I
In geometry, the set of all points in three-dimensional space lying the same distance (the radius) from a given point (the centre), or the result of rotating a circle about one of its diameters.
The components and properties of a sphere are analogous to those of a circle. A diameter is any line segment connecting two points of a sphere and passing through its centre. The circumference is the length of any great circle, the intersection of the sphere with any plane passing through its centre. A meridian is any great circle passing through a point designated a pole. A geodesic, the shortest distance between any two points on a sphere, is an arc of the great circle through the two points. The formula for determining a sphere's surface area is 4π r 2 ; its volume is determined by (43)π r 3 . The study of spheres is basic to terrestrial geography and is one of the principal areas of Euclidean geometry and elliptic geometry .
II
[c mediumvioletred] (as used in expressions)
celestial sphere
{{link=sphere of influence">sphere of influence