Definition of Astronomy

Astronomy is a branch of science which deals with the study of position, composition, motion and other related facts of heavenly bodies.


The universe

It is the limitless expanse of space around us consisting of solar system, stars, galaxies etc. Main constituent of universe are:
(a) Solar system (b) Stars (c) Galaxies.

Solar System

1) It is the name given to the family of sun and is part of the universe in which we live.

2) The sun is situated at the central position of the solar system around which nine planets revolve in elliptical orbits along with some other heavenly bodies.
3) The various planets of solar system in the order of increasing distance from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
The order of planate can be recollected by the phase ?My Very Educated Mother Just Served Us Nine Pizzas?. The first later of each word gives the name of the planate.
4) The velocity of the planets their distance from the sun keeps on changing during rotation. The speed of a planet is the highest when it is closest to the sun and is the lowest when farthest away from the sun.
5) The gravitational pull of the sun on the planets control their motion.
6) The sun is the only luminous body in the solar system.
7) There are other heavenly bodies which revolve around the planets called satellites (or moons) of the planets.
8) The solar system also includes asteroids, comets and meteors.

Windows of the Astronomy

The visible light and radio waves coming from the heavenly bodies are two studies about the sun, stars i.e. about the universe, hence called windows of the astronomy.

Heliocentric theory of planetary motion

1) It was proposed by Coprenicus.
2) As according to heliocentric theory, the sun is in the centre of the universe and the various planets revolve around the sun in various orbits.

Geocentric theory of planetary motion

1) It was proposed by a Greek astronomer Ptolemy.
2) According to geometric theory, the earth lies in the centre of the universe, the sun, moon etc. revolve round the earth in complex orbits.


1) These are groups of more than 1600 tiny rock pieces moving around the sun in between Mars and Jupiter.
2) The largest asteroid is called Seres. Its radius is 350 km and period of revolution around sun is 4.6 years.
3) The radius of smallest asteroid is 50m.
4) The density and chemical composition of asteroids is similar to that of moon.
5) These are also called small planets.


1) These are composed of rock like materials surrounded by large masses of easily vaporized substance like, ice, water, ammonia and methane.
2) They revolve around the sun in highly elliptical orbit.
3) Their time period of revolution around the sun is very large.
4) Comets appear having a bright head and a long tail while passing close to the sun and when away from sun generally they show no tail.
5) The tail of comet is formed when the comet is passing close to the sun and heat of sun exerts a pressure on the material which gets evaporated due to heat of sun.
6) Halley comet was seen in early 1986 and is expected to be seen again in 2062 i.e.


1) These are smaller pieces of stones and metals which may be produced due to breaking up of comet while approaching the sun.
2) When these pieces come close to earth, and enter the earth’s atmosphere, they burn due to friction of the atmosphere and look like a bright line of fire. Such bright meteors are called fire balls or shooting stars.
3) Sometimes, the large pieces of stones (acting as meteors) do not burn completely and reach the surface of the earth as stony, iron balls resulting in crators on the earth surface. These are called meteorites.