There are three moons orbiting Roshar. Each waxes and wanes separately from the others.
The first moon is Salas, which is the smallest and the dimmest of the moons, casting a violet light. The middle moon is Nomon, which is a bright, pale blue. The third moon is Mishim, which is small and green, rising in the east. (Easterners consider the moons to be male.)
The Shin call the moons the 'Three Sisters', in the order described. So Salas is 'First Sister', Nomon is 'Second Sister' and Mishim is 'Third Sister'. The Shin believe the time between the first and second moon is the darkest portion of the night, called "the hateful hour ... for it was one of the only times when the gods did not watch men."