<sect1 id="ai-precession">
<sect1info>
<author>
<firstname>Jason</firstname>
<surname>Harris</surname>
</author>
</sect1info>
<title>Precession</title>
<para>
<firstterm>Precession</firstterm> is the gradual change in the direction of the
Earth's spin axis. The spin axis traces a cone, completing a full circuit in
26,000 years. If you've ever spun a top or a dreidel, the
<quote>wobbling</quote> rotation of the top as it spins is precession.
</para><para>
Because the direction of the Earth's spin axis changes, so does the location of
the <link linkend="ai-cpoles">Celestial Poles</link>.
</para><para>
The reason for the Earth's precession is complicated.  The Earth is not a
perfect sphere, it is a bit flattened, meaning the
<link linkend="ai-greatcircle">Great Circle</link> of the equator is longer
than a <quote>meridonal</quote> great circle that
passes through the poles.  Also, the Moon and Sun lie outside the Earth's
Equatorial plane.  As a result, the gravitational pull of the Moon and Sun on
the oblate Earth induces a slight <emphasis>torque</emphasis> in addition to a
linear force. This torque on the spinning body of the Earth leads to the
precessional motion.
</para>
<tip>
<para>Exercise:</para>
<para>
Precession is easiest to see by osberving the <link
linkend="ai-cpoles">Celestial Pole</link>.  To find the pole, first switch to
Equatorial Coordinates in the <guilabel>View Options</guilabel> window, and
then hold down the Up arrow key until the display stops scrolling.  The
declination displayed in the center of the <guilabel>Info Panel</guilabel>
should be +90 degrees, and the bright star Polaris should be nearly at the
center of the screen.  Try slewing with the left and right arrow keys. Notice
that the sky appears to rotate around the Pole.
</para><para>
We will now demonstrate Precession by changing the Date to a very remote year,
and observing that the location of the Celestial Pole is no longer near Polaris.
Open the <guilabel>Set Time</guilabel> window
(<keycombo><keycap>Ctrl</keycap><keycap>s</keycap></keycombo>), and set the date
to the year 8000 (currently, &kstars; cannot handle dates much more remote than
this, but this date is sufficient for our purposes).  Notice that the sky
display is now centered at a point between the constellations Cygnus and
Cepheus.  Veryify that this is actually the pole by slewing left and right: the
sky rotates about this point; in the year 8000, the North celestial pole will no
longer be near Polaris!
</para>
</tip>
</sect1>
