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Oort constants

(two constants characterizing local Milky-Way motion)

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. 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 (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:

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

(constant,Milky Way,stars,kinematics,rotation)
Further reading:
https://en.wikipedia.org/wiki/Oort_constants
https://www.oxfordreference.com/display/10.1093/oi/authority.20110803100250872
https://www.astro.umd.edu/~richard/ASTRO421/A421_Dynamics_lec11.pdf
http://icc.dur.ac.uk/~tt/Lectures/Galaxies/TeX/lec/node42.html
https://ui.adsabs.harvard.edu/abs/1927BAN.....3..275O/abstract
https://ui.adsabs.harvard.edu/abs/2017MNRAS.468L..63B/abstract
https://ui.adsabs.harvard.edu/abs/2019ApJ...872..205L/abstract

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