Retrograde motion

Although the orbital motion of the objects in the sky is relatively simple and regular as observed from some fixed point in space, planetary motion appears uneven and somewhat complex when observed from the moving Earth.
As seen from the Earth, the Sun, Moon, and planets all appear to move along the ecliptic. Unlike the Sun and Moon, however, the planets don't always make steady progress along the ecliptic. They usually move in the same direction as the Sun (i.e., eastward), but from time to time they seem to slow down, stop, and reverse direction (i.e., move westward). This apparent reversal is called retrograde motion.
Apparent retrograde motion occurs when the observer (Earth) and the retrograding object (e.g., Mars) align with the Sun. This applet demonstrates this effect for Mars. The period between such retrogradations is the synodic period of the object.
The outer planets--Mars, Jupiter, Saturn, Uranus, Neptune and Pluto--spend 2-6 months of the year in retrograde motion. The farther the planet, the longer the duration of the retrogradation.
Retrograde motion is an illusion related to to the movement of the Earth-based observer. A familiar example of retrograde motion occurs when you pass a car on the freeway, the car being passed appears to move backwards relative to you. This applet demonstrates the retrograde motion of Mars as it marches across the Zodiac.
The retrograde motion of Mercury and Venus is difficult to observe directly because of their tight orbits around the Sun (too much glare).
Smaller bodies in the solar system (e.g., asteroids) also retrograde.
This chart shows the path of Mars in the sky during the opposition of 2007. Each dot represents one day's motion.
Click here for animation showing the retrogradation of Saturn over a 3-year observation.
Retrograde motion was a great puzzle to ancient astronomers. Copernicus offered the correct explanation: all planets, including the Earth, move around the Sun in the same direction; retrograde motion is an illusion created when we observe other planets from the moving planet Earth.
Asteroids and Kuiper Belt Objects (including Pluto) also exhibit apparent retrogradation.
However, objects such as the orbiter of the Space Shuttle and many artificial satellites appear to move from west to east. These are direct satellites (they actually orbit Earth in the same direction as the Moon), but they orbit Earth faster than Earth itself rotates, and so appear to move in the opposite direction.

The path of Mars, as viewed from the Earth. (Reload page to 're-animate' the gif.)

In this correct (heliocentric) perspective, when Earth and Mars are near each other and Earth overtakes Mars, Mars appears to move backwards from Earth's perspective. In this incorrect (geocentric) explanation, Mars is actually moving backwards on its epicycle.

Epicycles and Deferents

In the geocentric cosmology of the ancient Greeks, orbits were presumed to be uniform on perfectly circular orbits.
Since retrograde motion is clearly nonuniform, it was a problem for the ancient Greek astronomers (e.g., Ptolemy). They attempted to explain it with epicycles and deferents.
This animation shows how an epicycle centered on a deferent produces retrograde motion.


In this correct (heliocentric) perspective, when Earth and Mars are near each other and Earth overtakes Mars, Mars appears to move backwards from Earth's perspective.	In this incorrect (geocentric) explanation, Mars is actually moving backwards on its epicycle.