It would be fairly easy, and quite helpful, to have the moon icon for night-time weather indicate the current moon phase. Since the program already has longitude and latitude data, it would be easy to calculate. In fact, there is a separate lunar clock applet for gnome:

http://glunarclock.sourceforge.net/

Most users of the weather applet will not have need of the more esoteric statistics in that applet (moon distance from earth, apparent/mean/sidereal time) but the use of accurate moon phases in the weather report would not only be helpful, it would add a slick and classy touch of realism to the display, and implementation would not be difficult.