The earth is the only planet where life exists. We live on the outer hard surface of the earth called the crust. Below the crust is a layer of rock called the mantle. The centre of the earth is called the core, which is very hot and contains molten matter. It is divided into an inner core and outer core.

THE MOVEMENT OF THE EARTH:

The earth is always moving. It spins on its own axis. This movement of the earth is called rotation. The completes one rotation in 24 hours. The earth also revolves around the sun in a fixed orbit. This movement of the earth is called revolution. The earth takes 365.25 days to complete one revolution.

The equator is another imaginary line drawn around the earth that divides it into two equal halves. The upper half is called Northern hemisphere and the lower half is called Southern hemisphere.

DAY AND NIGHT:

The rotation of the earth causes day and night. As the earth rotates, one half of it faces the sun and the other half remains away from it. The part of the earth facing the sun has day and the other half which is away from the sun has night. The earth rotates from west to east on its axis. Thus the sun appears to rise in the east and set in the west.

SEASONS:

       The revolution of the earth causes changes in seasons. We know that the axis of the earth is not straight but tilted or inclined. Even during the revolution of the earth around the sun, its axis remains inclined in the same direction.

When the north pole is tilted towards the sun, the northern hemisphere has longer hours of sunshine than the southern hemisphere. The northern hemisphere, therefore, has summer while the southern hemisphere has winter. Similarly, hemisphere and winter in the northern hemisphere. During summer, days are long and nights are short whereas in winter, days are short and nights are long.

When the sun is vertically above the equator, the northern hemisphere experiences autumn and the southern hemisphere experiences spring. There occurs no change in seasons in the places on the equator as this part receives the same amount of sunlight throughout the year. These places experience summer throughout the year.

THE MOON:

The Moon is the natural satellite of the earth. It revolves around the earth. The Moon takes 27.3 days to revolve around the earth. The average distance from earth to the moon is 3, 84,403 kilometres.

The surface of the moon is uneven.

There are high mountains, flat plains and many big, round hollow spaces called craters.

There is no life on the moon because of the absence of air and water on it.

The moon’s surface is not surrounded by any atmosphere. Thus there is no protection from the strong rays of sun.

ARTIFICIAL SATELLITES:

    Satellites which have been launched by man are called artificial satellites. Sputnik-1 was the first man-made satellite launched by Russia in 1957. Aryabhatta was the first satellite sent by India into space.

Some other important Indian satellites are Bhaskara, Rohini, Apple, INSAT-2E.

These artificial satellites have many advantages.

• They help in the weather forecasting.
• They help in the live transmission of programmes through radio or television.
• They help in telephone communication.
• They help in locating ground water and mineral resources on the earth.

SPACE PROBES:

      In order to get sufficient information about the planets, scientists have sent probes into space. They do not carry astronauts. Space probes travel very fast. Some fly past the planets whereas others orbit or land on them. They send back information to a space centre on the earth where scientists analyse it.