The Oort constants describe local differential galactic rotation: and . Thus , the radial epicyclic frequency obeys , and the epicyclic ellipse has axis ratio .
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The Oort constants are a pair of values used in astrophysics to describe the rotation of the Milky Way galaxy. Named after the Dutch astronomer Jan Oort, these constants help characterize the distribution of orbital velocities of stars in the galaxy. Specifically, they refer to: 1. **Oort Constant A (A)**: This constant is related to the differential rotation of the galaxy. It indicates how the rotational velocity of stars varies with distance from the center of the galaxy.