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0001 <sect1 id="ai-epoch">
0002 <sect1info>
0003 <author>
0004 <firstname>Akarsh</firstname>
0005 <surname>Simha</surname>
0006 </author>
0007 </sect1info>
0008 <title>Epoch</title>
0009 <indexterm><primary>Epoch</primary></indexterm>
0010 <para>
0011 An epoch is a certain instant of time (usually past) used as a
0012 reference for time measurement in astronomy. In order to make
0013 calculations easier, astronomers usually use the number of days that
0014 have passed since a certain moment in time as a way of representing
0015 time in astronomical calculations. This arbitrarily chosen "starting
0016 point" reference for time is called an epoch.
0017 </para>
0018 <para>
0019 An epoch is usually cited along
0020 with <link linkend="equatorial">equatorial
0021 coordinates</link> in astronomical catalogs, since the equatorial
0022 coordinates of sky objects change (although slowly) with time. Printed
0023 star charts / atlases are usually marked as being valid for a certain
0024 epoch.
0025 </para>
0026 <para>
0027 The most popular epoch used currently is J2000.0,
0028 (the <link linkend="ai-julianday">Julian year</link> 2000.0), which
0029 (almost exactly) corresponds to 1st January 2000, 12:00 noon
0030 UTC. Other epochs include B1900 and B1950.
0031 </para>
0032 <para>
0033 &kstars; supports catalogs based on J2000.0 and B1950.0 epochs.
0034 </para>
0035 </sect1>