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34 lines
1.4 KiB
34 lines
1.4 KiB
<sect1 id="ai-hourangle">
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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>Hour Angle</title>
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<indexterm><primary>Hour Angle</primary>
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<seealso>Local Meridian</seealso>
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<seealso>Sidereal Time</seealso>
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</indexterm>
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<para>
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As explained in the <link linkend="ai-sidereal">Sidereal Time</link> article,
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the <firstterm>Right Ascension</firstterm> of an object indicates the Sidereal
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Time at which it will transit across your <link linkend="ai-meridian">Local
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Meridian</link>. An object's <firstterm>Hour Angle</firstterm> is
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defined as the difference between the current Local Sidereal Time and the Right
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Ascension of the object:
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</para><para>
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<abbrev>HA</abbrev><subscript>obj</subscript> =
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<abbrev>LST</abbrev> - <abbrev>RA</abbrev><subscript>obj</subscript>
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</para><para>
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Thus, the object's Hour Angle indicates how much Sidereal Time has passed
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since the object was on the Local Meridian. It is also the angular distance
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between the object and the meridian, measured in hours (1 hour = 15 degrees).
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For example, if an object has an hour angle of 2.5 hours, it transited across
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the Local Meridian 2.5 hours ago, and is currently 37.5 degrees West of the
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Meridian. Negative Hour Angles indicate the time until the
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<emphasis>next</emphasis> transit across the Local Meridian. Of course, an Hour
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Angle of zero means the object is currently on the Local Meridian.
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</para>
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</sect1>
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