The constituents of the universe are galaxies, stars, planets, comets, asteroids, meteoroids. The force which keeps them bound together is called gravitational force.
A seemingly unrelated phenomenon, observation of stars,planets and their motion has been the subject of attention inmany countries since the earliest of times. Observations sincearly times recognised stars which appeared in the sky with positions unchanged year after year.
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The more interesting objects are the planets which seem to have regular motions against the background of stars. The earliest recorded model for planetary motions proposed by Ptolemy about 2000 years ago was a ‘geocentric’ model in which all celestial objects,stars, the sun and the planets, all revolved around the earth.The only motion that was thought to be possible for celestial objects was motion in a circle.
Complicated schemes of motion were put forward by Ptolemy in order to describe the observed motion of the planets. The planets were described as moving in circles with the centre of the circles themselves moving in larger circles. Similar theories were also advanced by Indian astronomers some 400 years later.
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