Hi Sam,
I tried to reply to your email several times from two different systems but every time speakeasy rejected my email. You might want to check into this.
But anyway, I don't have much useful info for you. Yours is the first report I've seen about trouble with XEphem on El Capitan. I run it here all the time with no problems also. The only thing I can think of is an issue with your xquartz installation. Can you run xlogo ok? You might try removing it and installing fresh from xquartz.org.
Maybe running XEphem from a terminal would give more clues. Cd to where you copied the XEphem.app folder, then run?XEphem.app/Contents/Resources/xephem/bin/osx/xephem and see what that says.
If you can't get it working, let me know and I'll refund your purchase in full, no worries.
Elwood
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