A sphere is the set of all points in three-dimensional space at a fixed distance (radius) from a center point.
A sphere is the 3D analogue of a circle. Its surface area is 4πr² and its volume is 4πr³/3. A hemisphere is half a sphere. The Earth is approximately a sphere (oblate spheroid). Spheres are used in astronomy, ball bearings, optics (lenses), and sports ball design. Geodesics on a sphere are great circles.
Spheres were studied by ancient Greek mathematicians. Archimedes proved that the volume of a sphere is 2/3 of the circumscribed cylinder's volume. He considered this his greatest achievement and requested a sphere and cylinder engraved on his tomb. The word "sphere" comes from Greek "sphaira" meaning "ball."