Inastronomy, anequinox is either of two places on thecelestial sphere at which theecliptic intersects thecelestial equator.[1][2][3] Although there are two suchintersections, the equinox associated with theSun'sascending node is used as the conventional origin ofcelestial coordinate systems and referred to simply as "the equinox". In contrast to the common usage ofspring/vernal andautumnal equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.
In a cycle of about 25,800 years, the equinoxmoves westward with respect to thecelestial sphere because ofperturbing forces; therefore, in order to define a coordinate system, it is necessary to specify the date for which the equinox is chosen. This date should not be confused with theepoch.Astronomical objects show real movements such asorbital andproper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical object requires both the date of the equinox and of the epoch.[4]
The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.[5]
The equinox moves, in the sense that as time progresses it is in a different location with respect to the distant stars. Consequently, star catalogs over the years, even over the course of a few decades, will list differentephemerides.[6] This is due to precession and nutation, both of which can be modeled, as well as other minor perturbing forces which can only be determined by observation and are thus tabulated in astronomical almanacs.
Precession of the equinox was first noted byHipparchus in 129 BC, when noting the location ofSpica with respect to the equinox and comparing it to the location observed byTimocharis in 273 BC.[7] It is a long term motion with a period of 25,800 years.
Nutation is the oscillation of the ecliptic plane. It was first observed byJames Bradley as a variation in the declination of stars. Bradley published this discovery in 1748. Because he did not have an accurate enough clock, Bradley was unaware of the effect of nutation on the motion of the equinox along the celestial equator, although that is in the present day the more significant aspect of nutation.[8] The period of oscillation of the nutation is 18.6 years.
A Besselian epoch, named after German mathematician and astronomerFriedrich Bessel (1784–1846), is an epoch that is based on aBesselian year of 365.242198781 days, which is atropical year measured at the point where theSun's longitude is exactly 280°. Since 1984, Besselian equinoxes and epochs have been superseded byJulian equinoxes and epochs. The current standard equinox and epoch isJ2000.0, which is a Julian epoch.
Besselian epochs are calculated according to:
The previous standard equinox and epoch wereB1950.0, a Besselian epoch.
Since theright ascension anddeclination of stars are constantly changing due toprecession, astronomers always specify these with reference to a particular equinox. Historically used Besselian equinoxes include B1875.0, B1900.0, B1925.0 and B1950.0. The official constellation boundaries were defined in 1930 using B1875.0.
A Julian epoch is an epoch that is based onJulian 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:
The standard equinox and epoch currently in use areJ2000.0, which corresponds to January 1, 2000, 12:00Terrestrial Time.
The J2000.0 epoch is preciselyJulian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT. This is equivalent to January 1, 2000, 11:59:27.816TAI or January 1, 2000, 11:58:55.816UTC.
Since theright ascension anddeclination of stars are constantly changing due toprecession, (and, for relatively nearby stars due toproper motion), astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was theB1950.0 epoch.
When themean equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000. This is different from theInternational 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 thatnutation is averaged out or omitted. This means that the Earth's rotational North pole does not point quite at the J2000celestial pole at the epoch J2000.0; the true pole of epoch nutates away from the mean one. The same differences pertain to the equinox.[9]
The "J" in the prefix indicates that it is a Julian equinox or epoch rather than a Besselian equinox or epoch.
There is a special meaning of the expression "equinox (and ecliptic/equator)of date". This reference frame is defined by the positions of the ecliptic and the celestial equator as of the date/epoch on which the position of something else (typically asolar system object) is being specified.[10]
Other equinoxes and epochs that have been used include:
Epochs and equinoxes fororbital elements are usually given inTerrestrial Time, in several different formats, including:
Sidereal time is thehour angle of the equinox. However, there are two types: if the mean equinox is used (that which only includes precession), it is called mean sidereal time; if the true equinox is used (the actual location of the equinox at a given instant), it is called apparent sidereal time. The difference between these two is known as the equation of the equinoxes, and is tabulated in theAstronomical Almanac.[14]
A related concept is known as the equation of the origins, which is the arc length between theCelestial Intermediate Origin and the equinox. Alternatively, the equation of the origins is the difference between theEarth Rotation Angle and the apparent sidereal time at Greenwich.
In modern astronomy the ecliptic and the equinox are diminishing in importance as required, or even convenient, reference concepts. (The equinox remains important in ordinary civil use, in defining the seasons, however.) This is for several reasons. One important reason is that it is difficult to be precise what the ecliptic is, and there is even some confusion in the literature about it.[15] Should it be centered on the Earth's center of mass, or on the Earth-Moon barycenter?
Also with the introduction of theInternational Celestial Reference Frame, all objects near and far are put fundamentally in relationship to a large frame based on very distant fixed radio sources, and the choice of the origin is arbitrary and defined for the convenience of the problem at hand. There are no significant problems in astronomy where the ecliptic and the equinox need to be defined.[16]
