<sect1 id="ai-cpoles">
<sect1info>
<author
><firstname
>Jason</firstname
> <surname
>Harris</surname
> </author>
</sect1info>
<title
>The Celestial Poles</title>
<indexterm
><primary
>Celestial Poles</primary>
<seealso
>Equatorial Coordinates</seealso>
</indexterm>
<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
>Celestial 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 <quote
>Pole Star</quote
>). 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 action="simul"
>&Ctrl;<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>
