What is Retrograde Motion?

Retrograde Motion

Retrograde motion is the orbital motion of a spatial body in a direction that is opposite of what is normal in a given system. “Retrograde” is derived from the Latin words retro, backwards, and gradus, step.

In the Solar system, all major planets orbit the Sun counterclockwise as seen from Polaris. Most planets spin in the same sense, including Earth. These motions are called “direct” or “prograde.”

Retrograde

Rotation in the opposite sense is called retrograde. Venus and Uranus spin clockwise, so they have a retrograde rotation. Some small moons orbit clockwise around their planet, and are called retrograde satellites. Some comets and small asteroids orbit the Sun in retrograde orbits.

When we observe the sky, we expect most objects to move in a particular direction with the time. The motion of most bodies in the sky is from east to west. However, it is possible to observe a body moving west to east, such as an artificial satellite or space shuttle.

This orbit also may be considered retrograde motion. However, since the space shuttle and satellites that are moving eastward would appear from Polaris to be orbiting the Earth counterclockwise, they are considered direct satellites. There are also artificial satellites that orbit clockwise as seen from the pole star. These retrograde satellites can be seen in the sky going westward.

Retrograde motion should not be confused with retrogradation. The latter term is used in reference to the motion of the outer planets (Mars, Jupiter, Saturn, and so forth). Though these planets appear to move from east to west on a nightly basis in response to the spin of Earth, they are most of the time drifting slowly eastward with respect to the background of stars.

Direct motion

This motion is normal for these planets, so its name is a direct motion (not retrograde). However, since Earth completes its orbit in a shorter period of time than these outer planets, we occasionally overtake an outer planet, like a faster car on a multiple-lane highway.

When this occurs, the planet we are passing will first appear to stop its eastward drift, and it will then appear to drift back toward the west. This is retrogradation, since the planet seems to be moving in a opposite direction to typical direction for planets. Finally as Earth swings past the the planet in its orbit, it appears to resume its normal west-to-east drift on successive nights.

Mars goes through retrogradation about every 22.5 months. The more distant outer planets retrograde more frequently. The period between such retrogradations is the synodic period of the planet.

Retrogradation puzzled ancient astronomers, and was one reason why they named these bodies ‘planets’ which in Greek means ‘wanderers.’ In the geocentric model of the solar system, this motion was accounted for by having the planets travel in deferents and epicycles.

Some significant examples of retrograde motion in the solar system:

• Venus rotates slowly in the retrograde direction.
• The moons Ananke, Carme, Pasiphae and Sinope all orbit Jupiter in a retrograde direction. Many other minor moons of Jupiter orbit retrograde.
• The moon Phoebe orbits Saturn in a retrograde direction.
• The moon Triton orbits Neptune in a retrograde direction.
• The planet Uranus has an axial tilt which is very near to 90. This planet may have retrograde motion depending on one’s interpretation.