The Warhammer world is orbited by two moons.
The larger of the two moons is called Mannslieb and it acts and appears as any normal moon, having a cycle of 25 days from full to full, and orbits the planet upon a predictable orbit. Mannslieb means "Beloved of Manann" in Reikspiel and this moon is held sacred to Manann, god of the Sea.
Morrslieb, the smaller moon, is all but predictable, appearing sometimes closer, and other times farther away than its partner. Its position has major influence upon the Winds of Magic. It is named Morrslieb, meaning "Beloved of Morr", in honour of Morr, god of the dead.
A common legend tells that daemons once preyed upon the people of the world from a gateway in the sky, until Morr inflicted a great death upon them. Afterwards he fashioned the second moon from the wreckage of the daemons, so that mortals would never forget how he had saved them.
The second legend, which is mainly held by followers of Chaos, is that upon the collapse of the polar warp gates, a great block of Warpstone was hurled high into the sky, where it now circles the planet endlessly, spreading Chaos over the lands which it passes.
From time to time, meteors of Warpstone coming from Morrslieb fall upon the planet.
There are two nights in the year in which both moons are full at the same time. The first is called Hexensnacht ("Witching Night") and the other one Geheimnisnacht ("Night of Mystery"). To most, these are greatly feared and ill-omened nights of the year, and even the most cynical sceptic will avoid the eerie light cast by the full moons. On the other hand, gatherings and ritual sacrifices are held by the worshippers of Chaos on these nights.