Ptolemy assumed that all the planets revolved around the Earth in a circular path, including the moon. Also, each planet moves in a small circle which is called as an epicycle. The Earth is at the center of all the planets, the sun and the moon. All these bodies are surrounded by a ring of infinite stars which also revolve around the Earth.

Copernicus observed that all the planets have the same annual movement, except the sun. Thus, he hypothesized the sun standing at the centre. Also, this idea is strengthened by the characteristics of the sun (emission of heat and light). The stars are distant and are motionless. But, as the Earth rotates on its own axis, it appears that stars are moving. He proposed that the planets while making a number of circular movements, gradually created a non-circular path.

The geocentric theory and heliocentric theory explain the relative movement of stars and planets with respect to the Earth, the sun and the moon.

The geocentric theory states that the Earth being in the centre of universe, and the sun and the planets orbit around the Earth. The whole set that is the Earth along with the sun and the planets, exists within the revolving globe of the stars. This theory was proposed by Ptolemy.

The heliocentric theory states that the sun is at the centre of the solar system that the planets and stars are orbiting around. This was proposed by Nicholas Copernicus.