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Epoch
=====

         **Akarsh Simha**

         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.

         An epoch is usually cited along with :ref:`equatorial
         coordinates <ai-skycoords-equatorial>` 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.

         The most popular epoch used currently is J2000.0, (the :doc:`Julian
         year  <ai-julianday>` 2000.0), which (almost exactly)
         corresponds to 1st January 2000, 12:00 noon UTC. Other epochs
         include B1900 and B1950.

         KStars supports catalogs based on J2000.0 and B1950.0 epochs.