Epoch
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 equatorial coordinates 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 Julian year 2000.0), which (almost exactly) corresponds to 1st January 2000, 12:00 noon UTC. Other epochs include B1900 and B1950.
supports catalogs based on J2000.0 and B1950.0 epochs.