Part of the "Megastructures: The Visual Encyclopedia" book project, on sale now at http://www.neilblevins.com/megastructures/megastructures.htm
A Hoopworld would be a structure that would place humans on the outside surface of a giant spinning torus, or donut. The hoop would be spun along the axis of revolution to produce gravity and stabilization, and to provide night and day to different areas on the torus. Centrifugal force would cause stronger gravity on the inner equator, since you’d be pulled toward the outside of the hoop in the direction of the ground. The outer equator would have less gravity, since you’d be pulled toward the outside of the hoop in the direction of the sky.
Tipping the torus toward or away from the sun would create seasons, affect weather, and could create a wide variety of climates over different parts of the torus’ surface.