<sect1 id="ai-epoch">
<sect1info>
<author>
<firstname>Akarsh</firstname>
<surname>Simha</surname>
</author>
</sect1info>
<title>Epoch</title>
<indexterm><primary>Epoch</primary></indexterm>
<para>
An epoch is a certain instant of time (usually past) used as a
reference for time measurement in astronomy. In order to make
calculations easier, astronomers usually use the number of days that
have passed since a certain moment in time as a way of representing
time in astronomical calculations. This arbitrarily chosen "starting
point" reference for time is called an epoch.
</para>
<para>
An epoch is usually cited along
with <link linkend="equatorial">equatorial
coordinates</link> in astronomical catalogs, since the equatorial
coordinates of sky objects change (although slowly) with time. Printed
star charts / atlases are usually marked as being valid for a certain
epoch.
</para>
<para>
The most popular epoch used currently is J2000.0,
(the <link linkend="ai-julianday">Julian year</link> 2000.0), which
(almost exactly) corresponds to 1st January 2000, 12:00 noon
UTC. Other epochs include B1900 and B1950.
</para>
<para>
&kstars; supports catalogs based on J2000.0 and B1950.0 epochs.
</para>
</sect1>