The pattern of natural light caused by the two different light sources in the star system.
One source of light is The Central Star. Most celestial bodies in the system slowly turn towards its center and then away again, either around their own axis or by orbiting a planet.
The second factor influencing light on celestial bodies is their orbital position. During summer, when they stand between That Which Changes and The Central Star, the planet and any moons it may have receive light from both sides. In contrast, light only arrives from one direction during winter.
This means that for any given location on a celestial body the ratio between light and darkness slowly changes from 50% to 100% and back.
This pattern occurs on nearly every celestial body in the system. The only exceptions are The Outer Planetoid, which is nowadays kept in a fixed location, and The Burnt Planet, which is in tidal lock with The Central Star.