... In fact, all navigation is really about time. We invented longitude for this reason.
To find out where you are, you need latitude and longitude. Latitude is easy: You can measure your location in relation to the equator or the poles—how far north or south you are—using the stars or Earth’s own magnetic field. But longitude—how far east or west you are—is essentially made up. First, we needed to invent a prime meridian, an imaginary line drawn from pole to pole along the planet’s surface. Then, for navigation, you need to figure out where you are relative to that fixed line. The simplest way to do this is to use a time scale that is the same at both locations. So longitude is really about clocks.
Timekeeping, the way we do it now, is as much about making sure you show up at the right place as the right time.
This is also true for GPS. Every satellite in a GPS constellation carries a super-precise atomic clock. These clocks measure seconds not according to the arc of the Sun across the sky, as our ancestors did, but according to the movement of an atom, usually of the element cesium, which wiggles at a reliable rate. Each satellite’s internal clock of wiggling atoms is synced to a standard time reference. GPS satellites bounce signals to their receivers—say, your smartphone—and during that process measure the time, or the amount of atomic wiggling, it takes for the signals to reach the receivers.
Time dilates, or slows, at the satellites because they experience lower gravity than the receivers on Earth, and because they’re speeding around us at hundreds of miles per hour (remember that a moving clock ticks more slowly than one that’s stationary). Clocks on the satellites fall behind clocks on Earth by about 7 microseconds per day, but the receivers are programmed before launch to account for the tiny changes in time. Taken together, the set of GPS signals sent from the satellites and their built-in corrections can determine a receiver’s position and its local time.
But we can’t just port this system over to the Moon. Clocks on the lunar surface actually tick faster than Earth clocks by almost 58 microseconds per day. It’s not much in a given day, but over the months it will add up, and it’s enough to disrupt the precise timing of GPS.
