Planets move relative to the stars always moving from west to east in the sky. Well almost always. Every once in a while, a planet will slow down in its apparent slide across the constellations each night. For a few nights it may even appear to have stopped. Then it will start moving in the opposite direction, to the west. Eventually, it will stop once again, and begin its movement back towards the east, as it had done initially.
This is called retrograde motion and it confounded astronomer for centuries. Their confusion was due to their incorrect modelling of the solar system. If you have the Earth in the centre of everything, then the planets simply won’t move as they should all the time. There is an excellent, fuller description of retrograde motion, and its history, on the Science U website.
Yesterday’s APOD was of Mars undergoing retrograde motion. It is a lovely image, and one I shall be using to illustrate the concept next time I do a talk. Thanks to Tunc Tezel from Turkey for taking it and sharing with us all.