少年百科全書

 P6

What is the Earth?

The Earth is a huge ball of rock spinning in space. It is not a perfect ball as it is slightly flattened at the top and bottom. The equator is an imaginary line around the widest part. The top half of the Earth is called the northern hemisphere and the bottom half is called the southern hemisphere.

Earth Facts

The Earth measures about 40,000km around the equator. It would take a month to drive around it at 55km per hour.

The Earth weighs about 6,000 million million million tonnes.

The highest temperature ever recorded was 57.7°C in the Sahara Desert, in Africa. This is hot enough to fry an egg on the sand.

The lowest temperature ever recorded was minus 88.3°C in Antarctica. Antarctica is the coldest and windiest place in the world.

About one fifth of the land is desert. Deserts are the hottest and driest places on Earth. They have less than 250mm rain per year.

Near the equator it is hot all the year round and it rains nearly every day. The land around the equator is called the tropics.

Thick, green forests called rainforests grow in the tropics. Tropical rainforests contain over half of all the types of plants and animals in the world.

6

P7

Only about a quarter of the Earth's surface is land. There are seven big pieces of land, called continents. Most of the land is in the northern hemisphere.

Nearly three quarters of the Earth's surface is covered by sea. The four oceans are all joined together. You could sail right round the Earth without seeing land.

Many mountains are too high and cold for plants and animals to survive there. Mount Everest, the highest mountain on Earth, is 8,850 metres high.

About a tenth of the Earth's surface is always covered with ice. The further you travel from the equator, the colder it is. The Poles are the coldest. places on Earth.

Air and the atmosphere

The Earth is surrounded by a layer of air about 500 kilometres thick, called the atmosphere. It stops dangerous rays from the Sun burning up life on Earth.

We cannot see air but we feel it as wind. It is made up of many gases, such as oxygen, which we breathe, and carbon dioxide, which is used by plants.

The higher you go into the atmosphere, the less air there is and the harder it is to breathe. That is why people carry oxygen tanks when they climb mountains.

All our weather happens in the first twelve kilometres of the atmosphere. Above this, there are no clouds and Sun always shines in the daytime. Aircraft usually fly higher than the weather, about 13-22 kilometres above the Earth.

7

P8 P9

The Earth in Space

The Solar System 

The Earth is one of nine planets that circle the Sun. They are kept in orbit by the pull of the Sun's gravity*.

Together they make up the Solar System. It was probably formed from dust and gases billions of years ago.

Mercury is the planet closest to the Sun. It is a little bigger than our Moon.

Venus is so bright, it is often mistaken for a UFO- an unidentified flying object. Its surface is hidden by thick, white clouds.

Earth is probably the only planet which has water.

Mars is sometimes called the "red planet" because it is made of red rocks.

Jupiter is mostly gas with a tiny rocky core. It has sixteen moons.

Saturn is made of mainly gas and has rings of dust and ice circling around it. It has eighteen moons.

Uranus is circled by rings of dust.

Neptune sometimes travels beyond Pluto and becomes. the furthest planet from the Sun.

365 days

Pluto was first sighted in 1930. Scientists now consider it to be a dwarf planet.

The asteroids are thousands of rocks which go around the Sun between Mars and Jupiter.

Planet data

Use this chart to work out the order of the planets from biggest to smallest. Which one takes the longest to orbit the Sun? Which one is the furthest away from the Sun?

Name of Planet

Diameter

Distance from the Sun

Time taken to orbit the Sun

Pluto

Neptune

Uranus

Saturn

Jupiter

Mars

Earth

Venus 

Mercury

kilometres

2.300

49,530

51,120

120,540

142,800

6,790

12,600 

12,100

4,880

kilometres

5.950 million

4,497 million

2,870 million

1,427 million

778 million

228 million

150 million

108 million

58 million

years

248 years

165 years

84 years

29.5 years

12 years

687 days 

1 year = 365 days

225 days

88 days

What does the Sun do?

The Sun gives the Earth light and warmth. Without it, the Earth would be in constant darkness and too icy cold for anything to survive.

*You can read more about gravity on page 120.

The Sun is so far away that it takes eight minutes for its rays to reach Earth. It is so big that you could fit over a million Earths into it.

How hot is the Sun?

The Sun is a gigantic ball of burning gases. Its surface temperature is 5500°C. A pinhead as hot as this could kill you from 150km away.

Sunspots and solar flares

Sunspots are dark areas of gas which are slightly cooler than the rest of the Sun's surface. Solar flares are giant jets of gas that shoot up millions of miles.

WARNING Never look directly at the Sun as its brightness could damage your eyes.

Watching an eclipse

Sun

Moon 

Earth

A total eclipse happens when the Moon passes exactly in front of the Sun and blocks off its light for a few minutes. You can see a total eclipse of the Sun from any place on Earth, but you might have to wait for about 400 years.

*Approximate measurement

P10

Moons

A moon is a ball of rock that circles a planet. Most of the planets in our solar system have moons. You can see them on pages 8-9.

Our Moon has no light of its own and is lit up by light from the Sun.

Who is the man in the Moon?

It looks as if the Moon has a face because rocks on the surface make dark shadows.

Eclipse of the Moon

Sun

Earth

Moon

An eclipse of the Moon happens when a full Moon moves into the shadow behind the Earth.

During the eclipse, the Sun's light cannot reach the Moon so it looks as if it has vanished.

The dark side of the Moon

We never see one side of the Moon because the same side is always facing the Earth.

The Moon's far side was a mystery until photographed by a space probe in 1959.

Why does the Moon change shape?

The Sun lights up different parts of the Moon as it orbits the Earth. This makes the moon look as if it is changing shape.

It takes the Moon 27.3 days to circle the Earth. The different shapes of the Moon are called phases.

There is a new Moon when the Moon is between the Sun and the Earth and we see only a sliver of the lit-up part.

Each night for the next two weeks we see more and more of the lit-up surface as the Moon orbits the Earth.

There is a full Moon when the Moon has travelled halfway around the Earth and we can see all the lit-up surface in the sky.

Over the next two weeks, the Moon appears to shrink as we see less and less of the lit-up surface.

P11

Stars

A star is born

A star begins as a ball of gas that shrinks and becomes hot. The gas burns, which makes the star shine. It may shine for 10,000 million years.

As the gas is used up, the star explodes or shrinks. The tiny star, called a white dwarf, is almost dead.

Measuring the Milky Way

Our Solar System is somewhere here.

Our Sun is a star in a galaxy (star family) called the Milky Way. The Milky Way has 100,000 million stars. Scientists believe there may be 6000 other galaxies scattered. through space.

The huge distances in space are measured in light years. A light year is 9.5 million million km. A spacecraft travelling at the speed of light would take millions of years to reach Earth.

Comets and meteors

Comets sometimes appear as they orbit the Sun. They are balls of dirty ice and have long tails.

A shooting star is a meteor or rock burning as it enters the Earth's atmosphere.

How do we know? 

Astronomers are scientists who study the stars. Until recently, they only had their eyes or simple telescopes to rely upon.

Now, more powerful telescopes on satellites can "see" deep into space and send information back to Earth. Space probes are sent to explore the planets.

Manned space shuttles take off like rockets and land like gliders. They carry all sorts of useful scientific equipment into Outer Space.

P12

How the land moves

How old is the Earth?

The Earth was formed about 4,600 million years ago. At first, it was a fiery mixture of boiling rock and poisonous gases.

As millions of years went by, the Earth grew cooler and a thin crust of solid rock formed on its surface.

It is 6,300km to the centre of the Earth. The temperature there is as hot as the Sun.

The very middle of the Earth is called the core. It is a solid ball of melted iron.

The mantle is a layer of soft, flowing rock. Towards the surface, it is hot and liquid.

The Earth's crust is made of hard rock which is split into sections called plates.

What happened to the supercontinent?

Scientists think the continents may once have been joined as a giant piece of land called Panagaea.

About 190 million years ago movements in the Earth's crust made it break up. The pieces of land are still moving.

How mountains are made

Mountains take millions of years to form. Sometimes the Earth's crust is squeezed up to form fold mountains.

Block mountains form when the Earth's crust splits and one side is slowly pushed up.

When melted rock below the Earth's crust slowly forces land upwards, dome mountains are formed.

What is inside the Earth?

12

P13

Earthquakes and volcanoes

When the ground trembles and shakes, it is called an earthquake. Earthquakes can split and crack the ground, destroy buildings and cause floods and fires.

Geysers and mudsprings are found near volcanees.. Steam the Earth's heat. and boiling mud are escaping.

A volcano is a place where red-hot liquid rock, called lava, forces its way up a crack from deep inside the Earth. The lava hardens as it cools, to form a mountain.

Many active volcanoes are under the sea. Scientists try to predict when the

sleeping (dormant) ones. will erupt again.

Earthquakes and volcanoes often happen close. together where the plates that form the Earth's crust join.

Eruptions

The volcano Paricutin suddenly erupted in Mexico in 1943. In two years, it had grown 457 metres high and 16 kilometres wide. Every day, it blew out 100,000 tonnes of lava.

A fault line is where two plates of the Earth's crust rub against each other deep down and make the ground move.

Tidal waves

After lying dormant for over 200 years, a volcano called Krakatoa, in Indonesia, exploded in 1883. The explosion caused a tidal wave 41 metres high which killed over 36,000 people.

13

P14

Under the ground

Burrowing down

The Earth's crust has several layers. The topt? layer is soil, which is made of crushed rock and the remains of plants.

Below the soil there is al small layer of stones.

How are rocks made?

Rocks are formed from the minerals that are found in the Earth's crust. Different rocks contain different minerals and they are grouped into three families depending on how they are formed...

Igneous rock

Hot, molten rock bubbled up from deep inside the Earth, then cooled and hardened into rocks such as granite and basalt.

Sedimentary rock Rock that has built up in layers over millions of years. It is made from tiny bits of rock, sand or shells which have been washed into the sea or a lake.

These bits of rock sink to the bottom and over millions of years they harden together into solid rock.

Metamorphic rock Mainly sedimentary rocks that have changed in some way by being squeezed or heated deep inside the Earth.

Earthquakes sometimes force sedimentary rock up from the sea-bed so it becomes part of the land.

Small animals, plants and insects burrow into the soil and help break it up.

Underneath everything there Is a layer of solid rock.

Minerals

Some minerals do not form rocks. They grow into mineral crystals under the ground. Each mineral has different shaped crystals.

Everyday minerals

Minerals are used to make all these everyday objects.

Precious minerals

Gemstones are very beautiful and rare. They are used to make jewellery.

P15