It's a 15" diameter but you don't know the diameter or radius?
If you have a piece of string, you can run it along the circumference, mark the length, fold that into thirds, and retrace.
You can try this search:
http://www.google.com/search?q=geometry+trisect+circle and try 180 or 90 for your angle. Once you get a 30 or 60, you'll be good with the steel ruler.