The EJS Differential Galactic Rotation Model illustrates the model of galactic rotation proposed by Jan Oort in 1927.  In Oort's model most of the mass of the galaxy is concentrated at the center.  Stars move in circular orbits around the center, with orbital speeds that are inversely proportional to the square root of their distance from the center.  Oort realized that this model produced a specific pattern of radial velocities in the stars near the sun, and that this pattern matched the observational data fairly well.  The simulation is intended to illustrate both Oort's model and the resulting pattern in the radial velocities.  The simulation can also depict the so-called "high-velocity" stars, which are stars that have very low orbital speeds (treated as zero in the simulation) around the galactic center and therefore have high speeds relative to the sun.

One window depicts stars (including the sun) orbiting counterclockwise around the galactic center as seen from above the galactic North pole.  The velocity vectors of the stars can be displayed, and stars near the sun can be highlighted.  Another window shows the highlighted stars (and, optionally, the high-velocity stars) in the region near the sun.  The radial velocity and relative velocity vectors for these stars can be displayed.  Finally, a plot of radial velocity versus galactic longitude (using either the modern longitude system in which the galactic center is at longitude 0, or the pre-1958 system with the galactic center at longitude 325 degrees) can be shown.  The plot can display data for the highlighted stars, the high-velocity stars, and two sets of Cepheid variables studied by Alfred Joy in 1939.  Using the Cepheid period-luminosity relation Joy found distances to these stars that fit with the distances derived from Oort's rotational model, provided that the apparent magnitudes of the stars were corrected for interstellar absorption.

Please note that this resource requires at least version 1.5 of Java (JRE).