Question

Can you describe or explain retrograde motion with respect to our perspective? Also, provide examples. How would Ptolemy explain or describe such motion? (answer in ~250 words)

Answer 1

Retrograde motion is always not an illusion in earth sky where the motion of an object seems to be reverting or moving backward. This illusion is experienced in our day to day life when two cars are moving in the same direction and the car moving with greater speed if pull alongside the slower car seems to be moving in the backward direction for just a moment, and the faster-moving car pulls ahead the car appears to resume its normal motion.

The same thing happens when the earth passes through the slower moving outer planets When the earth moving passes slower moving planet Jupiter or Mars or Saturn they are in the much outer orbit than earth and moves more slowly than earth in their orbit appears to reverse course in our sky for a couple of months,

There are two types of motion in a planet one is orbital rotation along its axis and the other rotation is around the sun in its orbit, For example, the motion of Mercury when it speeds through closest to the sun its orbital speed overtakes its rotational speed an astronaut on the surface will see sun partially rise and again dip back in the horizon and again rise and move from east to west.

Ptolemy is an ancient astronomer. Geographer and mathematician. He used a geocentric model and considered the earth as the center and other planets revolving around it in circular orbits, The motion in the perfect circle could not explain the phenomenon of retrograde motion he had used. This was accepted until the 16th century, Ptolemy had used epicycle and epicycles that are small circle the center of which moves around in the circumference of the larger circle.