A sphere is, roughly speaking, a ball-shaped object. In mathematics, a sphere is a quadric consisting only of a surface and is therefore hollow. In non-mathematical usage a sphere is often considered to be solid; mathematicians call this the interior of the sphere.

More precisely, a sphere is the set of points in 3-dimensional Euclidean space which are at distance r from a fixed point of that space, where r is a positive real number called the radius of the sphere.

In coordinate geometry a sphere with centre (x₀, y₀, z₀) and radius r is the set of all points (x,y,z) such that

(x - x₀)² + (y - y₀)² + (z - z₀)² = r²

x = r cos(φ) sin(θ)

y = r sin(φ) sin(θ)       (0 ≤ θ < π and -π < φ ≤ π)

z = r cos(θ)

A sphere of any radius centered at the origin is described by the following differential equation:

The circumscribed cylinder for a given sphere has a volume which is 3/2 times the volume of the sphere, and also a surface area which is 3/2 times the surface area of the sphere. This fact, along with the volume and surface formulas given above, was already known to Archimedes.

A sphere can also be defined as the surface formed by rotating a circle about its diameter. If the circle is replaced by an ellipse, the shape becomes a spheroid.

Spheres can be generalized to other dimensions. For any natural number n, an n-sphere is the set of points in (n+1)-dimensional Euclidean space which are at distance r from a fixed point of that space, where r is, as before, a positive real number. A 2-sphere is therefore an ordinary sphere, while a 1-sphere is a circle and a 0-sphere is a pair of points. The n-sphere of unit radius centered at the origin is denoted Sⁿ and is often referred to as "the" n-sphere.

An n-sphere is an example of a compact n-manifold.

See also

• Riemann sphere
• 3-sphere, hypersphere
• ball
• torus

Sphere Books was a British paperback publisher of the 1960s - 1980s.