Also, there is a subtlety to do with the "transform_to" method - since in general sky coordinates systems may move with respect to each other over time, you need something to indicate which reference frame is to be considered "fixed". For instance, say you have  two skyframes both representing Helioecliptic longitude and latitude (i.e. the Sun defines zero longitude) but they have different epoch of observations. If you use "transform_to" to transform a position from one SkyFrame to the other, does the position change? Since both SkyFrames represent HelioEcliptic, you could argue that the position should not change. But on the other hand, that position *will* change when viewed against the background of distant stars (i.e. if you convert the position from the first skyframe, to ICRS, and then convert back to the second SkyFrame). The AST library addresses this issue by means of the "AlignSystem" attribute attached to the base Frame class (see http://starlink.jach.hawaii.edu/docs/sun211.htx/node454.html).