# Epoch (astronomy)

As epoch (from ancient Greek ἐποχή Era "stop", "stop point [in the era];", "astronomy specifically" fixed time, to the reference positions defined and their changes are calculated " constellation ") is used in astronomy a The point in time to which the details of celestial coordinates , orbital elements or ephemeris refer.

## Basics

The epoch can be understood as a point in time at which a “ snapshot of the macrocosm ” is created. With this snapshot, all relevant numerical values ​​are recorded.

For other, not too different points in time, the celestial coordinates of astronomical objects outside the solar system , especially stars , can be derived from the information relating to a specific epoch by taking precession into account , i.e. H. the slow gyroscopic motion of the earth's axis and the proper motion of the object can be calculated. This is important for all precise measurements in astronomy and geodesy . B. also for orbital calculations, space travel and the alignment of telescopes .

Therefore, the mean equinox (the vernal equinox ) is used as the reference point of all given coordinates at this time and is called the equinox of the date or the equinox of the coordinates .

## Bessel epoch

In astronomy, it is often desirable to be able to relate time-dependent quantities to a point in time that occurs simultaneously for all observation locations. For this purpose, Friedrich Wilhelm Bessel introduced a convention for standard epochs (see below) in the 19th century , which are therefore called Bessel epochs .

A Bessel epoch is a point in time at which the mean right ascension of the mean ephemeris sun (i.e. the mean position of the true sun projected onto the celestial equator ) is subject to the constant part of the aberration (−20.5 ″) and from the mean equinox of the date measured, is exactly 280 ° (or hourly : 18 h 40 m ). These times are close to January 1, but fall on a slightly different date from year to year. They are identified by adding “.0” to the year. The equinoxes and epochs of the star catalogs , orbital data etc. related to these Bessel standard epochs (see below) up to 1984 : The indication "Equinox 1950.0" thus meant the beginning of the Bessel solar year 1950.

To distinguish it from the later Julian epochs, a Bessel epoch is nowadays marked by a "B" placed in front of the year: B1950.0.

### Bessel solar year

The Bessel solar year or “ Bessel year ” for short begins at the time of a Bessel epoch. The length of the year corresponds roughly to that of the tropical year of about 365.2422 days. Some calculations use the Bessel year fraction (usually marked with ), which is to be counted from the beginning of the Bessel solar year. Sometimes the fraction of the year relates to the nearest beginning of a Bessel solar year and is then to be counted negatively in the second half of the year. ${\ displaystyle \ tau}$

### Conversion between Bessel's epoch and Julian day number

If the Julian date is any point in time, then this is given by in Bessel's year counting ${\ displaystyle JD}$

${\ displaystyle t_ {B} = 1900 {,} 0 \, + \, (JD-2 {\,} 415 {\,} 020 {,} 313 {\,} 52) / 365 {,} 242 {\ ,} 198 {\,} 781}$.

It is the other way around

${\ displaystyle JD = 2 \, 415 \, 020 {,} 313 \, 52 \, + \, 365 {,} 242 \, 198 \, 781 (t_ {B} -1900 {,} 0)}$.

The time scale is the terrestrial time TT. The value entered here for the length of the tropical year results directly from the definition of the second from 1960 . The decrease in the length of the year by about 0.5 s per century is not taken into account, so that these two equations do not exactly reflect the above definition of Bessel's epoch.

## Julian epoch

Since the "Bessel start of the year" does not fall exactly on January 1st at midday (12 noon UT ) due to the leap days , the IAG and IUGG decided in 1984 to define the future epochs in Julian terms. From 1985 this is characterized by the leading letter J instead of B (for Bessel). There are exactly 365.25 days between two Julian epochs (a Julian year ). Between two epochs 100 years apart, i.e. 36525 days, a Julian century .

For the conversion to another Julian epoch, the Julian year length of exactly 365.25 days must be used.

## Standard epochs

In order to be able to compare measurement data obtained at different points in time with one another, a standard epoch is defined, and the equinox of this date has the same name as the standard equinox .

For this time of observation, on the one hand, particularly precise star catalogs are created, which enable a fundamental system (as an approximation to an idealized inertial system ), and all other astronomical base quantities for space, time and movement are cataloged as precisely as possible. All variable quantities can then be reduced to this , i.e. H. be unified. Standard epochs used to be set every 25 Bessel epochs, today all 50 Julian epochs.

For the standard epochs it results

Gregorian date Bessel epoch Julian date
1849 Dec 31 4:52 pm London time B1850.0 2,396,758.203
1899 Dec. 31 7:31 PM GMT B1900.0 2,415,020,313
1949 Dec. 31 22:09 UT B1950.0 2,433,282,423
2000 Jan. 01 11:59 UTC no longer common 2,451,545.0 = J2000.0 00

The default epoch currently recommended for star catalogs and dynamic theories is J2000.0. J2000.0 corresponds to the definition of the fundamental system on January 1, 2000 12:00 TT = JD 2451545.0, which corresponds to January 1, 2000, 11: 58: 55.816 UTC . Julian standard epochs are only set every 50 Julian years (18262.5 days), the next is expected to be J2050.0.

With the transition from B1950.0 to J2000.0, the fourth precision star catalog was also updated to the new version of the fifth catalog and the approximately 2000 fundamental stars were supported by interferometric VLBI measurements from 500 extragalactic radio sources ( quasars ). This enables modern astrometry to define the cosmic reference system and its changes to better than 0.01 ″. With this accuracy it now corresponds to a real, unmoved inertial system .

It is possible to calculate the Bessel epochs beyond 1984 and the Julian epochs for points in time before that. However, this is not common.