Domino [2]
The distinction between night and day in Eien was strange, but the night did eventually pass.
Although, the darkness of night wasn't necessarily a concept existing in a flat plane like Eien.
The sun and moon hung in the sky, but they only ever remained on the Grand Heavens Boundary half of Eien, given that the Abyss would consume them instantly if they crossed into its territory.
This strange situation led to both celestial bodies existing together at one time. The distinction between night and day was determined by which celestial body was closer to the earth at a given point in time, as the two rotated around each others' axes.
There were a true sun and moon dedicated to Eien, but since they traveled around the ring lengthwise, seeing them was a once in a countless hundred years experience.
When they met at the two extremes poles of Eien, mystical phenomena were said to occur.