Which sentence in the passage describes retrograde motion of a planet? Astronomers had long struggled to figure out why Mars appeared to move backward across the night sky. If you observe a planet over any two consecutive nights, it will seem to move from west to east against the background of the stars, most of the time. However, occasionally, the planet will seem to reverse direction, and for a short time appears to move from east to west. The elliptical orbit of planets was also the basis of the first of his laws of planetary motion. Kepler’s three laws helped to accurately describe the motion of the planets across the sky.