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0001 <sect1 id="ai-cpoles">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>The Celestial Poles</title>
0009 <indexterm><primary>Celestial Poles</primary>
0010 <seealso>Equatorial Coordinates</seealso>
0011 </indexterm>
0012 <para>
0013 The sky appears to drift overhead from east to west, completing a full circuit
0014 around the sky in 24 (<link linkend="ai-sidereal">Sidereal</link>) hours.  This
0015 phenomenon is due to the spinning of the Earth on its axis.  The Earth's
0016 spin axis intersects the <link linkend="ai-csphere">Celestial Sphere</link> at
0017 two points.  These points are the <firstterm>Celestial Poles</firstterm>.  As the
0018 Earth spins; they remain fixed in the sky, and all other points seem to rotate
0019 around them.  The celestial poles are also the poles of the <link
0020 linkend="equatorial">Equatorial Coordinate System</link>, meaning
0021 they have <firstterm>Declinations</firstterm> of +90 degrees and -90 degrees
0022 (for the North and South celestial poles, respectively).
0023 </para><para>
0024 The North Celestial Pole currently has nearly the same coordinates as
0025 the bright star <firstterm>Polaris</firstterm> (which is Latin for <quote>Pole Star</quote>).
0026 This makes Polaris useful for navigation: not only is it always above the North
0027 point of the horizon, but its <link
0028 linkend="horizontal">Altitude</link> angle is always (nearly)
0029 equal to the observer's <link linkend="ai-geocoords">Geographic Latitude</link>
0030 (however, Polaris can only be seen from locations in the Northern hemisphere).
0031 </para><para>
0032 The fact that Polaris is near the pole is purely a coincidence.  In fact,
0033 because of <link linkend="ai-precession">Precession</link>, Polaris is only near
0034 the pole for a small fraction of the time.
0035 </para>
0036 <tip>
0037 <para>Exercises:</para>
0038 <para>
0039 Use the <guilabel>Find Object</guilabel> window
0040 (<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to locate
0041 Polaris.  Notice that its Declination is almost (but not exactly) +90 degrees.
0042 Compare the Altitude reading when focused on Polaris to your location's
0043 geographic latitude.  They are always within one degree of each other.
0044 They are not exactly the same because Polaris isn't exactly at the Pole.
0045 (you can point exactly at the pole by switching to Equatorial
0046 coordinates, and pressing the up-arrow key until the sky stops scrolling).
0047 </para><para>
0048 Use the <guilabel>Time Step</guilabel> spinbox in the toolbar to accelerate time
0049  to a
0050 step of 100 seconds.  You can see the entire sky appears to rotate around
0051 Polaris, while Polaris itself remains nearly stationary.
0052 </para><para>
0053 We said that the celestial pole is the pole of the Equatorial coordinate
0054 system.  What do you think is the pole of the horizontal (Altitude/Azimuth)
0055 coordinate system?  (The <link linkend="ai-zenith">Zenith</link>).
0056 </para>
0057 </tip>
0058 </sect1>