<sect1 id="ai-retrograde">
<sect1info>
<author>
<firstname>John</firstname>
<surname>Cirillo</surname>
</author>
</sect1info>
<title>Retrograde Motion</title>
<indexterm><primary>Retrograde Motion</primary>
</indexterm>

<para>
<firstterm>Retrograde Motion</firstterm> is the orbital motion of a body in a
direction opposite that which is normal to spatial bodies within a given system.
</para><para>
When we observe the sky, we expect most objects to appear to move in a
particular direction with the passing of time. The apparent 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 that is orbiting eastward.  This orbit is
considered Retrograde Motion.
</para><para>
Retrograde Motion is most often 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 the Earth, they are actually drifting
slowly eastward with respect to the stationary stars, which can be
observed by noting the position of these planets for several nights in a
row. This motion is normal for these planets, however, and not
considered Retrograde Motion.  However, since the 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 Retrograde Motion, since
it is in a direction opposite that which is typical for planets. Finally,
as the Earth swings past the the planet in its orbit, they appear to
resume their normal west-to-east drift on successive nights.
</para><para>
This Retrograde Motion of the planets puzzled ancient Greek
astronomers, and was one reason why they named these bodies <quote>planets</quote>
which in Greek means <quote>wanderers</quote>.
</para>
</sect1>
