Two parameters defined by J. H. Oort to describe the major features of our Galaxy's differential rotation in the Sun's neighbourhood. They are usually expressed in units of kilometres per second per kiloparsec. The two parameters are given the symbols A and B. Values derived from Hipparcos proper motions are A = 14.82 km/s/kpc and B = −12.37 km/s/kpc. Subtracting B from A gives the angular velocity of the local standard of rest around the centre of the Galaxy, which corresponds to a period of about 200 million years.
Subjects: Astronomy and Astrophysics.