|The three yellow lines are parallel.
||That's because they represent the sun's rays which are
assumed to be parallel.
|Sun Angle A = <ADC
||If you have two parallel lines (the sun's rays) intersected
by a straight line, then alternate interior angles are equal. A and
F are such angles and they are equal.
|Sun Angle B = <BDC
||For the same reason as above.
|That means that A - B =
<ADC - <BCD
|But <ADC - <BCD = <ADB (which is the central angle)
|So A - B = <ADB
||What this means is that the central angle) = A (sun angle
#1) - B (sun angle #2) by substitution.