Essential Math

Here I will provide what I think are some of the most essential things from geometry needed for understanding the material on this site. If you are interested in proofs, the following site is an interactive recreation of Euclid's Elements.

Euclid's Elements -- Another creation of David Joyce, (see Trig Essentials), and highly recommended.

Equal Angles

Often in astronomy one is dealing with angles.  It's important to be able to recognize that if you know one angle, you often know many other angles too.  Below are two parallel lines with a third line intersecting the first two.  All of the angles highlighted in the same color are equal to each other.  The equal angles are shown in pairs because it is often easiest to remember them this way.

So, for example, if one can measure the angle A, then one knows every other angle shown:

A = B = C = D = E = F

G = 180 - A

G = H

Etc.

Below is an interactive diagram.