Your phone has never seen the sky. So how does it find you?

How GPS finds you, in 33 seconds. Your phone never talks to the satellites. It only listens. Each GPS satellite, about 20,000 km up, keeps sending the time; the delay in a signal's arrival is that satellite's distance. Two satellites give two possible places, three give one: you. But the phone's own clock is cheap, and a millionth of a second of error puts you about 300 metres off, on the wrong street. A fourth satellite corrects the clock. Where you are is a question of when. Sources: gps.gov (the satellites orbit about 20,200 km up); Scientific American, "How do GPS devices work?" (four satellites: three for position, one for the clock).