The Schwarzschild metric is a static, spherically-symmetric solution of the Einstein vacuum equation R μν = 0, such as would apply exterior to a spherical, nonrotating mass 1 where the mass density is presumed small enough that T μν ≈ 0 (and Λ = 0). The Schwarzschild metric is one of a handful of exact solutions of the Einstein equation, and is undoubtedly the most important. There are small but measurable effects that occur in Schwarzschild spacetime, and such measurements constitute a crucial part of the extent to which GR has been quantitatively tested.