I have not seen anyone mention xephem. It is written for the X windows system. It runs on Mac OS X as well as other UNIX/Linux platforms, and can even run on windows with sufficient help (e.g. cygwin/X). Graphically, it is not the fanciest by a long shot, but has capabilities other packages lack. I am able to isolate variables and graph them and solve equations based on them. For example, I have used the solving capability to compute contact times for a transit of Mercury at my location. It has telescope control through the INDI interface (I have not used telescope control and cannot vouch for it). It has a facility for registering an image with the star database (another capability I have not used). The web site is http://www.clearskyinstitute.com/ And here is a blurb "It takes some getting used to, but it's worth the effort. Once you understand its peculiarities, you'll find that XEphem can do just about anything you ask." Quoted with permission from Astronomy Hacks <http://www.oreilly.com/catalog/astronomyhks>, by Robert and Barbara Thompson. I am also fond of Starry Night Pro on the Mac. I will have to give The Sky a try sometime. I did not know it had been ported to the Mac. Jim Cobb james@cobbzilla.net