John Thorstensen's Skycalc software re-factored into a library so that it can be used from other software, in particular lsosc.
As you'd expect it's based on John Thorstensen's Skycalc software from a long time ago... v4 of the code appears to be available here: ftp://[email protected]/contrib/skycal.tar.Z.
See also http://www.briancasey.org/artifacts/astro/skycalc_notes.html.