Through the fathomless deeps of space swims the star turtle Great A’Tuin, bearing on it’s back the four giant elephants who carry on their shoulders the mass of the Discworld. A tiny sun and moon spin around, on a complicated orbit to induce seasons, so this is probably the only place in the Multiverse where it is sometimes necessary for an elephant to move a leg to allow the sun to go past.
Exactly why it is this way, may never be known.
Possibly the Creator of the universe got bored with all the usual business of axial inclination, albedos and rational velocities, and decided to have bit of fun.

There was, for example, the theory that A'Tuin had come from nowhere and would continue at a uniform crawl, or steady gait, into nowhere, for all time. This theory was popular among academics.

An alternative, favoured by those of a religious persuasion, was that A'Tuin was crawling from the Birthplace to the Time of Mating, as were all the other stars in the sky which were, obviously, also carried by giant turtles. When they arrived the would briefly and passionately mate, for the first and only time, and from that fiery union new turtles would be born to carry a new pattern of worlds. This was known as the Big Bang hypothesis.

—  The Colour of Magic, Terry Pratchett