What is Retrograde Motion?

The Solar System was once thought to be Earth Centered. This model was called Geocentric. However, one thing the geocentric model did not explain very well was retrograde motion. Retrograde motion is the apparent backward motion of a planet viewed from Earth. To clarify, the normal direction of Planets from our perspective is west to east. Retrograde motion is when a planet starts moving in the opposite direction[1]. It is easier to see than explain so click on the following links and answer these questions:


  1. Why does retrograde motion occur?
  2. Why is this evidence for a heliocentric model of the universe?

Animation: Retrograde Motion This animation explains why we see other planets experiencing retrograde motion.

Animated Photo
This set of photographs shows the retrograde motion of Jupiter and Saturn. The pictures were taken at two week intervals for almost a whole year.

Animation: Heliocentric Epicycle Model of Solar System Before the heliocentric (sun centered) view of the solar system was widely accepted, some people believed that epicycles could explain retrograde motion. Here is an animation that demonstrates this.

Picture: Retrograde Motion of Mars

Nicolaus Copernicus was the first to suggest the heliocentric view of the solar system. In 1543 he proposed that the Sun was in the center of the solar system and earth and the other planets revolved around it. This is a much better explanation for retrograde motion because the inner planets move faster than the outer planets. This theory was not initially accepted by the scientific community. However, within a century more support such as Kepler's laws were adding credibility to this theory[1].

In the next section we will look at Kepler's Laws!


[1]Arny, Thomas T. (2006) Explorations: An Introduction to Astronomy, Fourth Edition , The McGraw-Hill Companies, Inc. pp775-776, ISBN 0-07-302580-1.

Never Stop Exploring!!!