When a map on a phone shows a small blue dot and says "you are here," it is not guessing. It is doing rapid calculations using signals from space.

High above Earth, more than 30 satellites circle the planet as part of a system called GPS, short for the Global Positioning System. They orbit about 20,000 kilometres up. Each satellite carries an extremely accurate atomic clock and constantly broadcasts a message giving its exact position and the exact time the message was sent.

Your phone listens for these messages. Because the signals travel at the speed of light, the phone can work out how far away each satellite is by measuring how long its message took to arrive. Knowing the distance to one satellite places you somewhere on a huge sphere around it. With three satellites, those spheres overlap at a single point: your location. This method is called trilateration. A fourth satellite is used to correct the phone's own clock.

There is a surprising twist. Because the satellites move quickly and sit far from Earth's gravity, their clocks tick very slightly faster than clocks on the ground, exactly as Einstein's theories predict. If engineers did not correct for this tiny difference, GPS locations would drift by several kilometres every single day.

So the next time a map guides you around a corner, remember that your position is being fixed by a fleet of atomic clocks racing through space, and by a scientist's ideas about time itself.