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It is also usual now to specify on what time scale the time of day is expressed in that epoch-designation, e.g. often Terrestrial Time. In addition, an epoch optionally prefixed by "J" and designated as a year with decimals (2000 + x), where x is either positive or negative and is quoted to 1 or 2 decimal places, has come to mean a date that is ...
The currently used standard equinox and 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 equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used.
J2000: One commonly used ECI frame is defined with the Earth's Mean Equator and Mean Equinox (MEME) at 12:00 Terrestrial Time on 1 January 2000. It can be referred to as J2K, J2000 or EME2000. The x-axis is aligned with the mean vernal equinox.
An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits. A common astronomical epoch is J2000, which is noon on January 1, 2000, Terrestrial Time. An epoch in Geochronology is a time period, typically in the order of tens of millions of years. The current epoch is the Holocene.
Julian within its name indicates that other Julian epochs can be a number of Julian years of 365.25 days each before or after J2000.0. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0 at 12:00 TT on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have ...
Decimal time; Epoch (astronomy) Epoch (reference date) Era; J2000 – the epoch that starts on JD 2451545.0 (TT), the standard epoch used in astronomy since 1984;
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. Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, and B1950.0. [8]
The epoch for which M 0 is defined is often determined by convention in a given field or discipline. For example, planetary ephemerides often define M 0 for the epoch J2000, while for earth orbiting objects described by a two-line element set the epoch is specified as a date in the first line. [5]