A so-called Moiré pattern occurs when two similar repeating patterns are almost but not quite superimposed. The Moiré pattern is a kind of interference pattern. In this case, the basic pattern consists of lines radiating out from a common center. One copy of this pattern is fixed, and the other drifts about, creating a changing interference pattern.
You can start and restart the applet by shift-clicking on the pattern. You can also click-and-drag to control the motion of the pattern yourself.
Click here to see the source. This applet is configurable with a number of parameters. See the source for details.
You can find more of my Java work at http://math.hws.edu/xJava/.