The Earth and Space

Everything we see above the Earth is the sky. The Sun, the Moon, the planets and the stars are all in the sky. There are countless big and small things up there. They are called heavenly bodies (or celestial bodies).

Heavenly bodies are not all alike.

• Some are close to the Earth, and look big. Others are far away, and look small.
• Some have their own light. Others shine only because the light of another body falls on them.
• Some move across the sky. Others seem to stay in place.

By these differences, heavenly bodies are sorted into stars, planets, satellites, comets, asteroids (small rocky “baby planets”) and meteors.

The Earth we live on is a heavenly body too. As far as we know, it is the only one with the right air, water and temperature for living things.

Long ago, people believed that the Earth stood still, in the middle of everything. They thought the Sun, the Moon and the stars all went round it. Then scientists looked carefully and measured. They found that the Earth goes round the Sun.

Then why does the Sun seem to move across our sky? Think of riding in a bus. The houses beside the road seem to rush past you. But the houses are still. The bus is moving. In the same way, the Earth spins, and that makes the Sun seem to move round us.

The heavenly body that we see in different shapes, at different times of the night, is the Moon. Just as the Earth goes round the Sun, the Moon goes round the Earth. The Moon has no light of its own. It shines because sunlight falls on it. So moonlight gives us hardly any heat. It rises and sets at a different time each day.

Stars

A star is a huge, very hot ball of glowing gas. It shines with its own light. On a dark night, you can see thousands of them. There are far more than anyone could count.

Stars are enormous. The Sun is a star too. It is only a medium-sized one. It looks much bigger and brighter than the other stars because it is so much closer to us. The next nearest star is about 270,000 times farther away than the Sun.

Planets

A planet is a big, round heavenly body that goes round the Sun along a fixed path. Its path is called its orbit. A planet has no light of its own. It shines because sunlight falls on it.

There are eight planets going round the Sun. The Earth is one of them.

Satellites

Some planets have smaller bodies going round them. A body that goes round a planet is called a satellite. Like a planet, a satellite moves along a fixed path, its orbit.

The Moon is the Earth’s only natural satellite.

The solar system

The Sun is in the middle. Eight planets go round it, and many satellites go round the planets. The Sun and everything that goes round it make a family. This family is called the solar system. (Sol means the Sun.)

Here are the eight planets, in order from the Sun, and their satellites (moons).

To remember the order, say: “My Very Eager Mother Just Served Us Noodles.”

The time the Earth takes to go round the Sun once is called one year. It is about 365 days. The time the Moon takes to go round the Earth once is called a month. It is about 30 days (29½ days, from one full moon to the next).

1. True or false?

2. Fill in the blanks

star, Moon, Sun, satellite, planet, Earth

3. Match the planets with their number of satellites

4. Choose the best answer

5. Answer these questions

The Earth is a moving heavenly body. It moves in two ways at the same time.

• It spins on its own axis, like a top. This is its rotation.
• It goes round the Sun, along a fixed orbit, with the Sun in the middle. This is its revolution.

Rotation of the Earth

The Earth spins on its axis all the time, from west to east, like a grinding stone. This spinning is called the rotation of the Earth.

As it spins, the half of the Earth facing the Sun gets sunlight. The other half gets none. It is day in the sunlit half, and night in the dark half.

A globe has a rod through its middle, from the North Pole to the South Pole. The globe turns around this rod. The rod is the globe’s axis. The Earth spins around an imaginary line like it. The Earth’s axis runs through its center, from the North Pole to the South Pole.

The Earth’s axis is not straight up and down. It is tilted. It makes an angle of 66.5° with the flat plane of the Earth’s orbit. (That is 23.5° away from straight up.) A school globe is tilted in the same way.

The Earth takes one day (24 hours) to spin once on its axis. This movement is called its daily motion (or daily rotation). It gives us day and night.

Because the Earth spins, it is not the same time everywhere. When the Sun is rising in one place, it is setting in another. Places to the east see the Sun first. So their clocks are ahead. For example, when it is 3:00 PM in Nepal, it is 6:15 PM in Japan, which is far to the east.

Revolution of the Earth

The Earth goes round the Sun along a fixed orbit. This movement is called the revolution of the Earth, or its annual motion. (Annual means once a year.) One trip round takes about 365 days: one year.

The equator is an imaginary line round the middle of the Earth, halfway between the North Pole and the South Pole. The half of the Earth north of it is the northern hemisphere. Nepal is in it. The half south of it is the southern hemisphere.

All through the year, the Earth’s axis leans the same way in space. So in June, the northern half (where Nepal is) leans towards the Sun. In December, it leans away.

The Earth goes round the Sun, and spins on a tilted axis. So sunlight does not fall equally on every part of the Earth all year. This is why:

• the length of day and night changes through the year;
• the seasons change: summer, autumn, winter and spring.

In Nepal, the longest day is around 21 June, and the shortest around 21 December. Around 21 March and 23 September, day and night are equal all over the Earth.

1. True or false?

2. Fill in the blanks

faster, 66.5°, axis, orbit, equal, slow, 23.5°

3. Choose the best answer

4. Answer these questions

Do you see the Moon in the sky every evening? Look for a few days and you will see that you don’t. Some evenings it is round and bright. Some evenings it is a thin curve. And some nights it is not there at all.

The Moon goes round the Earth along an oval (elliptical) orbit. It takes about a month. This time is called a lunar month. (Luna means the Moon.) Each day of the lunar month is called a tithi.

Each day, the Moon moves about 12° further round its orbit: about one thirtieth of a full circle. So it goes all the way round in about a month. As it goes round, it is sometimes between the Sun and the Earth, and sometimes on the far side of the Earth.

• New moon (Aunsi): the Moon is between the Sun and the Earth. Its sunlit half faces away from us. Its dark half faces the Earth. So we can’t see it, by day or by night. A few days before the new moon, a thin Moon can be seen in the morning. A few days after, a thin Moon shows in the evening.
• Full moon (Purnima): the Earth is between the Sun and the Moon. The whole sunlit half of the Moon faces us. We see a round, bright Moon all night long. It rises at sunset and sets at sunrise.

After the full moon, the Moon rises about 50 minutes later each day. It also gets smaller and less bright. Its rising gets later and later until, on the new moon day, the Moon rises at almost the same time as the Sun.

The changing shapes of the Moon that we see are called the phases of the Moon.

Bright half and dark half

From the new moon to the full moon, the lit part of the Moon grows bigger each day. This half of the lunar month is called the bright half (Shukla Paksha).

After the full moon, the lit part shrinks each day, until the new moon. This half is called the dark half (Krishna Paksha).

The tithis of the bright half have Shukla in front of their names. The tithis of the dark half have Krishna in front. For example, Shukla Pratipada is the first day after the new moon (Aunsi). Krishna Pratipada is the first day after the full moon (Purnima).

The time from one full moon to the next is one lunar month.

1. True or false?

2. Fill in the blanks

bright half (Shukla Paksha), 60, new moon (Aunsi), full moon (Purnima), 24, increase, decrease

3. Choose the best answer

4. Answer these questions