# Wikijunior:Solar System/Earth

Earth is the planet we live on. It is the only planet in the Solar
System with liquid water on its surface. It is also the only planet we
know to have *life* on it. Earth is also known as Terra.

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!🜨.svg "🜨"){width="80"} **Earth Facts**

-   The Earth is the only planet we know to have life on it (such as
    ourselves.)
-   There is oxygen on Earth, and oxygen is necessary for life.
-   The Earth is the third planet from the Sun.
-   The Earth is the only planet we know that has liquid water on the
    surface, but scientists are trying to find others.
-   The Earth\'s axis is tilted which is why we have four different
    seasons.
-   The Earth is 4.6 billion years old.
-   The Hoba meteorite in Namibia is the biggest known meteorite to
    crash on Earth in one piece.

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!A picture taken by Apollo 17, a space
mission.

\
== How big is the Earth? == The Earth is nearly 13,000 km wide. It\'s
the largest *terrestrial planet* in the Solar System.

The Earth\'s mass is about 5,973,700,000,000,000,000,000,000 kg. That\'s
a lot. But it is little compared with Jupiter (319 Earths) and tiny
compared with the Sun (335,789 Earths) or other stars!

## What is the Earth\'s surface like ?

The Earth\'s surface is made of rock. Most of it is underwater, but not
all. Islands of rock rise up out of the water. The biggest islands are
called *continents*, of which there are seven: North
America, South
America, Europe,
Asia, Africa,
Australia, and
Antarctica. The largest bodies of water are
called *oceans*, of which there are five:
Pacific,
Atlantic,
Indian, Arctic
and Antarctic or
Southern.

thumb\|right\|upright=2\|Anawhata beach, west of Auckland, New
Zealand The Earth\'s surface is made up
of huge *plates*. They are like huge jigsaw pieces made of rock. These
plates move very, very slowly, carrying the continents with them. They
can rub beside each other, push against each other, or even move away
from each other. If there are gaps between them, hot molten rock can
rise up and make *volcanoes*. Where the plates rub or push against each
other, *earthquakes* may happen. When two plates push each other\'s rock
upwards, *mountains* are formed.

!Zabriske Point, Death Valley National Park,
CaliforniaEarth
has many kinds of *environments*. It is cold and icy in places like
Antarctica. It is hot and dry in deserts like the Sahara in Africa and
Death Valley in the United States. It is cold and dry in deserts like
Siberia in Russia. Where it is warm and wet, rainforests grow.

## Why is there life on Earth?

Wherever we have looked on Earth, we have found living things. They may
be very small, like *bacteria*, but they are there. We have found
bacteria where it is very cold, very hot, very deep, very high or very
dark.

!Galileo being deployed after being launched by the Space Shuttle
*Atlantis*.jpg "Galileo being deployed after being launched by the Space Shuttle Atlantis")
What all living things on Earth seem to need is *liquid water*. Wherever
you can find some water, there are almost always living things there
too, even if you can\'t see them. If we find liquid water somewhere else
in the Solar System, scientists think we might find some living things
there too. If we don\'t, there is always the rest of the universe to
explore!

There is another possibility. All the living things we know need liquid
water. But maybe somewhere else there are living things that don\'t need
water. Perhaps we will need to learn how to recognize them.

\
== What about the Earth\'s moon? == Earth has one moon we call\... the
Moon!  Sometimes it is called
by its name in Latin, *Luna*, so we don\'t get confused with other
planets and their *moons*. The Moon has also been called Selene
(pronounced \"suh-LEE-nee\") which is Greek for moon and was the name of
the Greek moon goddess.

Recently we have also found some other objects that are said to go
around the Earth. The largest one, called
Cruithne (pronounced \"cru-EE-nyuh\"), is
three miles wide. In fact, it *orbits* (goes around) the Sun in a way
that makes it keep coming close to Earth.

There are various ideas about where the Moon came from (no-one was
around to see it happen, after all), but the most widely held theory is
that when Earth was young, a large body hit Earth and split off a
section of the Earth that is now the Moon.

## How long is a day on this planet?

A day on Earth is 24 hours long. That\'s daytime *and* night time. A 24
hour day is how long it takes the Earth to spin around once. On the half
of the Earth that is facing the sun it is daytime and on the half of the
Earth that is facing away from the sun it is nighttime.

The spin of the earth is also the reason why the sun appears to rise in
the east and to set in the west. Although it looks like the sun is
moving from the surface of the earth, it is really the surface of the
earth that is moving. The reason we do not feel like we are spinning is
because the earth is so big compared to the size of people.

Also, the Earth is tilted at about 23°, so there are times that the
North or South pole is always facing or turned away from the sun. If you
live at one of the poles of Earth it can be light or dark throughout the
whole day!\
{{  }}

## How long is a year on this planet?

A year on Earth is about 365 and 1/4 days long. That\'s how long it
takes the Earth to orbit the Sun once. Approximately every four years we
have a leap year. A leap year contains an extra day in our calendar on
February 29th in order to account for the 1/4 of a day left over each
year.

## What is the Earth made of?

![](Inside_the_Earth.PNG "Inside_the_Earth.PNG")\
When a planet is made of rock, we call its surface the *crust*. Below
the Earth\'s crust is hot rock, some of which is molten. It is in a
layer called the *mantle*. The hot molten rock is what comes out of
volcanoes. It\'s then called *lava*.

Under the mantle is the *core* of the Earth. We think it is made from
solid iron and nickel, surrounded by hot molten iron. The temperature
there is very very hot!

The Earth\'s crust is very thin compared to the mantle and the core. But
it is very thick to us. Nobody has drilled all the way through it yet.
Even the deepest underground mines are far away from reaching its
deepest base.

## How much does the Earth\'s gravity pull on me?

It\'s easy to find your weight on Earth by using a scale. You have
weight because the Earth\'s **gravity** pulls you towards its center.
Normally, the ground or the floor get in the way, making you feel
\'stuck\' to them.

There are several kinds of scales:

!A pan balance{width="160"} 1) Comparing
of 2 masses (weights). You put the thing(s) you want to weigh on one pan
(like some marbles), and then you put several \"weights\" on the other
pan until the pointer shows that both pans have equal weights on them.
Then you look at the pan with the known weights on it, and add them all
up. The total is the mass of the thing(s) you want to weigh. !A spring
balance{width="60"} 2) A spring
balance usually has a hook on it, with a pan. You put the thing(s) you
want to weigh on the pan, the spring is pulled, and the greater the
weight, the further the spring is pulled. That distance, calibrated in
pounds or kilogram (or whatever), is usually shown either on a dial or
on a linear scale. !electronic
balance{width="100"} 3)
There are also electronic scales that give a properly calibrated
reading---grocery stores, for example, use these.

NOTE: **Gravity** varies slightly depending on the location where you
want to get the weight; spring balances and some electronic scales can,
in theory, read slightly different weights at different places because
of that, but usually in practice that difference is too small to be
noticed. But, because the balance type of scales work differently from
the spring or electronic types, they will always read the true, correct
mass. They would even give the same mass on the Moon, where gravity is
much less than on Earth.

![](Isaac_Newton_cartoon.png "Isaac_Newton_cartoon.png")

**Did you know?** that *Sir Isaac Newton* was the first person to
realize that the force pulling you down to the ground was the same force
that keeps the planets going around the Sun? The story goes that he
thought of this when he saw an apple fall from a tree.

Gravity is a very important force. As well as keeping you firmly stuck
to the Earth, it keeps the Moon going round the Earth, The Earth going
around the Sun and the Sun going around the center of the Milky Way
**galaxy**. Gravity also makes stars and planets a nice round ball
shape. In fact without gravity there wouldn\'t even *be* a Sun, Moon or
Earth because the material that they are made of would just float away
into space.

\
== Who is it named after? == !The Earth seen from the
Moon

The word earth is used for both planet Earth and soil. Other names had
been used for Earth such as Gaia, Tellus and Terra. Gaia is the Greek
goddess (meaning Mother Earth) and Terra is the Roman name of the same
goddess. Tellus is the Latin for \"Earth\", many scientific words
dealing with Earth will be derived form Latin.

Next Topic: Moon

## References

\"It is also the only planet we know\...\"
1
2\
\"The Earth is nearly\...\"
3
4\
\"The Earth\'s mass\...\"
5\
\"Earth\'s surface is made of\...\"
6
7

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