Z is for Mars

As seen on Astronomy Picture of the Day the motion of Mars in the Earth sky during the course of 7 months is captured on the multi-exposure digital composite shown on the right. This sequence follows Mars from July 2005 (right) through February 2006 (left). The images are spaced some 5 to 8 days apart and they trace the retrograde motion of Mars in the Earth’s night sky. On the left Mars appears in a single-exposure image in August 2005, which was taken from Uludag NP near Bursa. Move the slider on the image to see labels and dates on the both images.

Why would Mars appear to move backwards in the sequence? Mars do not actually reverse the direction of its orbit. Instead, the apparent backwards motion with respect to the background stars is a reflection of the 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. About every two years the Earth passes Mars (during opposition). On 2005 November 7 the Red Planet was opposite the Sun in Earth’s sky (at opposition). That date occurred at the center of this series with Mars near its closest and brightest. The familiar Pleiades star cluster lies at the upper left. TWAN photographer Tunc Tezel has a special interest to capture long-term motions of the planets in the night sky and his collection of Mars retrograde motions made in the last two decades is a unique effort.