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 <sect1 id="ai-precession">
 <sect1info>
 <author>
 <firstname>Jason</firstname>
 <surname>Harris</surname>
 </author>
 </sect1info>
 <title>Precession</title>
 <indexterm><primary>Precession</primary>
 </indexterm>
 <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 have 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 observing the <link
 linkend="ai-cpoles">Celestial Pole</link>.  To find the pole, first switch to
 Equatorial Coordinates in the <guilabel>Configure - &kstars;</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 action="simul">&Ctrl;<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.  Verify 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>