You may know that the sun, moon, and the planets move eastward against the background stars. The moon averages about 13 degrees of motion per day, the sun about 1 degree while the planets vary considerably because some are close to the sun and some are far away. The next month gives you a rare opportunity to see the other kind of motion – retrograde (westward). Mars is directly opposite the sun (180 degrees away) on May 22. So during May (and into June) Mars goes backwards from it usual motion. Now, all the planets do this, so why is this one so special. Well, Mars is currently located in the constellation Scorpius, which has several bright stars, including a first magnitude star, Antares. These stars provide a ‘fixed’ background to compare the motion of Mars. This is augmented by the fact that there is a second planet in Scorpius, Saturn. Currently Mars is the brightest of all the objects there. This is also a favorable opposition of Mars and it will reach magnitude -2.0 on May 22!
Mars is closer to us than Saturn, so it will appear to move more in the same amount of time. Retrograde motion was one of the hardest phenomena for the ancients to explain. An earth centered view of the universe proved an unworkable platform for this and ultimately lead Copernicus to publish De Revolutionibus and present his sun centered model, which moved in the right direction but had to be improved upon.