Retrograde motion

Retrograde motion is one of the apparent motions of planets relative to the background of the stars. If we continue to track a certain planet for a period of time, we will find that it sometimes moves to the east, sometimes stops for a short time, sometimes moves to the west, then makes a short stay, and then moves to the east as it did at the beginning. In order to explain this phenomenon, the epicycle-deferent model was born. This geocentric model dominated for two thousand years until Kepler’s three laws were proposed.

So, what causes the retrograde motion of the planets? Actually, they are not really retrograde. The earth is closer to the sun than many planets, so the earth moves faster. Whenever Earth overtakes a planet, the outer planets appear to start receding in the sky. From Earth, Mercury and Venus appear to oscillate on either side of the Sun. These planets all do circular motion around the sun, it’s just that here on Earth, it looks like the planets are going backwards.

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