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0001 <sect1 id="ai-precession">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Precession</title>
0009 <indexterm><primary>Precession</primary>
0010 </indexterm>
0011 <para>
0012 <firstterm>Precession</firstterm> is the gradual change in the direction of the
0013 Earth's spin axis. The spin axis traces a cone, completing a full circuit in
0014 26,000 years. If you have ever spun a top or a dreidel, the
0015 <quote>wobbling</quote> rotation of the top as it spins is precession.
0016 </para><para>
0017 Because the direction of the Earth's spin axis changes, so does the location of
0018 the <link linkend="ai-cpoles">Celestial Poles</link>.
0019 </para><para>
0020 The reason for the Earth's precession is complicated.  The Earth is not a
0021 perfect sphere, it is a bit flattened, meaning the
0022 <link linkend="ai-greatcircle">Great Circle</link> of the equator is longer
0023 than a <quote>meridonal</quote> great circle that
0024 passes through the poles.  Also, the Moon and Sun lie outside the Earth's
0025 Equatorial plane.  As a result, the gravitational pull of the Moon and Sun on
0026 the oblate Earth induces a slight <emphasis>torque</emphasis> in addition to a
0027 linear force. This torque on the spinning body of the Earth leads to the
0028 precessional motion.
0029 </para>
0030 <tip>
0031 <para>Exercise:</para>
0032 <para>
0033 Precession is easiest to see by observing the <link
0034 linkend="ai-cpoles">Celestial Pole</link>.  To find the pole, first switch to
0035 Equatorial Coordinates in the <guilabel>Configure - &kstars;</guilabel> window, and
0036 then hold down the &Up; arrow key until the display stops
0037 scrolling.  The declination displayed in the center of the
0038 <guilabel>Info Panel</guilabel> should be +90 degrees, and the bright star
0039 Polaris should be nearly at the center of the screen.  Try slewing with the left
0040 and right arrow keys. Notice that the sky appears to rotate around the Pole.
0041 </para><para>
0042 We will now demonstrate Precession by changing the Date to a very remote year,
0043 and observing that the location of the Celestial Pole is no longer near Polaris.
0044 Open the <guilabel>Set Time</guilabel> window
0045 (<keycombo action="simul">&Ctrl;<keycap>S</keycap></keycombo>), and set the date
0046 to the year 8000 (currently, &kstars; cannot handle dates much more remote than
0047 this, but this date is sufficient for our purposes).  Notice that the sky
0048 display is now centered at a point between the constellations Cygnus and
0049 Cepheus.  Verify that this is actually the pole by slewing left and right: the
0050 sky rotates about this point; in the year 8000, the North celestial pole will no
0051 longer be near Polaris.
0052 </para>
0053 </tip>
0054 </sect1>