If all goes well, we'll be sending astronauts  back to the Moon in just a couple years.

And scientists have a lot  to figure out before then, including the answer to a seemingly  simple question: what time is it up there? Astronauts and mission control need to  be able to coordinate with each other.

But it’s not as easy as just syncing  up their watches before takeoff. The Moon’s weaker gravity and  its motion relative to Earth throw a weird wrench into the works. So as straightforward as that question  sounds, it doesn’t have any easy answer.

But that hasn’t stopped the U. S. government from setting a timer for NASA  scientists to figure it out. [♪ INTRO] This isn’t the first time we’ll  be sending astronauts to the Moon, so it’s not the first time people  have had to tell time there. When the Apollo 11 astronauts  landed on the Moon back in 1969, they unofficially kept their watches  set to the time it was in Houston because that’s where Mission Control was.

But officially, both they and Mission  Control would refer to Apollo 11’s mission clock, which started ticking  upwards from zero at the moment of launch, and didn’t stop ticking until  the astronauts got back to Earth. And to this day, if you wander  the halls of a NASA building, you’ll probably find at least  one clock that tells you how long a current mission has been running. But right next to that proverbial  stopwatch, you’ll also probably see something that looks a lot more  like a standard 24-hour clock.

It’s not telling you the time in  some fancy space time zone, though. It’s actually keeping track of a time standard known as Coordinated Universal Time. UTC for short.

And a time standard is a  kind of benchmark we can use to measure the passage of  time and set other clocks. It’s kind of like how, for hundreds  of years, people used church bells or other public clocks to set their watches. These acted as a local time standard.

But the concept of a global time standard  has been around since the late 1880s. And UTC came onto the scene in the 1960s, which by complete coincidence is also the decade we started shooting humans into outer space. UTC measures the passing of  time using atomic clocks.

These are super dependable time  keepers that work by measuring changes inside atoms that happen  at extremely precise intervals. They’re as close as we can get to knowing exactly how much time has passed between  one instant and the next. But, every once in a while, we  need to make a small adjustment so that this time stays synced with  the Earth’s rotation, which changes ever-so-slightly over time due to things  like tides and the movement of the core.

Once we make those adjustments,  we get the time we call UTC. Then, to figure out what time it is  anywhere on Earth, we just take UTC and add or subtract a certain number of hours,  depending on which time zone we're in. It works well on Earth because  no matter where we are, a day lasts the same amount of time, and the timing of day and night are all  that shift from one time zone to the next.

And so far, we’ve also managed to make  do by using UTC on space missions. The timing of day and night up in orbit  don’t align with day and night on Earth, but aside from being a little  unintuitive, it’s worked well enough. But scientists don’t see UTC as a long-term  solution for telling time on the Moon, especially if humans are going to spend  significant amounts of time there.

Because time on the Moon actually passes  faster than it does here on Earth. Pardon the interruption. I’ve got some good news.

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According to Einstein’s  theory of general relativity, space and time are part of  a single four-dimensional cosmic fabric called spacetime. And spacetime is shaped by  the masses sitting in it. You can somewhat picture this by imagining how a bowling ball would curve  the surface of a trampoline.

But the key is that it’s not just  three-dimensional space that gets warped. There’s that fourth dimension,  time, that gets warped too. And the more warped spacetime gets,  the slower time seems to move.

This is called time dilation. Earth is 81 times more massive than the Moon, so it curves spacetime a whole  lot more than the Moon does. And that means that if you’re standing on Earth, time moves slightly slower where  you are than it does on the Moon.

But there’s another source  of time dilation, and it has to do with the motion of these two bodies. From our perspective here on Earth, Earth is still and the Moon is whizzing around it. That means another relativistic  effect comes into play here.

If two objects are moving relative to each other, from the perspective of the stationary object, time on the moving object passes slower. For instance, if a beacon  were flashing on the Moon, here on Earth the flashes would appear slower than they would appear to someone on the Moon. This time dilation only becomes  noticeable for objects moving really fast, which is why we don’t deal with this every  time a car drives by or a plane takes off.

But we can’t completely ignore it  when we’re telling time on the Moon. Now, even after accounting for  both these forms of time dilation, the difference in time between the  Earth and the Moon is minuscule. On the surface of the Moon, one Earth day passes 58.7 microseconds faster than it does on Earth.

But it adds up. After five decades, clocks on the Earth and the Moon will be off by about one second. But when it comes to operations in  space, there’s no room for error.

Equipment and maneuvers have  to be extremely precise. It wasn’t a big deal for the Apollo astronauts, who just spent a few days on the Moon, max. But if humans are going to spend  any extended time on the Moon, and that is NASA’s plan, we’re going  to need a solid time standard up there.

So the White House has directed NASA  to solve this problem before 2027. NASA hasn’t figured out yet  exactly how they’re going to do it, but one idea is just to take  some atomic clocks to the Moon. These clocks could measure time up  there the same way they do on Earth, giving us a lunar time standard  called LTC, or Coordinated Lunar Time.

The idea is for LTC to be traceable to  UTC, just like all our time zones on Earth, but reliable enough to hold steady if  people on the Moon lose contact with Earth. Now, just because NASA develops  a time standard for the Moon, that doesn’t mean it’ll automatically  become an international standard like UTC. But it’s a start.

And once scientists figure this  one out, the White House eventually wants to have time standards for  other celestial bodies too, like Mars. It might take some time to iron out the wrinkles, but the fact that we need these time zones now is a cool sign that we’re entering  a new chapter in space travel. [♪ OUTRO]