Astronomers pin their star maps to noon on January 1, 2000
The sky's coordinate grid slowly drifts, because Earth's axis wobbles. So astronomers anchor their numbers to a fixed moment, an epoch. Most catalogues today use noon on January 1, 2000, a date so standard it has its own shorthand, J2000, while older charts were tied to 1950 or 1900.
An epoch is a reference instant for quantities that change over time. A planet's orbit is nudged constantly by other bodies, so values such as its mean anomaly, the orientation of its orbital node or the length of its major axis are only exact at one moment. Stating that moment lets celestial mechanics project the orbit forward and produce an ephemeris, a table of where objects will be and how fast they will move.
There are two common ways to use it. A quantity can be written as a polynomial in the time elapsed since the epoch, the usual modern approach, or held constant at its epoch value while tables describe the change, as was typical in the 17th and 18th centuries. Back then the word itself meant something else: the tabulated values, not the date, were called epochs, so correcting the epochs meant adjusting numbers, not moving to a new date.
Coordinates need a second date. Equatorial and ecliptic systems are measured from the vernal equinox, which shifts with the precession of Earth's axis and orbit, so every such frame must be labelled with its own date. Present-day standards use 2000 TT, noon on January 1, 2000 in Terrestrial Time, which fell about 64 seconds before noon by the UT1 clock. Before about 1984, frames dated 1950 or 1900 were the norm.
Minor planet (5145) Pholus shows both dates in use. Its orbital elements are given at a recent epoch, but the angles that depend on orientation are expressed in the J2000 frame. A frame like that can serve indefinitely, whereas snapshot orbital elements drift out of accuracy as future perturbations accumulate. Full analytical theories with periodic correction terms, by contrast, can stay valid for centuries or millennia either side of their epoch.
Source: Epoch (astronomy)
