### Oort constants

(two constants characterizing local Milky-Way motion)

The Oort constants are two constants that characterize the general velocities of nearby stars. The two constants characterize the rotation curve of the solar neighborhood (local region of the Milky Way), useful for calculating distances and velocities of individual stars. They are derived from measured values of the angular velocities and distances to stars in the solar neighborhood, including a determination of the Sun's position and velocity. They can be used to estimate the position and velocity of stars in certain directions (not too close to or 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 modern determinations are:

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

(constant,Milky Way,stars,kinematics,rotation)