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+<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>