Why was retrograde motion important in astronomy?

1 Answer
Jun 20, 2017

Retrograde motion is/was important because it needs explaining.


Most planets orbit and spin in the same direction. If a body orbits of spins in the opposite direction to the rest it is called retrograde.

The solar system was formed from a disk of material which was spinning. The Sun and planets formed from that disk and spin in the same direction.

If a body is retrograde it must have had an encounter with another objects otherwise it would violate the law of conservation of momentum.

In our solar system, Venus spins in the opposite direction to the other planets and so is retrograde. It is also spinning very slowly. Venus is described as having an axial tilt of #177.4^@#. This says it is upside down. Venus probably experienced major impacts from other bodies in its past which flipped it upside down.

Another phenomenon is that the other inner planets, and most noticeably Mercury appear to have a retrograde orbit some of the time. In fact none of the planets ever have retrograde orbits. The planets can appear to be retrograde when they move across the sky in the opposite direction to normal. This is simply a visual effect of the relative motions of the Earth and the planet.