<sect1 id="ai-cpoles">
<sect1info>
<author>
<firstname>Jason</firstname>
<surname>Harris</surname>
</author>
</sect1info>
<title>The Celestial Poles</title>
<para>
The sky appears to drift overhead from east to west, completing a full circuit
around the sky in 24 (<link linkend="ai-sidereal">Sidereal</link>) hours.  This
phenomenon is due to the spinning of the Earth on its axis.  The Earth's
spin axis intersects the <link linkend="ai-csphere">Celestial Sphere</link> at
two points.  These points are the <firstterm>Celetial Poles</firstterm>.  As the
Earth spins; they remain fixed in the sky, and all other points seem to rotate
around them.  The celestial poles are also the poles of the <link
linkend="equatorial">Equatorial Coordinate System</link>, meaning
they have <firstterm>Declinations</firstterm> of +90 degrees and -90 degrees
(for the North and South celestial poles, respectively).
</para><para>
The North Celestial Pole currently has nearly the same coordinates as
the bright star <firstterm>Polaris</firstterm> (which is Latin for "Pole Star").
This makes Polaris useful for navigation: not only is it always above the North
point of the horizon, but its <link
linkend="horizontal">Altitude</link> angle is always (nearly)
equal to the observer's <link linkend="ai-geocoords">Geographic Latitude</link>
(however, Polaris can only be seen from locations in the Northern hemisphere).
</para><para>
The fact that Polaris is near the pole is purely a coincidence.  In fact,
because of <link linkend="ai-precession">Precession</link>, Polaris is only near
the pole for a small fraction of the time.
</para>
<tip>
<para>Exercises:</para>
<para>
Use the <guilabel>Find Object</guilabel> window
(<keycombo><keycap>Ctrl</keycap><keycap>f</keycap></keycombo>) to locate
Polaris.  Notice that its Declination is almost (but not exactly) +90 degrees.
Compare the Altitude reading when focused on Polaris to your location's
geographic latitude.  They are always within one degree of each other.
They are not exactly the same because Polaris isn't exactly at the Pole.
(you can point exactly at the pole by switching to Equatorial
coordinates, and pressing the up-arrow key until the sky stops scrolling).
</para><para>
Use the <guilabel>Time Step</guilabel> spinbox in the toolbar to accelerate time
 to a
step of 100 seconds.  You can see the entire sky appears to rotate around
Polaris, while Polaris itself remains nearly stationary.
</para><para>
We said that the celestial pole is the pole of the Equatorial coordinate
system.  What do you think is the pole of the horizontal (Altitude/Azimuth)
coordinate system?  (The <link linkend="ai-zenith">Zenith</link>).
</para>
</tip>
</sect1>
