Paheli and Boojho visited their grandparents’ village during summer break. After dinner, they went on to the roof of the house. It was a clear cloudless night. They were surprised to see a large number of bright stars in the sky. They had never seen such a beautiful scene in their city (Fig. 17.1).
Is there a change in the shape of the
moon everyday? Are there days when
the shape of the moon appears to be
perfectly round? Are there days when
the moon cannot be seen at all even if
the sky is clear?
The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.
What other objects do you see in the night sky? There is a large number of stars in the sky. Observe carefully on a dark night and from a place away from a big city. Are all the stars equally bright? Are they of the same colour? In fact, stars emit light of their own. The Sun is also a star. Why does it appear so large compared to the other stars?
Look at the sky for some time. Can you
see some stars forming groups with
shapes like those in Fig. 17.11.
The stars forming a group that has a recognisable shape is called a constellation.
The Sun and the celestial bodies which
revolve around it form the solar system.
It consists of large number of bodies
such as planets, comets, asteroids and
meteors. The gravitational attraction
between the Sun and these objects
keeps them revolving around it.
The Earth, as you know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars , Jupiter, Saturn, Uranus and Neptune.
There are some other bodies which
revolve around the Sun. They are also
members of the solar system. Let us
learn about some of them.
There is a large gap in between the orbits of Mars and Jupiter (Fig. 17.25). This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. Asteroids can only be seen through large telescopes.