I have this working on my second display in xfce4
Place this in your ~/.xinitrc file
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## Start xfce Desktop Environment ##
########################################################################
while true; do xplanet -body moon -geometry 1024x768 -background /home/username/.xplanet/stars/stars_custom.png -glare 0.001 -output /home/username/Wallpaper/moon.xplanet.jpg -num_times 1 && /usr/bin/xfdesktop --reload; sleep 45m; done &
After you paste that, go into your xfce settings manager and tell xfce to use the jpg as a wallpaper (if it does not exist yet, run that command once in a terminal session so it can generate the file.
Note... Set the geometry to your monitor's value. The background file will not exist on your computer, if you want a background I suggest googling around a bit to find a background image you like (you do not need a background if you prefer no background image then you can remove that section of the code). Then edit that image so it is exactly the geometry of your display. The -glare 0.001 option kills the glare render during new moon and full moon. Trust me, you will want this unless you like seeing a very bright yellow glare for a few hours during full and new moon cycles.
For the moon body to work in xplanet, you will first need to download a moon map. Xplanet does not ship with moon maps... I have no idea why, it comes with almost every other planet and Sattlite in our solar system. Google around for the xplanet moon map. (if anyone finds a Deathstar map please let me know. I actually want one of the second incomplete deathstar).
(The above command is a single line command, copy paste on this blog may have added some hard returns).
**MAKE SURE YOU REPLACE 'username' WITH YOUR ACTUAL USERNAME**
