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 |
By subtraction |

But <ADC - <BCD = <ADB (which is the central angle) |
By substitution |

So A - B = <ADB |
What this means is that the central angle) = A (sun angle
#1) - B (sun angle #2) by substitution. |