For example, if Mars and a star are next to each other in the sky, the next night, Mars will have moved farther east compared to the star. We call this motion prograde or direct motion.
However, there are times when the planets will have move west with respect to the background stars. This is called retrograde motion.
It is because of this retrograde motion that Ptolemy introduced the concept of epicycle to his geocentric model of the solar system. There would be not other way for retrograde motion to be explained if the planets orbited around the Earth. It wasn't until Copernicus and later Kepler that retrograde motion could be easily explained without the use of epicycles.
A good way to imagine retrograde motion is two cars on a highway. In general, a car in the fast lane will be going faster than a car in the slow lane. As the car in the fast lane approaches the slower car, both cars are going in the same direction. But as the faster car passes the slower car, to occupants in the faster car, the slower car seems to moving backwards with respect to the background trees, mountains, signs, etc.
In planetary motion, the planets closer to the Sun move faster than the planets farther away. So as Earth approaches the position of Mars, we see Mars moving west to east with respect to the background stars. However, as we get "close" to Mars, its motion seems to change direction and we have retrograde motion.
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| Drawing of how retrograde motion works. Credit: Wikipedia User Rursus |
The apparent motions of Venus and Mercury are a little more complicated than that of the outer planets because they are closer to the Sun than Earth is. I'll discuss this in the next post.
