Retrograde Motion Of Planets. Planets also grow brighter or dimmer as they move through the sky. The retrograde motion of the planets is an optical illusion, which is caused by planets orbiting the sun at different speeds to the earth.
Through most of the year, mars appears to move in one direction against the background stars when viewed from earth. This application simulates the apparent retrograde movement that the planets perform at certain times of the year, considering, naturally, the point of view of observation from.