This Disc revolves in exactly eight hundred days, in order to equally distribute the weight upon the four Elephants. A day is constituted of two main parts: the day itself, when the sun is above the Disc, and the night, when the sun is under the Disc, passing between the Elephants from A'Tuin's left hand side to Her right hand side. The sun is actually a tiny shining ball whose eruptions are hardly bigger than "croquet hoops". Its orbit is fixed, and when it rises or sets, it pass near the Rimfall of Discworld and these are the hottest days of the year for these regions. Of course, there is two summers in a complete year (one near the rise and one near the set), and in fact two winters, two autumns and two springs. Except for the Hub which is never closely warmed by the sun and whose lands are locked in permafrost, and where the only inhabitants are the gods of Discworld.
There are two main directions in Discworld: Hubward and Rimward. Completing them are two other directions based on the movement of the Disc: these are Turnwise and Widdershins. The oceans are flowing Rimward, where they constantly boil over the Edge into space. That's why when you see a ship fade into the distance as if it has dropped off the Edge of the world, it is because it has actually dropped off the Edge of the world. Of course, some people rejected this explanation and went on their own voyage to prove this was nonsense. For some reason they never came back.