Specifying An Epoch or Equinox
Epochs and equinoxes are moments in time, so they can be specified in the same way as moments that indicate things other than epochs and equinoxes. The following standard ways of specifying epochs and equinoxes seem most popular:
- Julian Days, e.g., JD 2433282.4235 for 1950 January 0.9235 TT
- Besselian years (see below), e.g., 1950.0 or B1950.0 for 1950 January 0.9235 TT
- Julian years, e.g., J2000.0 for 2000 January 1.5000 TT
All three of these are expressed in TT = Terrestrial Time.
Besselian years, used mostly for star positions, can be encountered in older catalogs but are now becoming obsolete. The Hipparcos catalog summary, for example, defines the "catalog epoch" as J1991.25 (one quarter-year after the start of calendar year 1991).
Read more about this topic: Epoch (astronomy)
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—Ralph Waldo Emerson (1803–1882)