DAVID A. HARDY / SCIENCE PHOTO LIBRARY DAVID A. HARDY / SCIENCE PHOTO LIBRARY
Artwork showing the apparent retrograde motion of Mars in the sky. The orbits of the Earth (blue) and Mars (red) around the Sun are seen at bottom. Mars is generally seen from the Earth moving in an eastward direction (from right to left); when the Earth overtakes it Mars appears to slow to a stop and to move westward in a retrograde motion. As the Earth draws ahead of Mars, Mars resumes its eastward motion against the background stars. Because the orbits of the Earth and Mars do not lie in precisely the same plane the eastward motion of Mars does not resume in the same original path. This causes the loop-shaped feature (top centre) seen in the trajectory of Mars.
Model release not required. Property release not required.