What is a retrograde loop is astronomy?

Answer 1

You and I, little buddy, are going to take a little ride in my virtual spaceship. Whoosh! Of we go, and moments later, we are poised about 3 AU directly above the north pole of the Sun. We're going to sit here and watch the planets Earth and Mars for a couple of virtual years.....

We can see that the Earth is clicking along in its orbit, making one complete circle in 365 1/4 days. Mars is further out from the Sun, so Mars is moving slower and takes nearly two Earth years for one Martian year. We can see that both planets are moving in very slightly elliptical paths; not perfect circles, but close enough.

Now, back to Earth! We land on the Earth, and watch the planet Mars through the year. Remember, Earth is still going faster than Mars is, and will overtake the slower planet. So day by day we see Mars moving along through the stars, but it appears to be slowing down! Soon, as the Earth gets close to passing Mars, our view shows us that Mars is REALLY slowing down, and because Earth is moving faster, Mars appears to move backwards among the stars! Then, as Earth gets enough ahead, we see that Mars was moving straight ahead all along; that it looked to be going backwards was a trick, an illusion caused by the fact that the Earth was catching up to Mars in their orbits and pulling ahead.

So the whole thing was a trick, caused by our perspective of riding on a moving planet and watching a different planet that was moving in a different way!