In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the case of orbital elements, it is necessary to take account of perturbation by other bodies in order to calculate the orbital elements for a different time.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch.
Besselian epochs were used before 1984, however Julian epochs are now used.
An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years.
Fourth epoch in the history of the Universe, lasting from about 100 sec to 106 yr, in which matter came to dominate radiation as the principal constituent of the Universe.
In the current epoch, asteroids are the majority of impactors.4 A combination of asteroid-to-asteroid collisions, gravitational effects such as the pull of Jupiter and Mars,...
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