The EJS Spiral Nebula Rotation model illustrates a simplified version of Adrian van Maanen's method for measuring the rotations (or internal motions) of spiral nebulae. Van Maanen used a device called a "blink stereocomparator" that allowed him to rapidly switch between viewing one image of a spiral nebula and another image of the same nebula taken at a later date. After aligning the foreground stars (which were not part of the nebula) in the two images, van Maanen made measurements of the displacements of various points in the nebula from one image to the other.

The simulation provides two images of the spiral nebula Messier 101, derived from a Hubble Space Telescope image. The user can "blink" back and forth between these two images. Foreground "stars" (red dots) have been superimposed on the two images. The user must first align the foreground stars by rotating the second image of the nebula. Then the displacements and distances from center of various points in the nebula can be measured using the draggable arrows in the simulation, and these values can be used to determine the angle by which the nebula has rotated in the time interval between the two images.

It should be emphasized that the rotation effect illustrated in this simulation is NOT REAL. Although spiral galaxies do rotate, the angle by which they rotate during a human lifetime is so small as to be undetectable. The purpose of this simulation is simply to illustrate van Maanen's method, because van Maanen's (spurious) results were historically important in that they led several astronomers to reject the idea that spiral nebulae are independent galaxies (at least for a while).

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