Oort constants

The Oort constants are two constants that characterize the general velocities of stars in our general neighborhood. The two constants characterize the rotation curve of the solar neighborhood (our local region of the Milky Way), useful for calculating distances and velocities of individual stars. The constants are derived from measured values of the proper motions and distances (via parallaxes) to stars in the solar neighborhood, along with a determination of the Sun's position and velocity. They can be used to estimate the position and velocity of stars in certain directions (neither very close to nor very far from the direction of the galactic center) and distances (not across to the far side of the galaxy), given some measurements. The two constants are known as A and B and a recent determination of the two is:

• A = 15.3 ± 0.4 km/s/kpc
• B = -11.9 ± 0.4 km/s/kpc

These values indicate a nearly flat rotation curve over the regime described above, implying "there must exist dark matter". Example formulae making use of them:

Vr = A d sin(2l)
Vt = A d cos(2l) + B d

• V_r - observed radial velocity of a star.
• V_t - observed transverse velocity of a star.
• d - distance to the star.
• l - angle of the star from a line to the galactic center.