Retrograde Motion

One of the more interesting results of parallax appears in the Retrograde Motion of the planets. Mars shows the largest retrograde motion due to its smaller distance from Earth. The Earth moves in its orbit faster than Mars and therefore overtakes Mars about every 26 months. This causes the apparent motion of Mars across the background stars to actually change direction for a short period of time. The apparent motion of Mars is normally eastward due to its orbit about the Sun. But, while the Earth passes Mars due to its smaller and faster orbit, Mars appears to progress westward for a couple of months. Once the Earth overtakes Mars in its orbit, the motion of Mars returns to the normal eastward motion.

The retrograde motion of Mars

In the above figure, the Earth moves half way around the Sun in its orbit, which takes about six months. During the same time period, Mars only moves about 27% of its orbit around the Sun (its orbital period is 687 days). While the Earth and Mars are moving through the positions labeled 1, 2 and 3, Mars appears to be moving eastward in relation to the background stars, which is normal. But, while moving through 4, 5 and 6, the faster Earth is at its closest approach to Mars, so its relative speed is greatest. This causes the appearance of Mars moving westward (retrograde) against the starry background for a couple of months. Once the Earth overtakes Mars, the apparent motion of Mars returns to an eastward motion for approximately two years, until once again the Earth passes Mars.

The effects of parallax on the apparent positions of solar system objects can be seen in the following demonstrations.