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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>