What are the constants A and B used to describe local Milky Way rotation called?

The constants A and B used to describe local Milky Way rotation are called the Oort constants. Jan Oort introduced them in the 1920s while studying stellar motions near the Sun.

They summarize the first-order behavior of Galactic rotation in the solar neighborhood. Constant A measures the local shear caused by changing orbital speed with Galactocentric radius, while constant B describes the local rotational or vorticity term. Together, they show that the Galaxy does not rotate as a solid body.

Astronomers determine the constants from the radial and transverse motions of nearby stars, along with their distances and Galactic longitudes. The measurements provide a local approximation rather than a complete rotation curve for the entire Milky Way.

The name can be confused with Hubble constants or relations such as Tully–Fisher, but those describe different problems. The Oort constants are specifically kinematic parameters for local Galactic rotation and can help estimate the Sun’s circular speed and constrain the Galaxy’s mass distribution.