In the Dungeons & Dragons roleplaying game, Oerth Script error: The module returned a function value. It is supposed to return an export table. is the name of the fictional planet on which one of the earliest campaign settings, the World of Greyhawk, is located. Oerth has an axial tilt of 30 degrees, which would cause greater seasonal temperature variation than on Earth, were it not for wizardly and divine magic that shifts weather patterns to be more favorable to the populace (at least the god-fearing folks).
Oerth is detailed in the 1980 version of the World of Greyhawk Fantasy Game Setting, which covers less than a quarter of the landmass of Oerth.
According to the paper by Gary Holian in Oerth Journal, Oerth's circumference is about 42,024 km, and its radius measures 6,714 km, about 1% more than that of Earth. There is no flattening of the planet sphere due to rotation, and divine power is again hinted at.
Oerth has at least four continents, the largest of which is Oerik, lying mainly in the Northern Hemisphere. Southeast of Oerik lies the much smaller landmass of Hepmonaland, lying in the tropics. Hyperboria, also called Telchuria (after Telchur, the god of winter), lies at the north pole, while an unnamed fourth continent lies in the southern hemisphere. Oerth is thought to have four oceans, including the Solnor Ocean, the Dramidj Ocean that circles Telchuria, the Ocean of Storms south of Oerik, and the Sea of Thunder encompassing Polaria.
Oerth also has a number of islands, the largest being Fireland in the northern Solnor, depicted on maps as being roughly half the size of Hepmonaland. Polaria is a chain of mountainous islands covered for at least part of the year by the southern polar ice cap.
The easternmost part of Oerik, the Flanaess, has received the lion's share of detail, though Hepmonaland has received significant attention. Western Oerik was the setting for TSR's original miniature line, Chainmail, but few regard the material written for that setting as canon, especially after the miniature line's cancellation in 2002.
Author Frank Mentzer, in a series of early modules, added a small territory called New Empyrea to the world across the Solnor Ocean from the Flanaess, but it has not been mentioned in subsequent Greyhawk products.
Telchuria is an ice-covered continent at the north pole of Oerth. Telchuria is sometimes referred to as "High Boros" or "Hyborea."
It is possible that, in the depth of winter, a natural ice bridge forms between Telchuria and the Land of Black Ice.
Hepmonaland is a tropical landmass to the southeast of Oerik. Hepmonaland is a very small continent compared to the other known continents.
Oerth in the "Solar" System
It takes exactly 24 hours for Oerth to make one revolution on its axis. Oerth lies at the center of Greyspace. Unlike most planets, Oerth does not orbit its sun, Liga. Instead, the Sun, and all other planets in the crystal sphere, orbit Oerth. Thus, Greyspace houses a geosystem, rather than a solar system. Oerth's "moons", Celene (Kule) and Luna (Raenei), are in fact the first and second planets in the geosystem, with Liga being the third. From Oerth, planets beyond Liga appear to the naked eye as wandering stars.
Liga orbits the Oerth once every 364 days, thus the Oerthly year is slightly shorter than that on Earth. Luna has a 28 day lunar cycle, while Celene's cycle is 91 days.
In a 1984 interview for Polyhedron Newszine, Gary Gygax revealed several "alternate Oerths" while explaining the setting for his HEROES CHALLENGE game books, co-written with author Flint Dille and published under the aegis of the Dungeons & Dragons Entertainment Corporation by the Wanderer Book division of Simon & Schuster.
"By the way, action takes place on Yarth, a place somewhat similar to Oerth, the setting of Greyhawk, et al. It has fewer magical properties than Oerth but more than Earth. It is not impossible that additional works will be contracted for in months to come, action being set on Yarth or perhaps another alternate world, Aerth. On Earth, magic is virtually non-existent. On Uerth, dweomers are weak, chancy things. Yarth has a sprinkling of things magical, and Oerth is pure magic."
This implies that the planets differ in both their geographies as well as the relative presence of magic; but also that they operate in divergent realities.
Other references to these alternate Oerths appear in the Gord the Rogue short story anthology Night Arrant as well as in Gygax's Epic of Aerth campaign setting for the Dangerous Journeys roleplaying game. The five parallel worlds of Oerth, Aerth, Uerth, Yarth and Earth were recently mentioned in the 3rd Edition adventure Expedition to the Ruins of Greyhawk and thus found their way into 3rd edition canon.
- Findley, Nigel. Greyspace (TSR, 1992).
- Gygax, Gary. World of Greyhawk Fantasy Game Setting (TSR, 1983).
- Holian, Gary. "Measuring up the Oerth." Oerth Journal, #4 (Council of Greyhawk, 1996). Available online: 
- Holian, Gary, Erik Mona, Sean K Reynolds, and Frederick Weining. Living Greyhawk Gazetteer (Wizards of the Coast, 2000).
- Mentzer, Frank. To the Aid of Falx, (TSR, 1982)
- Moore, Roger E. Greyhawk: The Adventure Begins (TSR, 1998).
- Williams, Skip, and David C Sutherland III. "Beyond the Flanaess." Dragon Annual #1.