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authortoma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da>2009-11-25 17:56:58 +0000
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+<sect1 id="ai-hourangle">
+<sect1info>
+<author>
+<firstname>Jason</firstname>
+<surname>Harris</surname>
+</author>
+</sect1info>
+<title>Hour Angle</title>
+<indexterm><primary>Hour Angle</primary>
+<seealso>Local Meridian</seealso>
+<seealso>Sidereal Time</seealso>
+</indexterm>
+<para>
+As explained in the <link linkend="ai-sidereal">Sidereal Time</link> article,
+the <firstterm>Right Ascension</firstterm> of an object indicates the Sidereal
+Time at which it will transit across your <link linkend="ai-meridian">Local
+Meridian</link>. An object's <firstterm>Hour Angle</firstterm> is
+defined as the difference between the current Local Sidereal Time and the Right
+Ascension of the object:
+</para><para>
+<abbrev>HA</abbrev><subscript>obj</subscript> =
+<abbrev>LST</abbrev> - <abbrev>RA</abbrev><subscript>obj</subscript>
+</para><para>
+Thus, the object's Hour Angle indicates how much Sidereal Time has passed
+since the object was on the Local Meridian. It is also the angular distance
+between the object and the meridian, measured in hours (1 hour = 15 degrees).
+For example, if an object has an hour angle of 2.5 hours, it transited across
+the Local Meridian 2.5 hours ago, and is currently 37.5 degrees West of the
+Meridian. Negative Hour Angles indicate the time until the
+<emphasis>next</emphasis> transit across the Local Meridian. Of course, an Hour
+Angle of zero means the object is currently on the Local Meridian.
+</para>
+</sect1>