This is actually an excerpt from the 'Colour of Magic' which is the first book in the Discworld series. It describes the discworld and is a great introduction (amongst other things) to the branch of science known as 'Astrozoology'.
An Extract From
The Colour of Magic
by Terry Pratchett
In a distant and second-hand set of dimensions, in an astral plane that was never meant to fly, the curling star-mists waver and part...
Great A'Tuin the turtle comes, swimming slowly through the interstellar gulf, hydrogen frost on his ponderous limbs, his huge and ancient shell pocked with meteor craters. Through sea-sized eyes that are crusted with rheum and asteroid dust He stares fixedly at the Destination.
In a brain bigger than a city, with geological slowness, He thinks only of the Weight.
Most of the weight is of course accounted for by Berilia, Tubul, Great T'Phon and Jerakeen, the four giant elephants upon whose broad and startanned shoulders the disc of the World rests, garlanded by the long waterfall at its vast circumference and domed by the baby-blue vault of Heaven.
Astropsychology has been, as yet, unable to establish what they think about.
The Great Turtle was a mere hypothesis until the day the small and secretive kingdom of Krull, whose rim-most mountains project out over the Rimfall, built a gantry and pulley arrangement at the tip of the most precipitous crag and lowered several observers over the Edge in a quartzwindowed brass vessel to peer through the mist veils.
The early astrozoologists, hauled back from their long dangle by enormous teams of slaves, were able to bring back much information about the shape and nature of A'Tuin and the elephants but this did not resolve fundamental questions about the nature and purpose of the universe.
For example, what was Atuin's actual sex? This vital question, said the Astrozoologists with mounting authority, would not be answered until a larger and more powerful gantry was constructed for a deep-space vessel. In the meantime they could only speculate about the revealed cosmos.
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 stars in the sky which were, obviously, also carried by giant turtles. When they arrived they 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.
Thus it was that a young cosmochelonian of the Steady Gait faction, testing a new telescope with which he hoped to make measurements of the precise albedo of Great A'Tuin's right eye, was on this eventful evening the first outsider to see the smoke rise hubward from the burning of the oldest city in the world.
Later that night he became so engrossed in his studies he completely forgot about it. Nevertheless, he was the first. There were others...
. . .
 The shape and cosmology of the disc system are perhaps worthy of note at this point. There are, of course, two major directions on the disc: Hubward and Rimward. But since the disc itself revolves at the rate of once every eight hundred days (in order to distribute the weight fairly upon its supportive pachyderms, according to Reforgule of Krull) there are also two lesser directions, which are Turnwise and Widdershins. Since the disc's tiny orbiting sunlet maintains a fixed orbit while the majestic disc turns slowly beneath it, it will be readily deduced that a disc year consists of not four but eight seasons. The summers are those times when the sun rises or sets at the nearest point on the Rim, the winters those occasions when it rises or sets at a point around ninety degrees along the circumference. Thus, in the lands around the Circle Sea, the year begins on Hogs' Watch Night, progresses through a Spring Prime to its first midsummer (Small Gods' Eve) which is followed by Autumn Prime and, straddling the half-year point of Crueltide, Winter Secundus (also known as the Spindlewinter, since at this time the sun rises in the direction of spin). Then comes Secundus Spring with Summer Two on its heels, the three quarter mark of the year being the night of Alls Fallow - the one night of the year, according to legend, when witches and warlocks stay in bed. Then drifting leaves and frosty nights drag on towards Backspindlewinter and a new Hogs' Watch Night nestling like a frozen jewel at its heart.
Since the Hub is never closely warmed by the weak sun the lands there are locked in permafrost. The Rim, on the other hand, is a region of sunny islands and balmy days. There are, of course, eight days in a disc week and eight colours in its light spectrum. Eight is a number of some considerable occult significance on the disc and must never, ever, be spoken by a wizard.
Precisely why all the above should be so is not clear, but goes some way to explain why, on the disc, the Gods are not so much worshipped as blamed.