KMoon is a little applet that docks into the System Tray and shows an image of the current phase of the moon. Hovering the mouse cursor over the image brings up a tooltip to display more information.

KMoon can be added to the panel by right mouse button clicking on an empty space on the panel, selecting Add Applet to Panel... and choosing Moon Phase in the dialog. If you cannot find an empty space, you can access the panel menu from an right mouse button click on any existing applet or icon.

To close KMoon right click on the applet handle beside the image and choose Remove Moon Phase.