Different Theories About Planetary Motion. Since ancient times, scientists have studied the motion of celestial objects like the Sun, planets, and the Moon. Some significant theories about planetary motion are as follows:

(i) Geocentric Model
Around 100 A.D., the Greek astronomer Ptolemy introduced the geocentric model in his book The Almagest. This model suggested that the Earth is stationary at the center of the universe, and all celestial objects, including the Sun, Moon, and planets, revolve around it. The planets were believed to move in small circular paths called epicycles, whose centers followed larger circular paths known as *deferents*.

(ii) Aryabhata’s Contribution
In 498 A.D., Indian mathematician and astronomer Aryabhata proposed that the Earth rotates on its axis and revolves around the Sun, along with other planets. He explained various phenomena like solar and lunar eclipses, as well as the formation of days and nights. However, his groundbreaking ideas were not communicated to the Western world during his time.

(iii) Heliocentric Model
In 1543, Polish astronomer Nicolaus Copernicus proposed the heliocentric theory, suggesting that the Sun is at the center of the solar system, while the Earth and other planets revolve around it.

(iv) Contributions of Brahe and Kepler
To validate Copernicus’s heliocentric model, Danish astronomer Tycho Brahe (1546–1601) conducted detailed observations of planetary motion without telescopes. His data were later analyzed by his assistant, Johannes Kepler (1571–1630). Using Brahe’s observations, Kepler formulated three fundamental laws of planetary motion. These laws significantly supported the Copernican model and laid the groundwork for Newton’s law of gravitation.