I have Xephem plus other astronomy programs here:

http://www.astro.uson.mx/~favilac/downloads/IRAF/el5/RPMS/

If you want to add it as a yum repo install this rpm:

http://www.astro.uson.mx/%7Efavilac/downloads/IRAF/el5/RPMS/astro-uson.repo-1.0-1.el5.uson.noarch.rpm

Then all the programs will be available for easy install.