Newton's description of gravity is sufficiently accurate for many practical purposes and is therefore widely used. Deviations from it are small when the dimensionless quantities ϕ ϕ / c 2 and ( v / c ) 2 are both much less than one, where ϕ ϕ is the gravitational potential, v is the velocity of the objects being studied, and c is the speed of light in vacuum. For example, Newtonian gravity provides an accurate description of the Earth/Sun system, since