How did medieval individuals tell time before the atomic clock, Greenwich Mean, or even dependable mechanical clocks? There were lots of ways, but one of the most scientifically sophisticated was the astrolabe, a hand-held adjustable device for determining time and meteorological details based on the positions of the stars. It was an important tool in the astrologer's kit (remember that astrology was a branch of science in medieval Europe), and Geoffrey Chaucer translated the first English treatise on how to use an astrolabe for his young son, Lewis.
Tom Wujec, in a short TED talk, walks you through not just how to use an astrolabe, but also what it can do that your wristwatch or iPhone can't. It's an amazing phenomenon that anyone even vaguely interested in the medieval past, history of science, or history of time should know about.