A
sphere (from
Greek —
sphaira, "globe, ball") is a perfectly round
geometrical object in
three-dimensional space that is the surface of a completely round
ball, (viz., analogous to a circular object in two dimensions). Like a
circle, which geometrically is a two-dimensional object, a sphere is defined mathematically as the
set of points that are all at the same distance from a given point, but in three-dimensional space. This distance is the
radius of the ball, and the given point is the
center of the mathematical ball. The longest straight line through the ball, connecting two points of the sphere, passes through the center and its length is thus twice the radius; it is a
diameter of the ball.
