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0001 <sect1 id="ai-hourangle">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Hour Angle</title>
0009 <indexterm><primary>Hour Angle</primary>
0010 <seealso>Local Meridian</seealso>
0011 <seealso>Sidereal Time</seealso>
0012 </indexterm>
0013 <para>
0014 As explained in the <link linkend="ai-sidereal">Sidereal Time</link> article,
0015 the <firstterm>Right Ascension</firstterm> of an object indicates the Sidereal
0016 Time at which it will transit across your <link linkend="ai-meridian">Local
0017 Meridian</link>. An object's <firstterm>Hour Angle</firstterm> is
0018 defined as the difference between the current Local Sidereal Time and the Right
0019 Ascension of the object:
0020 </para><para>
0021 <abbrev>HA</abbrev><subscript>obj</subscript> =
0022 <abbrev>LST</abbrev> - <abbrev>RA</abbrev><subscript>obj</subscript>
0023 </para><para>
0024 Thus, the object's Hour Angle indicates how much Sidereal Time has passed
0025 since the object was on the Local Meridian.  It is also the angular distance
0026 between the object and the meridian, measured in hours (1 hour = 15 degrees).
0027 For example, if an object has an hour angle of 2.5 hours, it transited across
0028 the Local Meridian 2.5 hours ago, and is currently 37.5 degrees West of the
0029 Meridian.  Negative Hour Angles indicate the time until the
0030 <emphasis>next</emphasis> transit across the Local Meridian.  Of course, an Hour
0031 Angle of zero means the object is currently on the Local Meridian.
0032 </para>
0033 </sect1>