The Discworld is the fictional world where English writer Sir Terry Pratchett's Discworld fantasy novels take place. It consists of an interstellar planet-sized disc, which sits on the backs of four huge elephants, themselves standing on the back of a world turtle, named Great A'Tuin, as it slowly swims through space.
The Disc is the setting for all forty-one Discworld novels; it was influenced by world religions which feature human worlds resting on turtles, as a setting to reflect situations on Earth, in a humorous way. The Discworld is peopled mostly by three main races: men, dwarfs, and trolls. As the novels progress, other lesser known races are included, such as dragons, elves, goblins, and pixies.
Pratchett first explored the idea of a disc-shaped world in the novel Strata (1981).
Great A'Tuin Great A'Tuin is the Giant Star Turtle (of the fictional species Chelys galactica) who travels through the Discworld universe's space, carrying four giant elephants (named Berilia, Tubul, Great T'Phon, and Jerakeen) who in turn carry the Discworld. Great A'Tuin frequently rolls on its belly to avoid asteroid and comet collisions, and even snatches projectiles out of the sky to protect the Disc. These stunts do not affect the Disc's population, other than to induce severe seasickness on anyone who happens to be looking at the night sky at that time.
Great A'Tuin's sex is unknown to the inhabitants of Discworld, but the subject of much speculation by the Disc's scientific minds. The sex of A'Tuin is pivotal in proving or disproving a number of conflicting theories about its destination. If, as the Discworld version of the popular "Big Bang theory" states, Great A'Tuin is moving from the Birthplace to the Time of Mating, then at the point of mating the civilisations of the Disc might be crushed, simply slide off, or else the entire world will end. The hypothesis is that all stars in the sky are also worlds carried by giant turtles, and that when all the turtles meet they will mate, birthing new turtles to carry new worlds.
The other theory, akin to the steady state theory, is the "steady gait" theory, which states that Great A'Tuin came from nowhere and will perpetually swim through space. Eric shows Great A'Tuin being made instantly from nothing, seemingly in support of the "steady gait" theory. In The Light Fantastic, the Great A'Tuin attends the hatching of eight baby turtles, each with four elephants and a discworld of their own, which appears to support the Big Bang hypothesis.
A small sun and moon orbit Great A'Tuin, both described as in diameter at the start of the series. The description of their diameter is increased to at least later in the chronicles. The moon is slightly closer to the Disc than the sun. One half is covered with silvery glowing plants, which feed lunar dragons.The Last Hero The other half is burnt black by the sun. The moon rotates and completes a full revolution in about a month; the full moon occurs when the luminescent side is completely visible from the Disc; the new moon when the dark side is shown. The sun's orbit is so complex that one of the elephants has to cock its leg to allow the sun to continue on its orbit.Demonstrated in the animation sequence accompanying the credits of the Cosgrove Hall animated adaptations of Wyrd Sisters and Soul Music.
According to the wizards of Unseen University, the bodies of Chelys galactica, and thus Great A'Tuin, are composed largely of the fictional element chelonium. The element's properties are known to the wizards, but not to readers.
Magic
Magic is the principal force in Discworld and operates in a similar vein to real-world elemental forces such as gravity and electromagnetism. The Disc's "standing magical field" is essentially the local breakdown of reality, which allows a flat planet on the back of a turtle to exist. Magic is a function of the relative absence of reality in the local area, much in the same way that the absence of heat is described as coldness. Magic warps reality in much the same way as gravity warps space-time. The act of performing magic is, essentially, telling the universe what form it must take, in terms that the universe cannot ignore. This is draining to magic users, due to Discworld science's Law of Conservation of Reality, which states that magic and mundane methods require the same energy expenditure. This is why most wizards store magic in a staff, a capacitor for magical energy.
High-level background magic occurs when a powerful spell creates particles that distort reality. Entering a region where this has happened is extremely dangerous, as it is mentioned that an individual may not remain the same species, shape or level of sanity as they were when they entered. Medium levels cause odd effects, such as coins landing on their edges and objects transforming into caterpillars. Areas with large quantities of background magic tend to display unusual qualities, even for the Disc. High quantities of magic can knock a hole in Discworld reality, leading to an invasion by Lovecraftian monstrosities from the Dungeon Dimensions, or, almost as bad, the world of the Elves.
In the Discworld universe, magic is broken into elementary particulate fragments in much the same way that energy and other forces are in real-world quantum physics. The basic unit of magic is the thaum (from the Greek thauma, marvel), equal to the amount of mystical energy required to conjure a small white pigeon or three normal-sized billiard balls. Several SI-modifiers have been applied to it (e.g. millithaum, kilothaum). Magic can be measured with a thaumometer, a black cube with a dial on one side. A standard thaumometer can measure up to a million thaums; beyond that level, reality starts to break down. Parodying the introduction of the metric system, later novels refer to the introduction of the newer unit, the Prime, to avoid arguments over the standard sizing of pigeons. It is defined as the magical energy required to move a pound of lead one foot, referencing the joule.
The thaum also appears to be a particle, the equivalent of the atom. "Splitting the thaum" revealed that it was composed of numerous sub-particles, called resons ("thingies") which in turn are created from five "flavours": up, down, sideways, sex appeal, and peppermint (parodying quark flavours).
In the opening books, the number eight is significant and has magical properties on the Disc, (e.g. the number of the colour of Magic, octarine) and should never be spoken by wizards, especially in certain places. Doing so may allow the ancient dungeon dimension creature "Bel-Shamharoth the sender of eight" to break through (this is taken to an extreme end in one book, as even the narrator takes great pains to avoid saying the word). On the other hand, eight turns up in many places one would expect the number seven in the real world (e.g. the Discworld week contains eight days, not seven). After The Colour of Magic, both the colour and the number eight no longer appear as dangerous.
The Disc's magical field is centred on Cori Celesti. Everyday natural forces, such as light and magnetism, are muffled by the power of the Disc's magical field, and rather than a magnetised needle, navigators on the Disc use a compass with a needle of the magical metal octiron, which will always point towards Cori Celesti. Light is affected by magic, slowing it to moving hundreds of miles an hour rather than the speed of light.
The power of belief Reality is spread thinly on the Disc, so events may be affected by expectations, especially those of 'intelligent' species. As such, the Discworld is not governed by real-world physics or logic but by belief and narrative resolution. Essentially, if something is believed strongly enough, or by enough people, it may become true. Jokes such as treacle mines and drop bears are real on the Disc. These concepts of Discworld physics are also exploited in magic. For example, if one wishes to turn a cat into a human, the easiest way is to convince the cat that it is human.
More significantly, it is also belief that gives Discworld's gods their powers. Discworld gods start off as small spirits, and gain power as they gain believers; this is explored most thoroughly in Small Gods. A similar effect has led to the personalisation and "reification" in the Discworld universe of mythological beings symbolising abstract concepts, such as Death. It is also possible for humans to gain godly powers through the power of belief, typically venerated rulers. It is possible for gods to derive power from misotheists, those who hate gods. As demonstrated in Faust Eric and Mort, personal belief determines someone's fate in the afterlife, rather than divine judgement, whether it is going to paradise or Hell, or being reincarnated.
Such is the nature of belief on the Disc that temporary new gods come into being on a regular basis, and often for such mundane things as stuck drawers; Anoia, Goddess of Things That Get Stuck in Drawers, came into her powers in such a manner, though she was previously a volcano goddess.
Narrative causality The Disc's nature is fundamentally teleological; its basic composition is determined by what it is ultimately meant to be. This primary element, out of which all others spring, is known as narrativium, the elemental substance of Story. Nothing on the Disc can exist without a Story first existing to mould its destiny and determine its form. This is, perhaps, a take on the fact that nothing can ever happen on the Disc unless it is written in a story by Terry Pratchett. On the Disc, if a story or legend is told often enough and believed by enough people, it becomes true. This is known as the law of narrative causality.
Discworld witches often employ narrative in their magic, but consider it ethically tricky since it interferes with free will. Discworld wizards avoid doing so because narrative that severely strains credibility requires outright sources of magic to feed on, sometimes indiscriminately. Knowledge of stories, their use, and how to change them forms the basis of many forms of magical power in Discworld physics. Pratchett characters who use or change stories include Lilith, Black Aliss, and Granny Weatherwax. The habit of many Discworlders to take metaphor literally has combined with the power of belief to produce odd areas on Discworld.
Octarine Octarine is the colour of magic on Discworld, also referred to as the eighth colour. Octarine is strongly indicative of magic and can only be seen by wizards and cats, who both possess "octagon cells" in addition to the normal cones and rods possessed by humans. This is an effect akin to tetrachromacy. Rincewind, a wizard, describes octarine as a fluorescent greenish-yellow purple.
Substances The Disc contains magical substances. One such is octiron, a dense black metal that makes up much of Discworld's crust. Its melting point is above the range of metal forges. The gates of Unseen University are made out of it, as is Old Tom, the university's bell. It is used to make magic needles and bells. Octiron releases magical radiation, but if it becomes negatively polarised, it can be used to absorb the radiation. Octiron generates significant amounts of heat under pressure, accounting for most volcanic geological processes on Discworld. When struck, octiron briefly silences anything around it instead of producing sound.
Another is fingles, insinuated in Eric to be an important part of human psychology. Their absence, according to the Creator, can cause psychological problems. "On the surface they were all right, but deep down they knew something was missing", as he put it, referring to the inhabitants of a world where he forgot to include any. Since fingles do not exist on Earth, it is implied that Earth is the planet the Creator is referring to, and humanity is the species that is fundamentally incomplete because of their absence.