Installation 04
From Halopedia, the Halo wiki
Summary
Installation 04 is the official name of the fictional ring-shaped space station called Halo (or Alpha Halo in full) in Halo: Combat Evolved.
The Station
Halo is one of seven giant space stations built by The Forerunner. It is first and foremost a weapon, intended to destroy the parasitic Flood threat by wiping out all possible hosts, i.e. all intelligent life in its range. While that range is "only" approximately 25,000 light years, the activation is thought to trigger the other six Halo stations, extending the range to the whole Milky Way. Ironically, the Flood is apparently still alive only on those space stations. Aside from the Flood the only things "alive" on Installation 04 are its "Monitor", 343 Guilty Spark, and his Sentinels
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.
http://upload.wikimedia.org/wikipedia/en/4/4d/Lagrangepoint1.png
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.
