Finding the center of a circle on the other hand wasn't quite as intuitive to me, and until doing some searching of my own, I didn't know that there was a simple and easy trick. Time to share.
Similar circle finding Instructables can be found here:
Step 1: Draw Some Chords
They are easy to draw...simply take a ruler, place it down on the edge of your circle so that it crosses the outer edge in two places, and use a pencil to mark a line. You've just created a chord.
Technically to find the center of a circle you only need one perfectly drawn chord but since people aren't machines and there's some user error in the process, draw a couple so that you can average the results.
I've drawn five chords near the perimeter of the circle in the photo below. Disregard the lines pointing in towards for the center for the time being.
Step 2: Mark the Centers and Draw a Perpendicular Line
Mark that point.
Then, using a square, draw a line that is exactly 90 degrees to the chord pointing towards the center of your circle. Make it a little longer then where you think the center of the circle resides.
Do this for all of your chords.
Step 3: The Center Is the Point Where They Intersect
This means that the system works...ain't geometry cool?