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Astronomy

What Causes the Retrograde Motion of Mars in the Sky?

Quick fact

Retrograde motion of Mars was a major piece of evidence for the heliocentric model. Copernicus explained it simply by Earth overtaking Mars in its faster orbit, a concept later confirmed by Kepler's laws.

Why this is interesting

For weeks, Mars—normally a steady eastward traveler among the stars—seems to stop, reverse direction, and drift westward before looping back. Why does this fiery dot of light appear to break the cosmic rule of continuous eastward motion?