The J2000.0 epoch is used in astronomy. In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... Astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and monitoring of transient phenomena. ...

It is precisely Julian date 2451545.0 TT, or January 1, 2000, 12h TT. This is equivalent to January 1, 2000, 11:59:27.816 TAI or January 1, 2000, 11:58:55.816 UTC. The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC (in the proleptic Julian calendar; or November 24, 4714 BC in the proleptic Gregorian calendar). ... This article is about terrestrial time; for other meanings of TT, see TT (disambiguation). ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... 2000 is a leap year starting on Saturday of the Gregorian calendar. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... 2000 is a leap year starting on Saturday of the Gregorian calendar. ... Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... 2000 is a leap year starting on Saturday of the Gregorian calendar. ... UTC also stands for the University of Tennessee at Chattanooga Coordinated Universal Time or UTC, also sometimes referred to as Zulu time, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. ...

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. Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... Precession (also called gyroscopic precession) is the phenomenon by which the axis of a spinning object (e. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... The B1950. ...

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, but more properly it is the International Celestial Reference System (ICRS) as referenced below. 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 suffers nutation ("is nutated") away from the mean one. The same differences pertain to the equinox. Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a planet, due to the fact that the tidal forces which cause precession of the equinoxes vary over time so that the speed of precession... Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a planet, due to the fact that the tidal forces which cause precession of the equinoxes vary over time so that the speed of precession...

The "J" in the prefix indicates that it is a Julian epoch, as opposed to a Besselian epoch. A Julian epoch is an epoch that is based on Julian years of exactly 365. ... A Besselian epoch, named after the German mathematician and astronomer Friedrich Bessel (1784 _ 1846), is an epoch that is based on a Besselian year, which is a tropical year measured at the point where the Suns longitude is exactly 280°. Since 1984, Besselian epochs have been superseded by...

External links

• What is TT? (http://aa.usno.navy.mil/faq/docs/TT.html)
• International Celestial Reference System, or ICRS (http://aa.usno.navy.mil/faq/docs/ICRS_doc.html)