kstars/doc/retrograde.docbook

0001 <sect1 id="ai-retrograde">
0002 <sect1info>
0003 <author>
0004 <firstname>John</firstname>
0005 <surname>Cirillo</surname>
0006 </author>
0007 </sect1info>
0008 <title>Retrograde Motion</title>
0009 <indexterm><primary>Retrograde Motion</primary>
0010 </indexterm>
0011
0012 <para>
0013 <firstterm>Retrograde Motion</firstterm> is the orbital motion of a body in a
0014 direction opposite that which is normal to spatial bodies within a given system.
0015 </para><para>
0016 When we observe the sky, we expect most objects to appear to move in a
0017 particular direction with the passing of time. The apparent motion of
0018 most bodies in the sky is from east to west. However it is possible to
0019 observe a body moving west to east, such as an artificial satellite or
0020 space shuttle that is orbiting eastward.  This orbit is
0021 considered Retrograde Motion.
0022 </para><para>
0023 Retrograde Motion is most often used in reference to the
0024 motion of the outer planets (Mars, Jupiter, Saturn, and so forth).
0025 Though these planets appear to move from east to west on a nightly
0026 basis in response to the spin of the Earth, they are actually drifting
0027 slowly eastward with respect to the stationary stars, which can be
0028 observed by noting the position of these planets for several nights in a
0029 row. This motion is normal for these planets, however, and not
0030 considered Retrograde Motion.  However, since the Earth completes its
0031 orbit in a shorter period of time than these outer planets, we
0032 occasionally overtake an outer planet, like a faster car on a
0033 multiple-lane highway.  When this occurs, the planet we are passing will
0034 first appear to stop its eastward drift, and it will then
0035 appear to drift back toward the west. This is Retrograde Motion, since
0036 it is in a direction opposite that which is typical for planets. Finally,
0037 as the Earth swings past the planet in its orbit, they appear to
0038 resume their normal west-to-east drift on successive nights.
0039 </para><para>
0040 This Retrograde Motion of the planets puzzled ancient Greek
0041 astronomers, and was one reason why they named these bodies <quote>planets</quote>
0042 which in Greek means <quote>wanderers</quote>.
0043 </para>
0044 </sect1>