Short Video Series (SVS0021)
Apparent Retrograde Motion (Visual Explanation)

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If we observe the planet Mars in the sky over a period of 2 years, we see that it slows down, stops, goes back and then returns its original path.

Why does Mars move like that? In reality, Mars doesn’t actually slow down or stop or go back. The movement in the sky we see is just an illusion called apparent retrograde motion. It happens because Mars orbits the Sun at different distances and speeds than Earth does.

If we look at the solar system, planets that are closer to the Sun orbit faster. As you know, Earth orbits the Sun every 365 days. Mars, on the other hand, completes one orbit of the Sun in 687 days – almost twice as long. That means every 2 years Earth catches and passes Mars. If we look at Mars and Earth’s orbital movements from Earth’s perspective, we can see clearly that when Earth catches and passes Mars, Mars’ path looks like a loop.

Apparent retrograde motion is valid for other planets, including the inner planets Mercury and Venus. However Mercury and Venus always pass Earth during daytime, so their apparent retrograde motions are difficult to observe.

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