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 view
to 're-animate' the gif.)
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.
