A SkyFrame is a specialised form of Frame which describes
celestial longitude/latitude coordinate systems. The particular
celestial coordinate system to be represented is specified by
setting the SkyFrame's System attribute (currently, the default
is FK5) qualified, as necessary, by a mean Equinox value and/or
an Epoch.
All the coordinate values used by a SkyFrame are in
radians. These may be formatted in more conventional ways for
display by using AST_FORMAT.