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0001 <sect1 id="ai-csphere">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>The Celestial Sphere</title>
0009 <indexterm><primary>Celestial Sphere</primary>
0010 <seealso>Celestial Coordinate Systems</seealso>
0011 </indexterm>
0012 <para>
0013 The celestial sphere is an imaginary sphere of gigantic radius, centered
0014 on the Earth.  All objects which can be seen in the sky can be thought
0015 of as lying on the surface of this sphere.
0016 </para><para>
0017 Of course, we know that the objects in the sky are not on the surface of
0018 a sphere centered on the Earth, so why bother with such a construct?
0019 Everything we see in the sky is so very far away, that their distances
0020 are impossible to gauge just by looking at them.  Since their distances
0021 are indeterminate, you only need to know the <emphasis>direction</emphasis>
0022 toward the object to locate it in the sky.  In this sense, the celestial sphere
0023 model is a very practical model for mapping the sky.
0024 </para><para>
0025 The directions toward various objects in the sky can be quantified by
0026 constructing a <link linkend="ai-skycoords">Celestial Coordinate System</link>.
0027 </para>
0028 </sect1>