|
The class of decision problems solvable by a Turing machine. Often identified with the class of 'effectively computable' functions (the Church-Turing thesis).
In [[astronomy]], an '''epoch''' is a moment in time for which [[celestial coordinate]]s or [[orbital element]]s are specified.
In the case of celestial coordinates, the position at other times can be computed by taking into account [[precession]] and [[proper motion]].
In the case of orbital elements, it is necessary to take account of perturbation by other bodies in order to calculate the orbital elements for a different time.
Since we can decide any problem for which there exists a recogniser and also a co-recogniser, the class is equal to <math>RE \cap coRE</math>.
The currently used standard epoch is [[J2000.0]], which is [[January 1]], [[2000]] at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a [[Julian epoch]]. The previous standard epoch was [[B1950.0]], with the prefix "B" indicating it was a [[Besselian epoch]].
==External links==
Besselian epochs were used before 1984, however Julian epochs are now used.
* [http://qwiki.caltech.edu/wiki/Complexity_Zoo#re Complexity Zoo]
[[Category:Complexity classes]]
* The [[Henry Draper Catalog]] uses [[B1900.0]]
* [[Constellation]] boundaries were defined in 1930 along lines of [[right ascension]] and [[declination]] for the [[B1875.0]] epoch.
{{ComplexityClasses}}
Epochs for orbital elements are usually given in [[Terrestrial Time]], in several different formats, including:
* [[Gregorian Calendar|Gregorian]] date with 24-hour time: 2000 Jan. 1, 12:00 TT
* Gregorian date with fractional day: 2000 Jan. 1.5 TT
* [[Julian Day]] with fractional day: JDT 2451545.0
* [[NASA]]/[[North American Aerospace Defense Command|NORAD]]'s [[Orbital_elements#Two_line_elements|Two-Line Elements]] format with fractional day: 00001.50000000
<!-- [[Category:Astronomy]] too general -->
[[Category:Celestial coordinate system]]
[[de:Epoche (Astronomie)]]
[[fr:Époque (astronomie)]]
[[it:Epoca (astronomia)]]
[[nl:Epoche]]
[[nn:Epoch]]
[[sk:Epocha (astronómia)]]
[[zh:纪元 (天文学)]]
| 