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The J2000.0 epoch is used in astronomy. Wikipedia does not have an article with this exact name. ... A Julian epoch is an epoch that is based on Julian years of exactly 365. ... Look up expert in Wiktionary, the free dictionary. ... In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... Radio telescopes are among many different tools used by astronomers Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, law of the stars) is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as auroras and cosmic background radiation). ...

It 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. 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). ... Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... This article is about the year 2000. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... This article is about the year 2000. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... This article is about the year 2000. ... Coordinated Universal Time (UTC) is a high-precision atomic time standard which approximately tracks Universal Time (UT). ...

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 refers to a change in the direction of the axis of a rotating object. ... 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. 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 suffers nutation ("is nutated") away from the mean one. The same differences pertain to the equinox. The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ... The equinoxes are two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. ... The J2000. ... 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 largely axially symmetric object, such as a gyroscope or a planet. ... 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 largely axially symmetric object, such as a gyroscope or a planet. ...

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...

See also

• International Celestial Reference System
• International Celestial Reference Frame
• Astrometry

The J2000. ... The International Celestial Reference Frame (ICRF) is a quasi-inertial reference frame centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources (mainly quasars). ... This article does not cite its references or sources. ...

External links

• What is TT?
• International Celestial Reference System, or ICRS
• IERS Conventions 2003 (defines ICRS and other related standards)