The area (literal or figurative) influenced by a country, person, etc.

An instrument consisting of graduated metal circles used to represent the motions of celestial bodies around the earth.

A hypothetical system of orbiting solar power satellites meant to completely encompass a star and capture its entire energy output. / (science fiction) A solid shell of matter enclosing a star.

(astronomy) An abstract sphere of infinite radius which serves as the imaginary backdrop for celestial objects, and of which the visible sky is one hemisphere.

(astronomy, geography) The celestial or terrestrial sphere when its axis is oblique to the horizon of the place; or as it appears to an observer at any point on the earth except the poles and the equator.

(astronomy) The region in which a given astronomical body dominates the attraction of satellites.

(of a black hole) A region of around a black hole where photons travel in orbits around the singularity, just beyond the event horizon.

(mathematics) A sphere with a radius of 1 unit.

(topology, complex analysis) The complex numbers extended with the number ∞; the complex plane (representation of the complex numbers as a Euclidean plane) extended with a single idealised point at infinity and consequently homeomorphic to a sphere in 3-dimensional Euclidean space.

A manifold (of any dimension) homeomorphic to a sphere but not diffeomorphic to the standard sphere.