This explained the declining movement while the planets were kept in their circular orbits around the Earth. It was said that the planets move around the epicycles, which then moved along the deferents, forming a spiral orbit. He argued that planets move on two circles, a deferent and an epicycle. The Greeks insisted that the movement of the planets was completely circular.
For the moon, the final model uses a similar mechanism to create a moving shift, and in addition, the moon’s position in its epicycle is measured by a line that extends from the opposite side of the small crank circle to the epicycle’s center.
How did the Ptolemaic model explain the planet’s declining motion?
This is certainly the easiest to imagine for superior planets outside of Earth’s orbit, such as Mars. Retrograde motion is the apparent movement of a planet in a direction that is opposite to the other body in its system. When the Earth passes another planet, its gravitational pull slows the other planet, making it appear to move backwards. In fact, the question of apparent declining movement was even more difficult for the ancient Greeks because they also believed in an idea sometimes referred to as “heavenly perfection,” and they interpreted “perfection in a way that all movements in heaven must be in perfect circles.
In planets, the apparent movement in the sky is based on a combination of their inner movement (around the sun) and their reflex movement.
How did Ptolemy try to explain the apparent downward or declining movement of several planets?
Ptolemy used the idea of epicycles, or smaller circles, which were the paths for planets and then revolved around larger circles around the Earth to explain what he called retrograde motion. When used in this way, the backward movement is solely an illusion caused by the moving earth passing the outer planets in its orbits. Since one half of an epicycle runs counter to the general movement of the deviating path, the combined movement sometimes seems to slow down or even reverse (backward) direction. Meanwhile, real retrograde movements — the rotation of a planet around its axis, of moons orbiting planets, and even planets in distant solar systems — are a sign of long-forgotten collisions and conquests.
