The Epoch value is important in cases where the co-ordinate system changes with time. For instance, when considering celestial co-ordinate systems, possible reasons for change include: (i) changing aberration of light caused by the observer's velocity (e.g. due to the Earth's motion around the Sun), (ii) changing gravitational deflection by the Sun due to changes in the observer's position with time, (iii) fictitious motion due to rotation of non-inertial co-ordinate systems (e.g. the old FK4 system), and (iv) proper motion of the source itself (although this last effect is not handled by the SkyFrame class because it affects individual sources rather than the co-ordinate system as a whole).
The Epoch attribute is stored as a Modified Julian Date, and is not usually changed.
When enquiring Epoch values, the format used is the ``Year'' format described under ``Input Formats''. This is a value in decimal years, which will be a Besselian epoch if less than 1984.0, and a Julian epoch otherwise.
KAPPA --- Kernel Application Package