xephem-3.6.tar.gz does not exist on the server, specifically, the ftp site. When trying to download all of the ports, using the MAKE FETCH command, xephem will try to download version 3.6 from ftp://ftp.freebsd.org/pub/FreeBSD/ports/distfiles/ and will fail, because that file does not exist. Only fersion 3.5.1 is on the server.