Oort constants

The Oort constants (discovered by Jan Oort) and are empirically derived parameters that characterize the local rotational properties of our galaxy, the Milky Way, in the following manner:

where and are the rotational velocity and distance to the Galactic center, respectively, measured at the position of the Sun. As derived below, they depend only on the motions and positions of stars in the solar neighborhood. As of 1997, the most accurate values of these constants are = 14.82 ± 0.84 km s kpc and = -12.37 ± 0.64 km s kpc. From the Oort constants, it is possible to determine the orbital properties of the Sun, such as the orbital velocity and period, and infer local properties of the Galactic disk, such as the mass density and how the rotational velocity changes as a function of radius from the Galactic center.