Therefore, the Oort constants can tell us a great deal about motions in the galaxy.

Therefore, we can write the Oort constants and as:

So the Oort constants can tell us something about the mass density at a given radius in the disk.

The Oort constants can greatly enlighten one as to how the Galaxy rotates.

One of the major uses of the Oort constants is to calibrate the galactic rotation curve.

This is remarkably close to the actual measured Oort constants and tells us that the solar neighborhood is roughly rotating with the same linear velocity.

The flat rotation curve serves as an intermediate step between the two rotation curves, and in fact gives the most reasonable Oort constants as compared to current measurements.

The Oort constants (discovered by Jan Oort) and are empirically derived parameters that characterize the local rotational properties of our galaxy, the Milky Way, in the following manner:

The non-circular velocities will introduce scatter around this line, but with a large enough sample the true function can be fit for and the values of the Oort constants measured, as shown in figure 2.

The Hipparcos satellite, launched in 1989, was the first space-based astrometric mission, and its precise measurements of parallax and proper motion have enabled much better measurements of the Oort constants.

The Gaia spacecraft, planned for launch in 2012, is an updated successor to Hipparcos; when it comes online the quality of available data should allow new levels of accuracy in measuring the Oort constants.

Thus, the Oort constants can be expressed in terms of the radial and transverse velocities, distances, and galactic longitudes of objects in our Galaxy - all of which are, in principle, observable quantities.