For thousands of years, travelers venturing out beyond the realm of identifiable landmarks have called upon the assistance of celestial bodies to help them find their way. By measuring the angles between their visual horizon and those celestial bodies above, sailors, landgoers and, eventually, pilots could compare those figures to the known location of those stars, planets or the moon at a given time, and thus deduce their location on Earth. The method relied on spherical trigonometry and astronomical know-how, but demanded little in terms of physical equipment, requiring, at the very least, the help of a sextant, an almanac and a timepiece. While celestial navigation left plenty of room for human error, it became a more accurate practice over time, helping pilots take aviation to new heights during everything from the dawn of aviation to the Gemini space missions. Lest you think celestial nav is an antique discipline, you should be aware that the United States Air Force Academy conducted courses in celestial navigation well into the 1990s, though graduating pilots were no longer given a sextant to carry in their flight kit.