Celestial navigation is the practice of navigating from one place to another using celestial bodies as a guide. Prior to the invention of GPS, people would use the position of the stars in relation to themselves as a method of orienting oneself to their destination. The process is simple in concept – the navigator uses the angle created between the chosen celestial body (or bodies) and the visible horizon to narrow down the amount of possible places on Earth they could be. In modern times, this process is completed using a sextant, which is a tool to looked into by the navigator that can help measure the angle between the celestial body and the horizon. For each celestial body viewed at a certain time, the possible positions of the observer can be narrowed down into a large circle on Earth. This process works because every celestial body is directly above exactly one spot on Earth. To view that celestial body from directly below (90 degrees), you would have to be in exactly one spot. But to view that celestial body from a slight angle (perhaps 85 degrees), the observer would have to move a specific distance away from the 90 degree spot in ANY direction. This is what creates the circles used in the positional calculation! In order to improve accuracy, celestial navigators often consider multiple celestial bodies, each projecting their own large “possibility” circle onto Earth. Where multiple of these circles meet corresponds to the position of the observer! To be sufficiently accurate, most navigators use three to five celestial bodies, as using just one would only deduce the large circle, and using two would create multiple intersections between the circles.
Video on Celestial Navigation Math
Wikipedia article on Celestial Navigation
Bobby's Solar System Blog 