How to Calculate the Phase of the Moon
a One of the sides of the triangle has the length equal to the distance between the Moon and the Earth. The last side of this triangle will change in length depending on the angles between the Earth, Moon, and Sun. Now that we know the length of c, we can calculate š¯˛±, the angle between the Earth and the Sun from the Moonā€™s point of view.
Source: witchy.co