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 .
 The position information right ascension and declination ( equatorial coordinates ) in star catalogs always relate to an epoch.
 The orbital elements of celestial objects are also given for an epoch (in fact, the epoch is one of the 6 basic orbital elements of a Kepler orbit ): When calculating for other times, the orbital disturbances (perturbations) caused by other celestial bodies must be included. The orbital elements of the epoch are osculating , that is , the closest possible approximation to the current state.
 Ephemeris , which are tabulated positions of individual astronomical objects, mostly in azimuthal coordinates , are related to the respective epoch.
Bessel epoch
In astronomy, it is often desirable to be able to relate timedependent 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.
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
 .
It is the other way around
 .
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

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.
See also
 Astronomical coordinate systems , absolutely necessary for specifying an epoch
 Astronomical units of measure
 Observational Astronomy
literature
 Joachim Herrmann : dtv atlas astronomy . Dtv, March 2005. ISBN 3423032677 .
 Andreas Kamp: From Paleolithic to Postmodern  The genesis of our epoch system . Vol. I: From the beginning to the end of the 17th century , Amsterdam / Philadelphia 2010, ISBN 9789027287366 .
Individual evidence
 ^ Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language. 3rd edition, 6th impression, Vieweg & Sohn, Braunschweig 1914. 1914, accessed on August 13, 2018 .
 ^ Robert Scott , Henry George Liddell : A GreekEnglish Lexicon. Retrieved August 13, 2018 .
 ^ ^{A } ^{b } [Resolutions of the] XVIth General Assembly, Grenoble. (PDF) IAU, 1976, pp. 15, 16 f. , accessed on March 23, 2019 (Notes on Recommendations 2 and 5).
 ^ Definition of the unit of time (second). BIPM , 1960, accessed March 24, 2019 (Resolution 9 of the 11th CGPM).
 ↑ United States Naval Observatory (ed.): The Astronomical Almanac for the year 2009 . US Government Printing Office, 2007, ISBN 9780118873420 , pp. B3 ( limited preview in Google Book search).
 ↑ TT = TAI + 32.184 s and TAI = UTC + 32 s in 2000