Your phone uses GPS to find where you are. GPS satellites have very exact clocks. But clocks in space run a little faster than clocks on Earth. This is because of Einstein's theory of relativity.

Each day, the satellite clocks gain about 38 millionths of a second. That sounds very small, but without a fix, the position could be wrong by about 10 kilometers a day.

A smartphone finds its location using about 30 GPS satellites flying around 20,000 kilometers above Earth. Each satellite carries an atomic clock and sends signals showing the time they were sent. The phone calculates its distance to each satellite from how long the signals take to arrive. Because radio waves travel at the speed of light, even a tiny clock error causes a big distance error. According to Einstein, the satellites' high speed makes their clocks run about 7 microseconds slower each day.

However, weaker gravity in space makes them run about 45 microseconds faster. Together, the two effects make them gain about 38 microseconds per day. If this were not corrected, errors of about 10 kilometers would build up every day.

The map on your smartphone depends on Einstein's theories, though most users never think about it. GPS uses about 30 satellites orbiting roughly 20,000 kilometers up, each carrying an atomic clock. A receiver determines its distance to each satellite from the travel time of the signals, and because radio waves move at about 300,000 kilometers per second, an error of just one microsecond corresponds to about 300 meters.

Two relativistic effects act on the satellite clocks in opposite directions. Special relativity predicts that the satellites' speed of about 4 kilometers per second slows their clocks by around 7 microseconds per day, while general relativity predicts that the weaker gravity at their altitude speeds them up by about 45 microseconds. The net result is a gain of about 38 microseconds daily.

As physicist Neil Ashby explained in a 2002 article, ignoring these effects would cause navigation errors of more than 10 kilometers in a single day. Engineers therefore set the clocks to tick slightly slower before launch. Note, however, that the 10-kilometer figure expresses the size of the effect rather than a real risk, since ground stations continuously monitor the clocks and send corrections up to the satellites.