Epoch (astronomy) - Julian Years and J2000

Julian Years and J2000

A Julian year is an interval with the length of a mean year in the Julian calendar, i.e. 365.25 days. This interval measure does not itself define any epoch: the Gregorian calendar is in general use for dating. But, standard conventional epochs which are not Besselian epochs have been often designated nowadays with a prefix "J", and the calendar date to which they refer is widely known, although not always the same date in the year: thus "J2000" refers to the instant of 12h on 1 January 2000, and J1900 refers to the instant of 12h (midday) on 0 January 1900, equal to 31 Dec 1899. 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 positive or negative and quoted to 1 or 2 decimal places, has come to mean a date that is an interval of x Julian years of 365.25 days away from the epoch J2000 = JD 2451545.0 (TT), still corresponding (in spite of the use of the prefix "J" or word "Julian") to the Gregorian calendar date of 2000 Jan 1 at 12h TT (about 64 seconds before noon UTC on the same calendar day). (See also Julian year (astronomy).) Like the Besselian epoch, an arbitrary Julian epoch is therefore related to the Julian date by

J = 2000.0 + (Julian date − 2451545.0)/365.25 .

The IAU decided at their General Assembly of 1976 that the new standard equinox of J2000.0 should be used starting in 1984. Before that, the equinox of B1950.0 seems to have been the standard.

Different astronomers or groups of astronomers used to define epochs to suit themselves, but nowadays standard epochs are generally defined by international agreement through the IAU, so astronomers worldwide can collaborate more effectively. It is inefficient and error-prone if data or observations of one group have to be translated in non-standard ways so that other groups could compare the data with information from other sources. An example of how this works: if a star's position is measured by someone today, he/she then uses a standard transformation to obtain the position expressed in terms of the standard reference frame of J2000, and it is often then this J2000 position which is shared with others.

On the other hand, there has also been an astronomical tradition of retaining observations in just the form in which they were made, so that others can later correct the reductions to standard if that proves desirable, as has sometimes occurred.

The currently-used standard epoch "J2000" is defined by international agreement at 2000 January 1.5 (or January 1 at 12h on a defined time scale usually TT), or more precisely the Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT. The equivalent in International Atomic Time is 11:59:27.816; in Coordinated Universal Time, 11:58:55.816.