Retrograde Motion
The outer planets appear to make strange reversals
in their motion against the stars. This is due to the fact that the
Earth moves around the Sun faster than they do, causing us to overtake them periodically,
during which time they appear to move “backwards” in the sky. This caused a lot of
headaches
for those trying to explain the apparent motion of the planets. The “S”
shape is due to the fact that the orbital planes aren’t quite aligned.