_T_ = ((α)/(ε) × (_S_)/(4σ))(^¼).
[Illustration: _Although the total surface area, A, of a sphere is
4πr², light rays from the sun only strike half the surface at any
one time. This area, a, is effectively equal to the sphere’s
cross-section, πr²._]
This equation is well known in astronomy, and has been used for more
than 80 years to calculate the temperatures of various objects in the
sky. Today, we still find it useful for measuring the surface
temperatures of man-made satellites such as Telstar. Since both _S_ and
σ are known constants (in this case, we use the quantities _S_ = 445 and
σ = 0.173 × 10⁻⁸), you can see that temperature is dependent on the α/ε
ratio.