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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>