Mars and Saturn in Retrograde Motion

As seen on Astronomy Picture of the Day wandering Mars and Saturn spent much of 2016 remarkably close together in the Earth night sky. The right side image show them on May 31st when Mars was almost at closest distance to the Earth and Saturn was only 2 nights away from opposition. With a sequence of exposures spanning December 2015 to September 2016, the delicately planned photo composite on the left side was achieved. Click the second photo to see this larger with dates and labels. It traces the planets movements just north of bright star Antares in Scorpius, near the Milky Way’s central bulge. Saturn’s apparent motion is back and forth along the flattened, compact loop, while Mars follows the wider S-shaped track from upper right to lower left through the frame.

Mars and Saturn don’t actually reverse direction in their orbits. Instead, their apparent backwards or retrograde motion with respect to the background stars is a reflection of the orbital motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own relatively close-in orbit.