From 1d4chan
You can see how it earns its name.

An Orrery is a complex, clockwork-based model used to show the differing motions and states of celestial bodies, from as simple as the earth/sun/moon trinity to as complex as a whole solar system. It is also the common fan-nickname for the unique multiverse created for the Dungeons & Dragons setting of Eberron, which has never had an "official" name like its relatives.

Origins of the Orrery

As part of creating Eberron, to reinforce his desires for a unique setting that consciously abandoned many of the sacred cows of D&D, such as the extremely alignment-focused Great Wheel and the prescence of active deities, Keith Baker created his own cosmology. Because the planes of it are considered synonymous with the moons orbiting the planet of Eberron, allowing it to frequently illustrate its complexities through the use of an orrery design, fans went on to nickname it "The Orrery".

In some ways, it can be considered as a prototype for the World Axis; interestingly, it did not change greatly when 4th edition came out; the Astral Sea was considered synonymous with the dragon-god Siberys and the Elemental Chaos was considered synonymous with the dragon-god Khyber, but the actual planes of the Orrery themselves maintained all of their standard traits from before, as the World Axis also used the "planes within planes, like planets in space" model.

Planar Mechanics

Eberron spins within its own Material Plane, enfolded in three coexistent transitive planes: the Astral Plane, the Ethereal Plane and the Plane of Shadow. Within Eberron’s Astral plane, thirteen planes revolve in a complex orbit around the material plane. These planes are a combination of the features of Inner planes and Outer planes: Some have an elemental nature, some have alignment tendencies, and others are simply alien worlds. These planes are home to all the extraplanar creatures. None of these planes are the home of Deities and only one (Dolurrh) is a plane where mortal spirits go upon death. The thirteen planes are separate from each other, with no connections between them. They are coexistent with the astral plane, but separate from the Ethereal and Shadow planes, so certain spells are not available to casters on these planes. Each of the outer planes occasionally becomes coterminous with the Material Plane, allowing connections between the planes.

As the thirteen planes move through the Astral Plane, their paths take them closer to the material plane at times and farther away at other times. A plane's distance to the Material is described in one of three ways:

Coterminous: On rare occasions, a plane so near the Material Plane that it actually touches it. At these times it is possible to move freely between the Material Plane and the coterminous plane at certain locations on each plane. For example, when Fernia is coterminous, a character can travel there by means of a volcano or any extremely hot fire – and the native of Fernia can enter Eberron just as easily. Different planes’ paths through the Astral Plane bring them into a coterminous position with varying frequency: Fernia draws near fairly often (one month every five years), while Xoriat has not done so in over 7000 years. When a plane is coterminous, certain of its traits may bleed over into the Material, or at least certain areas of the Material. For example, when Risia is coterminous, spells with the cold descriptor are enhanced in certain areas. Not all planes have traits that spill over into the Material.

Waxing/Waning: As a plane draws close to becoming coterminous it is called waxing, and as it moves away it is waning. No special effects on the Material plane occur when a plane is waxing or waning.

Remote: When a plane is remote, it is difficult to establish any connection at all between it and the Material plane. When Fernia is remote, fire spells work less effectively, fire elementals are harder to summon, and it is nearly impossible to reach that plane via plane shift. Using plane shift to travel to a remote plane or using any spell to call or summon a native of a remote plane, requires a Spellcraft check (DC30 + Spell lvl). A character with 5+ ranks in Knowledge (The Planes) gains a +2 bonus on this check.

Certain locations on, within or above Eberron share a particularly close relationship to one of the outer planes. These locations are known as Manifest Zones. For example, the Gloaming, a region in the Eldeen Reaches, has a close connection with Mabar, the plane of Endless Night. This connection enhances the power of Negative Energy in that region. The Shadow Marches have a number of small magic zones tied to Xoriat, the realm of Madness, which both enhance transmutation spells and occasionally trigger Wild Magic effects. Parts of Aerenal have close ties to either irian and Mabar, which accounts (at least in part) for the prominent role of necromantic magic and the Undying Court on the Elven continent. To an extent, manifest zones defy the normal interrelationship among the planes. The effects of a zone may wane slightly when its connected plane is remote and wax equally slightly when it is coterminous but the connection never vanishes entirely. Manifest zones sometimes allow passage when another plane becomes coterminous with the Material. Specific, important manifest zones are permanent and, in most cases, have a profound effect on the land around them and the people that live there. Smaller and less important manifest zones are located across Eberron, and it is possible that new ones appear even as old one wink out of existence. The appearance and disappearance of manifest zones has no relationship to the linked planes cosmological position relative to the Material plane. No manifest zones linked to Dal Quor exists. Minor zones and occasionally major zones linked to every other plane exist somewhere on Eberron.

When the Fifth Edition launched and Eberron had its own sourcebook, the section on planar mechanics specifically pointed out that the function of the Orrery specifically prevents classical Great Wheel influence on Eberron. It names Asmodeus, for example, as a Fiend Outsider who would have no presence in the Eberron planes, despite being an overwhelming presence in most others. However, it also says that it is theoretically possible for the protections afforded Eberron by the Orrery to be potentially overcome if the DM decides to do so.

Planes of the Orrery

An alternative map of the Orrery, based on the World Axis model.