Here’s a script which writes the current phase of the moon (in emoji form) to stdout. I use it in my shell prompt, which I highly recommend per the first sentence of this paragraph.
The easiest way is to call the self-contained interpreted script, moon-phase. I’d recommend
symlinking it somewhere in your $PATH, but you do you. The code is identical to moon.el, which is
provided as an easier entry point in case you want to use/edit it with your own emacs instance on
your own terms. Go ahead, go nuts, go apeshit.
The moon-phase-fast wrapper script directly calls the byte-compiled version of the elisp file, which
I added to the repo. I don’t know how portable elisp bytecode is, though: it’s possible you’ll have
to crack open an emacs instance of your own and run byte-recompile-file on moon.el.
This uses functions adapted from the lunar.el library included in emacs. That file only names phases
to quarter moon precision, so I adapted it to split the lunar cycle into 8, not 4 (adding gibbous
and crescent moons (both waxing and waning)) to match the precision of the moon phase emojis.
Macs come bundled with a version of emacs that is ancient. Problem is, this script is only
compatible with emacs 24+ (I think: I wrote it using emacs 26). If you’re on macOS and you don’t
know what sort of emacs you have, crack open a shell and run emacs --version, followed by brew
install emacs if needed.
It would be cool to expand this into a patch for lunar.el adding this functionality for all emacs users.