Milky Way galaxy - Astrography: The vast spiral of the galaxy stretches across over 100,000 light years and swells to 12,000 light years thick at its hub. It contains hundreds of billions of stars and untold millions of habitable planets. Through long ages the galaxy has seen successive intelligent civilisations rise and fall without end. Some never reached other stars and died stillborn on their birth worlds, while others rose to dominate vast swathes of the galaxy before collapsing back into dissolution and anarchy.The first phase of Human expansion across the galaxy is lost in the distant past, obliterated by twenty thousand Terran years of subsequent regression and rebuilding. In the current era, the Age of the Imperium, the Imperium of Man is only the latest ascendant to mount a throne set upon the ruins of earlier, greater ages.The Imperium is mighty, centred around the high concentration of Human worlds in the galactic west and alleging a divine right to the fealty of all Mankind through its control of the cradle of Humanity itself: sacred Terra. A million worlds fall beneath the dominion of the divine Emperor of Mankind; it controls countless armies billions strong and fleets of starships beyond number.Yet for all the Imperium's tremendous strength its million worlds are but pinpricks of light and order in the midst of the howling darkness of the galaxy. For every world that feels the autocratic boot of the adepts of Terra upon its throat, a thousand others lie undiscovered in the void. There are alien empires thriving and multiplying in the outer reaches as well as fiercely independent Human domains that refuse to acknowledge the Emperor's laws.Beyond the reach of the zealots of the Imperial Creed it is also possible to find the signs of eldritch powers that pre-date the Age of the Imperium. Some say that the seeds of Humankind's salvation can be found there, the strength to win freedom from the Emperor on Terra and wreak terrible vengeance for the broken promises of His Great Crusade. Outside the feeble light cast by the Emperor's domains there are many who remember the old ways of worship and sacrifice, and many that still know of the power of the Dark Gods who rule the Warp.The Imperium of Man is currently the dominant power in the Milky Way galaxy, ruling over more than 1,000,000 settled Human worlds that stretch across the entire length and breadth of the galaxy. To effectively govern and protect this vast domain, the High Lords of Terra and the Adeptus Terra have divided the galaxy into five zones of military and political control known in High Gothic as the Segmentae Majoris (sing. segmentum).Each segmentum has its own Navis Imperialis battlefleet and Merchant Fleet, as well as a Segmentum Fortress or Segmentum Naval Base that forms the base of fleet and military operations for the Imperial Navy and Astra Militarum, respectively, within the segmentum. The segmentum battlefleet commanders are known as the Lord High Admirals. The Segmentum Fortress is controlled directly by a high-ranking official of the Adeptus Administratum referred to as the "Master of the Segmentum."Although originally intended for purposes of fleet administration and commercial shipping administration at the time of their creation by the Adeptus Terra, the segmentae have evolved into the primary political administrative divisions of the Imperium, with each one being further comprised of thousands of 200 light-year-square sectors.The sectors are in turn made up of sub-sectors each comprising a cube of space some 10 to 20 light years square. In general, despite actually being a three-dimensional space, Imperial astrocartographers refer to any location in the galaxy in reference to its cardinal direction in a two-dimensional plane from Terra. For example, the "Eastern Fringes" of the galaxy beyond the Ultima Segmentum is located to the galactic "east" of Terra and the Eye of Terror lies in the galactic "north-west."