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.).
KAPPA --- Kernel Application Package