The Summer Triangle and Celestial Navigation

The Summer Triangle (Vega, Deneb, Altair) dominates the northern sky in August. Its coordinates (right ascension, declination) are measured in hours and degrees. Celestial navigation uses spherical trigonometry: the altitude of a star gives a circle of position on Earth. By measuring two stars, you find two intersecting circles – usually two possible positions; a third resolves the ambiguity. This is how sailors found longitude before GPS. The mathematics is the same as that used for satellite positioning, but with stars instead of satellites.