THE RETROGRADE MOTION OF MARS

Video Credit:  NASA

The ancients observed that, like the Sun, each planet usually moved generally eastward through the background field of fixed stars.  However, each planet also looped "backwards" (westward) for a time!  Here we see an example of this with Mars.

Retrograde motion  is simply a function of our changing line of site to the other planets, as all orbit the Sun at different speeds.  Moreover, because the orbits of the planets are all slightly inclined to one another, and because their perihelia and aphelia are not aligned, the shape of a planet's retrograde motion in our sky changes from one occurence to the next.  That shape can be a loop, a "2", or a "Z" (as well as the inverted and mirrored forms of the same), all depending on where Earth and the other planet are in their respective orbits.

Here are the shapes of Mars's retrograde paths in 2003 and 2005.

Image Credit:  NASA/JPL-Caltech

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