26 Aug
2007
26 Aug
'07
9:23 p.m.
Hi Tony, x is called the sagitta. At least when making a telescope mirror it is[1]. By bisecting your angle with another radius, you'll see that you have a right triangle with hypotenuse a, and legs of length (b/2) and (a-x). Then sagitta a b = a - sqrt (a*a - b*b/4) Considered as a function of half the angle subtended by the points on the circumference, this is called the versine. http://en.wikipedia.org/wiki/Versine [1] http://www.stellafane.com/atm/atm_grind/atm_measure_sag.htm