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the plane perpendicular to the mean heliocentric orbital angular momentum vector of the Earth-Moon barycentre in the BCRS (IAU 2006 Resolution B1). In the past, there was no unique interpretation; an ecliptic was defined by means of the angles of the precession theory.
a fixed date used to reckon time for expressing time varying quantities. It is often expressed in the system of Julian date, marked by the prefix J (e.g. J2000.0), with the Julian year of 365.25 days as unit. The term is also used to designate the date and time of an observation, e.g. “epoch of observation”, which would be better expressed by ‘date of observation’.
the right ascension of the mean equinox referred to the true equator and equinox; alternatively the difference between apparent sidereal time and mean sidereal time (GAST − GMST).
distance between the CIO and the equinox along the intermediate equator; it is the CIO right ascension of the equinox; alternatively the difference between the Earth Rotation Angle and Greenwich apparent sidereal time (ERA − GAST).
either of the two points at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; i.e., when the apparent longitude of the Sun is 0° or 180°. When required, the equinox can be designated by the ephemeris of the Earth from which it is obtained (e.g. vernal equinox of DE 405). By 2100 the equinox will have moved 1.4° from the ICRS meridian, due to the precession of the equinoxes.
