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 coordinate origin (the intersection of the two planes termed the "mean equinox") move with time according to some model which removes the more rapid fluctuations. The SkyFrame class supports both the FK4 and FK5 models.
The position of a fixed source expressed in any of these coordinate systems will appear to change with time due to movement of the coordinate system itself (rather than motion of the source). Such coordinate 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 coordinate system on the sky to be determined. This is the role 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.).
The default Equinox value is B1950.0 (Besselian) for the old FK4-based coordinate systems (see the System attribute) and J2000.0 (Julian) for all others.
AST A Library for Handling World Coordinate Systems in Astronomy