Question

Describe how Copernicus explained the retrograde motion of the planets in his heliocentric model.

Answer 1

First of all, Let us know what a retrograde motion for planet is.

It is simply the apparent backward motion of planet in the sky as seen by observer on some other planet. Please mind the word "apparent". It means the planet does not actually move backward, it just looks like that

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How did Copernicus explained

Actually, In the heliocentric model of Copernicus, retrograde motion of planets is naturally explained. We now know that as we go away from sun, the speed of planets around the sun decreases.

Suppose, you are standing on earth and looking at the mars. Remember earth laps mars. It means mars's orbit is after the earth's orbit in solar system. As we observe from a fixed point on earth, we will notice that Mars appears to move backwards which is retrograde motion. Remember, only outer planets ( with respect to earth) will show this retrograde motion as they are moving slower than earth,

In this sense, Mercury and Venus will not show retrograde motion as they are moving faster than earth. However, someone standing on Venus will see retrograde motion of earth.

I hope this makes the concept clear !!!