Installation 04
From Halopedia, the Halo wiki
Summary
Installation 04 is the official name of the ring-shaped space station called Halo (or Alpha Halo in full) in Halo: Combat Evolved.
The Station
The name "Halo" is derived from Covenant battle transmissions, translated by Cortana. A more literal translations would be "Ring of Heaven", or "Ring of the Gods". Alpha Halo orbits a large gas giant (Earth Survey Catalogue Number B1008-AG, nick-named "Threshold"), suspended at the Lagrange point between the planet and it's largest moon, Basis. The ring has a 24-degree axial tilt from the gas giant's orbital plain, which means that any point on the surface receives sunlight for half of the time it is on the sunny side of Threshold, and for none of the time it is on the dark side. This gives the ring a somewhat unususal day-night cycle in which it is dark three-fouths of the time. There is of course, no sunrise or sunset in the traditional sense.
The Halos were built to study and contain the Flood organism, and to stop its spread. They are also massive weapons--they can destroy all life within a 25,000 lightyear radius. It is unknown how this weapon functions, and if it is even possible to survive within the blast radius. Halo is honeycombed with tunnles, caverns, rooms, facilities, canyons, ravines, and mountains. It was created with natural ecosystems, including small organisms, such as insects, and even small bird-like animals.
Halo is home to 343 Guilty Spark, an AI, and Monitor for Installion 04. He was somewhat misleading when he lead the Master Chief to activate Halo's "defenses" against the Flood and Covenant. If not for Cortana's intervention, the Chief would have inadvertantly triggered Halo's weapon. Halo was destroyed after the Pillar of Autumn's fusion core exploded. The pieces of the ring now drift in space, hurtling toward Basis and Threshold, being caught in their gravity wells.
Halo holds great religious significance for the Covenant. Soon after Halo's destruction, a Covenant flagship, with it's armada, exited Slipspace in the system. Cortana deciphered their transmissions, and learned from their data core that the "Guardian of the Lumious Key" was due to arrive there for some sort of ceremony. It is unknown why the Covenant held it in such high regard.
Astronomy
The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld. While the diameter of Niven's ringworld is close to the diameter of Earth's orbital of 300,000,000 km, the 10,000 km diameter of Halo is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our sun in its center, and Halo is orbiting a Planet, but does not encircle it, because it would be too small.
Besides the very beautifully sculpted landscape on Halo's surface, the sky is also worthy of attention. Depending on the viewpoint, you can see the star, a moon (called Basis), a gas giant (called Threshold), and the band of Halo itself, which looks like a street into the sky.
In the course of Halo 1, neither Threshold nor Basis is reachable, all action takes place on or in the Halo. In Halo 2 there is a campaign level on Threshold, and a multiplayer level on Basis.
A diagram showing the five Lagrangian points in a two-body system. Threshold is the yellow circle, Basis is the blue circle, and Alpha Halo is positioned at L1.
Halo is positioned on a Lagrange point of Threshold and Basis, directly between the two celestial bodies, but closer to Basis.
