Installation 04

Installation 04 is the official name of the fictional ring-shaped space station called Halo in Halo: Combat Evolved.

The station
Halo is one of six giant space stations built by the Forerunners. It is first and foremost a weapon, intended to destroy the 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 five 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 Flodd the only thing alive on Installation 04 is its "Monitor", 343 Guilty Spark.

Astronomy
The "Ringworld" of Halo is much smaller than Larry Niven's ringworld. While the diameter of Niven's ringworld is much closer 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, with a diameter of 26,000 km), a gas giant (called Threshold, its diameter is 175,000 km), and the band of Halo itself, which looks like a street into the sky.

The apparent size of Basis and Threshold is unfortunately depicted uncorrectly in the game. In the course of the game, neither Threshold nor Basis is reachable, all action takes place on or in Halo.

Halo is positioned on the Lagrange point L1 of Threshold and Basis, directly between the two celestial bodies, but closer to Basis. On Lagrange point L1 the gravity of Basis and Threshold, as well as the centrifugal force of the three objects, nullify ech other exactly. The rotational axis of Halo is near the line between Threshold and Basis, so that, when viewed from Halo, the moon and the Planet are alsways opposite to each other, and both are about 90 degrees left or right of the ring of Halo. All three bodies rotate with the same angular speed around their common center of gravity, which is inside of Threshold, but not exactly in the center of gravity of the planet. The common center of gravity rotates around the star.

Contrary to the lagrange points L4 and L5, the lagrange points L1, L2 and L3 are not stable, similar to a sphere in the center of a saddle-shaped plane. This indicates that Halo would have to stabilize its position actively, if it existed in the real world, as not to fall onto either Threshold or Basis. The Master Chief should have asked the Monitor of Installation 04 why the Forerunners did not build their fortress world on the L4 or the L5 position, instead of relying on their still-running energy generators. One possible answer would be that Halo was intended to be destroyed alongside the Flood spores in the impact, should its generators malfunction.