Equinox - Epoch of the mean equinox (SkyFrames only)

Description:

This attribute is used to qualify those celestial co-ordinate systems described by a SkyFrame  that are notionally based on the ecliptic (the plane of the Earth's orbit around the Sun) and/or the Earth's equator.

Both of these planes are in motion and their positions are difficult to specify precisely. In practice, therefore, a model ecliptic and/or equator are used instead. These, together with the point on the sky that defines the co-ordinate origin (the intersection of the two planes termed the `mean equinox') move with time according to some models that remove the more-rapid fluctuations. The SkyFrame class supports both the old FK4 and the current FK5 models.

The position of a fixed source expressed in any of these co-ordinate systems will appear to change with time due to movement of the co-ordinate system itself (rather than motion of the source). Such co-ordinate systems must therefore be qualified by a moment in time (the `epoch of the mean equinox' or `equinox' for short) which allows the position of the model co-ordinate system on the sky to be determined. This is the rôle of the Equinox attribute.

The Equinox attribute is stored as a Modified Julian Date, but when setting or getting its value you may use the same formats as for the Epoch  attribute (q.v.).

Type:

Floating point.

Notes:

• Care must be taken to distinguish the Equinox value, which relates to the definition of a time-dependent co-ordinate system (based on solar-system reference planes which are in motion), from the superficially similar Epoch value. The latter is used to qualify co-ordinate systems where the positions of sources change with time (or appear to do so) for a variety of other reasons, such as aberration of light caused by the observer's motion, etc.

• See the description of the System  attribute for details of which qualifying attributes apply to each celestial co-ordinate system.