A body moving in an orbit around a planet is called Satellite.
They are two types.
i) Man made or Artificial Satellites
ii) Natural Satellites.

Time taken by a satellite to complete one revolution round the earth is called time period.
Time period T = \(\frac { Distances covered }{ Orbital velocity }\)

The horizontal velocity that has to be imparted to a satellite at the determined height, so that it makes a circular orbit around the planet is called orbital velocity.

It is the time duration required for the Sun to orbit once around the centre of the Milky way galaxy. It takes 250 million years to do that.

The Sun and celestial bodies which revolve around it form the solar system.

h = 600 x 10³ m
V = 7616 x 10³ krn/s
R = 6371 x 10³ m
Substituting the values.
T = \(\frac {2 \pi (R + h)}{V}\)
= \(\frac { 2\times \frac { 22 }{ 7 } \left( 6371+600 \right) { 10 }^{ 3 } }{ 7616 } \)
= 5.748 x 10³
= 5748 seconds.
This is T ≈ 96 min

Radius of the earth = R = 6400 km
= 6.4 x 10⁶ m
Radius of the orbit of the Moon = r = 3.84 x 10⁵ km or 3.84 x 10⁸ m
g = 9.8 m/s²
To find period of revolution of the Moon.
T = 2π \(\sqrt {\frac {{r}^{3}}{GM}}\)
T = 2π \(\sqrt {\frac {{r}^{3}}{{R}^{2}g}}\)
T = 2 x 3.14 \(\sqrt { \frac { { \left( 3.84\times { 10 }^{ 8 } \right) }^{ 3 } }{ { \left( 6.4\times { 10 }^{ 6 } \right) }^{ 2 }\times 9.8 } } \)
T = 2 x 3.14 \(\sqrt { \frac { { 3.84 }^{ 3 }\times { 10 }^{ 24 } }{ { 6.4 }^{ 2 }\times { 10 }^{ 12 }\times 9.8 } } \)
T = 2.538 x 10⁶s
T = \(\frac {2.358 \times {10}^{6}}{24 \times 60 \times 60}\)
T = 27.3 days
Period of revolution of the earth to revolve around the moon is = 27.3 days.

After the Big Bang, clouds of gases began to compress under gravity to form the building blocks of galaxies. A galaxy is a massive collection of gas, dust and millions of stars and their solar systems.

1. Meteors are small pieces of rocks scattered throughout the solar system .
2. They travel with high speed, they come close to the earth's atmosphere and are attracted by the gravitational force of the earth .
3. Most of them are burnt up by the heat generated due to friction in the earth's atmosphere. They are called Meteors .
4. Some of the bigger meteors may not be burnt completely and they fall on the surface of the earth, these are called meteorites.

1. The four large planets Jupiter, Saturn, Uranus and Neptune spread out in the outer solar system, that slowly orbit the Sun are called outer planets.
2. They are known as gas giants and are called gaseous planets, because they have hydrogen, helium and other gases in huge amounts and have very dense atmosphere
3. These planets have rings around them. These rings are actually tiny pieces of rock covered with ice.

1. At the time of Big Bang, hydrogen gas condensed to form huge clouds which later concentrated and formed the numerous galaxies .
2. Some of the hydrogen gas was left free and started floating around in our galaxy .
3. This free floating hydrogen gas concentrated and paved way for the formation of the sun and solar system.
Rotation:
The Sun rotates on its own axis. Since the Sun is primarily made of very hot gases. The surface at the equator rotates once every 25.4 days. The rotation near the poles takes around 36 days.

1. Stars were the fundamental building blocks of galaxies .
2. Stars were formed, when the galaxies were formed during 'big bang' .
3. Stars produce heat light ultraviolet rays, X-rays and other forms of radiations .
4. They are largely composed of gas and plasma .
5. They are built by hydrogen gases. Hydrogen atoms fuse together to form helium atoms and in the process they produce large amount of heat.
6. Our universe contains more than 100 billion galaxies and each of those galaxies may have 100 billion stars.
7. The brightness of the stars depends on their intensity and distance from the earth.
8. Stars appear to be in different colours depending on their temperature.
9. Group of stars are called as constellations .
10. There are 88 formally accepted constellations such as Aries, Gemini, Leo etc.,