PBS
Pbs Learning Media: Finding the Circle's Center
In this video segment [3:10] from Cyberchase, the CyberSquad must find the exact center of a circle in order to save Motherboard.
Wolfram Research
Wolfram Math World: Diameter
This site shows what a diameter is, how to find one on a circle or a sphere, and provides the formula to find diameter of a circle or "the great circle of a sphere.