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29 lines
1.1 KiB
29 lines
1.1 KiB
<sect1 id="ai-csphere">
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<sect1info>
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<author>
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<firstname>Jason</firstname>
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<surname>Harris</surname>
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</author>
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</sect1info>
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<title>The Celestial Sphere</title>
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<indexterm><primary>Celestial Sphere</primary>
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<seealso>Celestial Coordinate Systems</seealso>
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</indexterm>
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<para>
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The celestial sphere is an imaginary sphere of gigantic radius, centered
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on the Earth. All objects which can be seen in the sky can be thought
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of as lying on the surface of this sphere.
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</para><para>
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Of course, we know that the objects in the sky are not on the surface of
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a sphere centered on the Earth, so why bother with such a construct?
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Everything we see in the sky is so very far away, that their distances
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are impossible to gauge just by looking at them. Since their distances
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are indeterminate, you only need to know the <emphasis>direction</emphasis>
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toward the object to locate it in the sky. In this sense, the celestial sphere
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model is a very practical model for mapping the sky.
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</para><para>
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The directions toward various objects in the sky can be quantified by
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constructing a <link linkend="ai-skycoords">Celestial Coordinate System</link>.
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</para>
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</sect1>
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