 To calculate a dynamical parallax, the angular semi-major axis of the orbit of the stars is observed, as is their apparent brightness.By using Newton's generalisation of Kepler's Third Law, which states that the total mass of a binary system multiplied by the square of its orbital period is proportional to the cube of its semi-major axis, together with the mass-luminosity relation, the distance to the binary star can then be determined.With this technique, the masses of the two stars in a binary system are estimated, usually as the mass of the Sun.