help me my troglodyte is winning
I have to be quick, work is calling.
I need to mark the outer perimeter of a 15" circle at three even places. I know that means 120* apart. I know where the center is, but, I do not have a protractor. I vaguely remember from Geometry class, 2 lifetimes ago, about using a ruler and measurements to get my point markers. I can always use trial and error. I was hoping someone might remember the way to get 120* intervals with the knowledge of Radius, or Diameter and thus save me an hour or two of measuring and marking and erasing and doing it all over again. I do not have a way to measure the perimeter accurately.( steel rulers won't bend the arc) Oh yeah all i have are steel rulers and straight edges. I may be able to find my wifes sewing tape measure. It is flexible, made of cloth and all. I will check back later and see what you fine Gentlemen have come up with. I may have to buy more tools. did I tell you my tool budget is already shot. Thank You, in advance, for all the help I have received. Schultz. 
I'd just take something like this, print it out, and use it as a template with your straight edge to mark the 120°.
http://www.smartbridges.com/new_images/image035a.gif A larger image would be better, but this will do if your not looking for incredible accuracy. 
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. 
lets call the three dots on the circle A, B, and C. If they are to be evenly spaced, an imaginary straight line would be the exact same length between AB, BC, CD, DA. Kind of trial and error, but easiest way since you only have steel rulers, also the way with the least amount of math involved.

Too late probably, but the chord of each segment of a circle divided in three is radius * (3)^1/2. Then, take that value and find it on your ruler. Draw a dot on the circle, put the 0 mark of the ruler on the dot, and swing the ruler until the value touches the circle. Mark that point and repeat the process on the new dot until all three points are drawn. Draw lines from the center to each point and bam, done.
 LostCause 
Thank You
Good suggestions all.
Thanx BoxChains link led me to the Archimedes Method which I believe is what I vaguely remember from some classroom in the seventies. TomO, that is the method I was planning on using. Trial and error. Measure, mark, rinse, repeat. LostCause if that is true then that will work. I am trying to make three holes around the perimeter of the new wheel covers. I want them as close to perfect as possible. So. I can transfer wheel to wheel without always having to line up the holes. Well off to the races. Thanx Again, S. 
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