Install wmmoonclock by entering the following commands in the terminal:
sudo apt update sudo apt install wmmoonclock
Description:
Window Maker moon phase dockapp
wmMoonClock displays a small pixmap of the current phase of the moon. You have to enter your latitude and longitude for it to work correctly. wmMoonClock is designed to work with the Window Maker dock, but will work with other window managers as well.
Homepage: http://www.dockapps.net/wmmoonclock
Version: 1.29-1
Section: universe/x11