Watch your friend and think about how they are moving relative to you. Start out standing side by side with a friend. You can experience this effect for yourself. So as we catch up to that planet in its orbit and then move beyond it, the motion appears to go through the pro-retro-pro cycle.
Notice that it is all due to the fact that the Earth moves faster in its orbit than does Mars. How the planet Mars would appear to have both prograde then retrograde then prograde motion is shown in the diagram below. The explanation for retrograde motion in a heliocentric model is that retrograde occurs roughly when a faster moving planet catches up to and passes a slower moving planet. This seemingly strange behavior is easily understood within the context of a Sun-centered ( heliocentric) solar system. Mercury retrograde happens three to four times per year, when the planet Mercury slows down and appears to stop (station) and move backward (retrograde). The retrograde motion continues for a short time and then the motion switches back to prograde. However, peridiocally the motion changes and they move east-to-west through the stars. Normally, the planets move west-to-east through the stars at night. It just appears to do so because of the relative positions of the planet and Earth and how they are moving around the Sun. It is not REAL in that the planet does not physically start moving backwards in its orbit. Retrograde motion is an APPARENT change in the movement of the planet through the sky. What does it mean for Mercury to be in retrograde? From the day it turns retrograde until the day it turns direct, this is the official Mercury retrograde period that people most often refer to. What does it mean for Mercury to be in retrograde? StarChild Question of the Month for June 2002 Question:
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