Segmentum
In Warhammer 40,000, the Imperium of Man has divided the Milky Way galaxy into five zones known as a Segmentum Majors (plural Segmenta Majoris). Originally designed as a means of fleet and shipping control, the Segmentae are administrative divisions of the Adeptus Terra, with each Segmentum ruled over by a Administratum adept, known as the Master of the Segmentum, while the Imperial Guard and Imperial Navy both have Lord Commanders overseeing their operations in those Segmenta. The Imperial Military machine is based at a Forge World in each Segmentum. Of course, interstellar communication being what it is, many of these Segmenta have a federated system where the individual sectors oversee their planets and report back to Segmentum command.
Segmentum Solar
The Imperium's creamy center, where the Sol System is located. As a result, the Segmentum is one of the most heavily industrialized and militarized, as the High Lords of Terra tend to start freaking out whenever a rebellion occurs in their own back yard.
It is approximately a 44,000 light year diameter circle centered on Sol System, and is approximately 1.52 billion square light years.
Planets:
- Sol Systems
- Armageddon
- Elysia
- Fenris
- Necromunda
Segmentum Tempestus
The galactic "south" (meaning south of Terra).
Planets:
Segmentum Pacificus
The galactic "west", and one of the least-explored regions of the galaxy. This part of the Imperium has regularly rebelled, starting with the Nova Terra Interregnum. There was also the smaller Sabbat Worlds Crusade, a major Chaos incursion that took lots of resources to quell. Much of the Segmentum was reconquered by Lord Commander Macharius in the Macharian Crusade. In 999.M41, mere months before the 13th Black Crusade, much of the Segmentum Pacificus went into open revolt against the Imperium in the "Night of a Thousand Rebellions".
It is approximately a quarter-torus 11,000 light years wide radially and with outer and inner arcs 52,000 light years and 35,000 light years long, respectively; it comprises roughly 475,000 square light years.
Planets:
- Sabbat Worlds
- Tanith
- Nova Terra
Segmentum Obscurus
The galactic "north", which has the misfortune of being the location of the Eye of Terror. As a result, the Segmentum Obscurus has one of the largest Segmentum fleets in the Imperium, in an effort to keep the Chaos Space Marines from breaking out. A large section of the Segmentum is "obscured" by the Eye of Terror, blocking the light of the Astronomicon, meaning navigation and communication do not function well (if at all) on that edge of the galaxy.
Planets:
Ultima Segmentum
The galactic "east" and largest of the Segmenta, clocking in at nearly a third of the galaxy. Includes the galactic core and the realm of Ultramar. The Segmentum marks the rough outer limits of the Astronomicon's influence, making any travel further treacherous for any Imperial. It is also bordered by a number of physical obstacles, such as the Maelstrom, the Tau Empire, and the Ork Empire of Octarius.
Planets: