text: Thus only the product of G and M is needed to predict the motion of the smaller body. Conversely, measurements of the smaller body's orbit only provide information on the product, μ, not G and M separately. The gravitational constant, G, is difficult to measure with high accuracy, while orbits, at least in the solar system, can be measured with great precision and used to determine μ with similar precision.
  For a circular orbit around a central body, where the centripetal force provided by gravity is :
  where r is the orbit radius, v is the orbital speed, ω is the angular speed, and T is the orbital period.
  This can be generalized for elliptic orbits:
  where a is the semi-major axis, which is Kepler's third law.
  For parabolic trajectories rv2 is constant and equal to 2μ. For elliptic and hyperbolic orbits magnitude of μ = 2 times the magnitude of a  times the magnitude of ε, where a is the semi-major axis and ε is the specific orbital energy.
  General case
  In the more general case where the bodies need not be a large one and a small one, e.g. a binary star system, we define:
  the vector r is the position of one body relative to the other