Epoch (astronomy)
From Wikipedia, the free encyclopedia
 |
This article may require cleanup to meet Wikipedia's quality standards. Please discuss this issue on the talk page or replace this tag with a more specific message. This article has been tagged since August 2006. |
 |
This article or section recently underwent a major revision or rewrite and needs further review. You can help! |
In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements 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.
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. The previous standard epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch.
Before 1984 Besselian epochs were used. Since that time Julian epochs have been used.
- 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.
Epochs for orbital elements are usually given in Terrestrial Time, in several different formats, including:
- 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/NORAD's Two-Line Elements format with fractional day: 00001.50000000
Contents |
[edit] Besselian epochs
A Besselian epoch, named after the German mathematician and astronomer Friedrich Bessel (1784 – 1846), is an epoch that is based on a Besselian year of 365.242198781 days, which is a tropical year measured at the point where the Sun's longitude is exactly 280°.
Since 1984, Besselian epochs have been superseded by Julian epochs. The current standard epoch is J2000.0, which is a Julian epoch.
Besselian epochs are calculated according to:
- B = 1900.0 + (Julian date − 2415020.31352) / 365.242198781
The standard epoch that was in use before the current standard epoch (J2000.0) was B1950.0, a Besselian epoch.
Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch.
[edit] B1875.0
The official constellation boundaries were defined in 1930 along the lines of right ascension and declination for this epoch.
The 1875 figure refers to the year 1875 whose mean equator and equinox are used to define right ascension and declination of celestial bodies. See precession.The "B" in the prefix indicates that it is a Besselian epoch, as opposed to a Julian epoch.
[edit] B1900.0
The B1900.0 epoch is used in astronomy.
Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. This standard was superseded by the succeeding epoch, B1950.0. The current one in standard use is the J2000.0 epoch.
The "B" in the prefix indicates that it is a Besselian epoch, as opposed to a Julian epoch.
[edit] B1950.0
Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. This standard was superseded by the succeeding epoch, which is currently in standard use, the J2000.0 epoch. Nevertheless, since many old star tables are still useful, B1950.0 cannot just be shelved. For comparison to modern J2000.0 positions, the old ones have to be corrected for precession and nutation.
The "B" in the prefix indicates that it is a Besselian epoch, as opposed to a Julian epoch.
[edit] Julian epochs
A Julian epoch is an epoch that is based on Julian years of exactly 365.25 days. Since 1984, Julian epochs are used in preference to the earlier Besselian epochs.
Julian epochs are calculated according to:
- J = 2000.0 + (Julian date − 2451545.0)/365.25
The standard epoch currently in use is J2000.0, which corresponds to January 1, 2000 12:00 Terrestrial Time.
[edit] J2000.0
The J2000.0 epoch is precisely Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT. This is equivalent to January 1, 2000, 11:59:27.816 TAI or January 1, 2000, 11:58:55.816 UTC.
Since the right ascension and declination of stars are constantly changing due to precession, (and, for relatively nearby stars due to proper motion), astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was the B1950.0 epoch.
When the mean equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000. Technically, this is different from, but similar to, the International Celestial Reference System (ICRS): the mean equator and equinox at J2000.0 are distinct from and of lower precision than ICRS, but agree with ICRS to the limited precision of the former. Use of the "mean" locations means that nutation is averaged out or omitted. Novices are sometimes confused by finding that the Earth's rotational North pole does not point quite at the J2000 celestial pole at the epoch J2000.0; the reason is that the true pole of epoch nutates away from the mean one. The same differences pertain to the equinox.
The "J" in the prefix indicates that it is a Julian epoch rather than a Besselian epoch.
[edit] See also
[edit] References
- S. Aoki, et al., "Conversion matrix of epoch B 1950.0 FK 4-based positions of stars to epoch J 2000.0 positions in accordance with the new IAU resolutions", Astron. Astrophys. 128(2), 263–267, (1983).
- E. M. Standish, "Conversion of positions and proper motions from B1950.0 to the IAU system at J2000.0", Astron. Astrophys. 115(1), 20–22, (1982).
[edit] External links
- What is TT?
- International Celestial Reference System, or ICRS
- IERS Conventions 2003 (defines ICRS and other related standards)
) • 
) •
) •
) •
) •
)
) • 
) •
) •
) •
) •
) •
)
