Every morning, princess Celestia raises the sun, and every evening, she lowers it.
This never changes.

Every night, princess Luna moves the constellations through the sky, and every month, the moon cycles through its phases.
This never changes.

Like clockwork, the movements of the heavens are constant, unwavering.

Most ponies never give it a second thought. Even Twilight Sparkle.

Until one day. . .