A lunar day and a lunar night each last two Earth weeks. What’s more, seconds tick by slightly faster than they do on our home world.

And as we continue to explore space by planning missions to the moon and beyond, time becomes critical for both knowing when to go to bed, and where to go to bed. Let me explain. 

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.

But in space, time moves differently. 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.

Precise navigation is increasingly important on the Moon, as astronauts from various countries and private companies seek to get back up there. Spacecraft landings must be precisely timed, to avoid dangerous terrain but also to avoid interfering with eventual lunar infrastructure. Astronauts need to pinpoint exact locations—maybe of an interesting rock, maybe of the right place to look for water in what will be the most futuristic version of X marking the spot.

To fix all this, scientists at the National Institutes of Standards and Technology recently devised a new coordinated Moon time, which will serve as a standard reference for any location on the Moon. Moon time can be checked against a standard Coordinated Lunar Time, the way we use Coordinated Universal Time (formerly called Greenwich Mean Time) to figure out our time zones.

“It’s like having the entire Moon synchronized to one ‘time zone’ adjusted for the Moon’s gravity, rather than having clocks gradually drift out of sync with Earth’s time,” NIST physicist Bijunath Patla said in a statement earlier this month.

The physicists say their efforts to develop Coordinated Lunar Time could be applied to other places in the solar system, simply by adapting the clock system for any other world’s gravity. 

It’s about time.