I saw that there were a couple of airplanes flying around Ann Arbor Airport as I was heading home, so I decided to stop there again.
This is a rotating beacon light. It indicates the position of an airport for pilots flying at night. This particular beacon alternates between green and white which indicates that it is a land based airport with lights. It is very similar to the beacon of a lighthouse. This is not to be confused with an airway beacon which was an early way to indicate air corridors. The first lighted airport was in North Platte, Nebraska (at the time it was lighted with gas powered lights).
The first plane I got a picture of was a Delta 767 that was flying over head.
Next was the Cessna 152 operated by the Michigan Flyers. It was doing touch and go landings for a couple passes when I got there.
Soon it was joined by the Cessna 172 which is also operated by the Michigan Flyers.