Explain how geocentric theory is required by helliocentric theory using the idea of retrograde motion of planets.

Explanation

(i) To explain this retrograde motion, Ptolemy introduced the concept of "epicycle" in his geocentric model. According to this theory, while the planet orbited the Earth, it also underwent another circular motion termed as "epicycle".
(ii) A combination of epicycle and circular motion around the Earth gave rise to retrograde motion of the planets with respect to Earth.
(iii) But Ptolemy's model became more and more complex as every planet was found to undergo retrograde motion.
(iv) In the 15th century, the Polish astronmer Copernics proposed.
(v) The helicentric model to explain this problem in a simpler manner. According to this model, the Sun is at the center of the solar system and all planets orbited the Sun.
(vi) The retrograde motion of planets with respect to Earth is because of the relative motion of the planet with respect to Earth.