobody had ever seen before. Using his telescope, Galileo was the first
person to see the rings around Saturn and draw a very detailed picture
of the moon. He also saw the four largest moons of Jupiter, sometimes
called the Galilean moons, and saw spots on the Sun. Telescopes on Earth
and in space are still used to explore the Solar System. There are
several types of telescopes. The most common ones are optical
telescopes, such as Galileo\'s (*optical* means relating to light, which
is what these telescopes see), and radio telescopes, that pick up radio
waves from outer space (radio waves occur naturally; they don\'t have to
be made by humans).

Until the 1950s, humans were limited to exploring the Solar System from
the ground. However, in 1957, the Soviet Union (now Russia and several
other countries) launched the very first satellite, *Sputnik 1*
(pronounced like *spoo-tneek*). Since then, humans have been launching
vehicles into space to explore the Solar System---some manned (with
people) and some unmanned (without people).

Now, the Solar System is full of human-made probes exploring the planets
and moons of the solar system. The probes send back information to Earth
that scientists study to figure out what it means. Every year,
scientists learn more about the Solar System. Sometimes they learn
things about other worlds that remind us of Earth. Other times, the
things they learn are very strange. Everything they learn helps us
understand more about Earth, Earth\'s history, and Earth\'s
neighborhood.

## How is the Solar System measured?

It's important that scientists use measurement to tell how big, how hot
or cold, or how far away something is. In science, people use the
**metric system**, which is named after its basic unit the metre. Below
is a description of all the types of measurement used in this book.

### Distance or length/width

For indications of measurements, such as how distant something is or how
long or wide it is, scientists use **kilometres** or **metres**. Units
of the metric system (a kilometre is 1000 metres, a metre is little more
than 3 feet in the old imperial unit system still in use in some
regions). Kilometres is often shortened to **km**, and metres is often
shortened to **m**. Kilometres and metres can also be spelled as
*Kilometers* and *Meters*, but the International Bureau of Weights and
Measurements uses the -re versions as the official spelling.

Since distances outside of the Earth get so vast, scientists have also
invented new units of measurement to make it easier to measure large
distances in space. They invented the **Astronomical unit** (㍳) which
is equivalent to 149 597 871 kilometres. One Astronomical Unit is the
approximate distance between the Earth and the Sun. The mean distance
between the sun and Neptune (the farthest planet from the sun) is 30.1
㍳, or 4.503 billion kilometres. This is why it\'s good to use ㍳ for a
big distance like that: 30 distances from the Earth to the sun is easier
to understand than four and a half billion kilometres. You might not
realize there was something wrong if someone told you the distance from
the sun to Neptune was 45.03 million kilometres, but if you thought of
it as 0.301 ㍳, you would know it couldn\'t be right.

In astronomy, they have leveling-up scales similar to the metric (10 mm
in 1 cm, 100 cm in 1 m) and customary (12 in. in 1 ft., 3 ft. in 1 yd.)
scales. Usually, these distances are not used within the solar system,
but they are important to know if you want to be an astronomer or
astrophysicist.

:   1 Light-year (ly) = 63241.077 ㍳
:   1 Parsec (pc) = 3.26 ly
:   1 Kiloparsec (kpc) = 1000 pc
:   1 Megaparsec (mpc) = 1000 kpc
:   1 Gigaparsec (gpc) = 1000 mpc

To help you visualise just how big some of these are,

:   4.22 ly = The distance from Earth to the nearest star (Proxima
    Centauri) other than the sun
:   1.3 pc = The distance from Earth to Proxima Centauri
:   34 kpc = The length of the Milky Way
:   0.76 mpc = The distance from Earth to the nearest Galaxy, the
    Andromeda Galaxy
:   14 gpc = The radius of the observable universe

### Mass

In order to measure how big something is, scientists measure the
**mass** of an object in kilograms or grams. There are 1000 grams in a
kilogram. Scientists do not use weight, because weight is a measurement
that means how hard gravity is pulling on an object. An object's
**mass** is the same wherever you are in the solar system because it
measures how much stuff, or matter, a thing is made up of. Your weight
will change because the amount of gravity there is varies from place to
place.

On Earth, mass and weight are the same. If you weigh 30kg (short for
kilograms) on Earth, your mass is 30kg. If you are floating around in
space, your weight if you try to stand on a set of scales will be 0kg,
but your mass is still 30kg. You are still made up of the same amount of
matter.

### Temperature

Temperature is a numeric reference, on a scale of degrees, on how hot or
cold something is in relation to a \"constant\" reference. There are
several scales. In our
everyday lives, we measure temperature in degrees Celsius, written °C
for short (the little circle ° means \"degrees\"), or degrees
Fahrenheit, written °F for short. But scientists, especially
astronomers, use **degrees Kelvin** to measure temperature, written K
for short (with no °). Don\'t use Celsius or Fahrenheit for temperatures
in astronomy!

Some important temperatures to know in Kelvin:

-   0K (−273.15°C) is maximum coldness, also called absolute zero. This
    is a great thing about measuring temperature in Kelvin: that the
    number is always positive and tells you how much warmer things are
    than the coldest they could possibly be.
-   The freezing point of water is 273.15K (0°C), and the boiling point
    of water is 373.15K (100°C).
-   A sunny day of 30°C (86°F) would be 303.15K. This is 273.15 + 30,
    because degrees change equally in Kelvin and Celsius.

## Introduction for Parents, Guardians, and Educators

`<i>`{=html}The Solar System`</i>`{=html} is a Wikijunior book written
by a group of volunteers and made freely available to Internet users,
printers, and distributors under the terms of its license. It is the
result of cooperation between The Beck Foundation, The Wikimedia
Foundation, and volunteer writers and editors.

The volunteer writers and contributors thank you for obtaining this
book. By making it available to a young person, you complete the goal of
the Wikijunior project, which is to encourage reading and literacy among
young people.

The original text and graphics are available at
<http://www.wikibooks.org> and printed versions may be available from
many different entities under license.

Again, thank you, and enjoy.

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