Imagine, your character is keeping watch while the rest of the party sleeps. You hear a noise and stare intently into the darkness to see what made it. You ask the DM, "How much light is there?" And the DM gets a look on his face like he never even thought about how bright the moons would make it.
Well, Rovewin of Athas.org made this awesome calendar of moon phases so this doesn't happen to your DM, or to you if you are the DM.
Special thanks to Rovewin, for allowing me to share this with all Dark Sun fans.