What time is it on the Moon?

January 26, 2023 5:21 PM

Defining lunar time is not simple. Although the definition of the second is the same everywhere, the special theory of relativity dictates that clocks tick slower in stronger gravitational fields. The Moon’s gravitational pull is weaker than Earth’s, meaning that, to an observer on Earth, a lunar clock would run faster than an Earth one. [...] “This is a paradise for experts in relativity, because you have to take into account so many things.” 1300 words from Elizabeth Gibney for Nature.
posted by cgc373 (60 comments total)

What a hassle. Almost makes me not want to go to the moon at all!
posted by aubilenon at 5:33 PM on January 26, 2023 [19 favorites]

It's almost as if our sense of time is intimately tied to our conceit that the sun orbits the Earth.
posted by mollweide at 5:47 PM on January 26, 2023 [3 favorites]

In 2010 NIST scientists were able to measure this effect with very accurate atomic clocks arranged so that one was 33 centimeters higher than the other. And then in a study published in 2022, more scientists measured it at millimeter scale.

I can't think about this too much or my brain will hurt.
posted by fedward at 6:14 PM on January 26, 2023 [9 favorites]

I'm a Star Wars guy so, while I've never seen this come up in SW lore (and there's just SO MUCH now, maybe too much for my liking) - maybe they cover this kind of nerdery in Star Trek? - I've always wondered why time is never discussed. No one ever says seconds, minutes, hours, days, weeks, years. When the 2nd Death Star is nearing completion, Vader simply refers to a vague "soon!" as for when the battlestation should be completed.

I mean, sure, it really would be impossible to compare, say, what day/time it is on Endor when you're on Tattooine. But surely if you're meeting a friend at the Mos Eisley Cantina Bar, you'd reference some version of time, yeah?

Anyway, moon time is pretty cool too.
posted by revmitcz at 6:20 PM on January 26, 2023 [5 favorites]

That is a great title. It is Feynmansian.
posted by NoThisIsPatrick at 6:37 PM on January 26, 2023

To answer revmitcz' question, in Star Trek, Federation starships use a 24-hour clock centered on (IIRC) San Francisco time, because that's where Starfleet Headquarters is located. To talk about time in other places, the stardate method was introduced specifically to avoid the problems inherent with other frames of reference. It has a nice technical sound to it.

As for Star Wars, it's more space fantasy than science fiction, and such niceties don't bother the writers. (N.B.: I am not knocking Star Wars.)

And now, back to the moon.
posted by bryon at 6:43 PM on January 26, 2023 [3 favorites]

Defining lunar time is not simple. Although the definition of the second is the same everywhere, the special theory of relativity dictates that clocks tick slower in stronger gravitational fields.

Shouldn't that be general theory of relativity and not special theory of relativity?
posted by Schmucko at 7:22 PM on January 26, 2023 [5 favorites]

[finance bro] hmmm what if there were such a thing as lunar time based arbitrage
posted by Fiasco da Gama at 7:29 PM on January 26, 2023 [2 favorites]

This 2006 article, "Time Too Good to be True", is a solid mind-blower of a companion piece:

The prospect of a major advance in clock accuracy brings me to the second issue concerning time. At accuracies beyond 1 part in 1016, the gravitational redshift or, more precisely, the gravitational blueshift, predicted by general relativity, scrambles time with Earth’s gravity in a rather unmanageable fashion that ultimately upsets what we mean by “keeping time.”

posted by whuppy at 7:31 PM on January 26, 2023 [1 favorite]

But surely if you're meeting a friend at the Mos Eisley Cantina Bar, you'd reference some version of time, yeah?

<ahem>
<flips through script>
<puts finger down and reads>

"It's the ship that made the Kessel Run in less than twelve parsecs."

<looks at camera>
posted by LURK at 8:35 PM on January 26, 2023 [19 favorites]

If the state of the art for atomic-clock-altimetry is now millimeters, that may mean that the 2005 mountain camping trip version (“it was the best 22 extra nanoseconds I’ve ever spent with my kids”) is now accessible to people who begin with a less impressive collection of clocks.
posted by fantabulous timewaster at 8:42 PM on January 26, 2023 [1 favorite]

When 900 years old you reach, look as good you will not.
posted by rube goldberg at 8:53 PM on January 26, 2023 [3 favorites]

No one ever says seconds, minutes, hours, days, weeks, years.
Now hold on a centon! Battlestar Galactica had a consistent set of equivalent terms.
posted by bartleby at 9:03 PM on January 26, 2023

"It's the ship that made the Kessel Run in less than twelve parsecs."

[pushes up glasses] Well they did kind of explain that if you enjoyed / could tolerate watching Solo. The Vessel Run required a lot of screwing around to get through a complex set of blockades inside of a storm or nebula or something. Han Solo found a much more direct route.

I have to admit I was amused that they found a way to explain that after presumable decades of ridicule.
posted by pkingdesign at 9:21 PM on January 26, 2023 [2 favorites]

Shouldn't that be general theory of relativity and not special theory of relativity?


In other words, "pffft, it's really not that special"
posted by aubilenon at 9:46 PM on January 26, 2023

We have no idea how time really works

Time is an illusion caused by the passage of history.
History is an illusion caused by the passage of time.

HTH HAND
posted by Greg_Ace at 10:10 PM on January 26, 2023 [2 favorites]

Time is what keeps everything from happening all at once.
Space is what keeps everything from happening to me.
Neither has been working well lately.
posted by straight at 10:17 PM on January 26, 2023 [23 favorites]

"It's the ship that made the Kessel Run in less than twelve parsecs."

Ahh c'mon. That's a unit of length not time. And it makes perfect sense that the bragging rights were about how to find and exploit that shortest length because of the speed and maneuverability of the Falcon.

Okay, I'm derailing this thread way too much already. I just thought it fascinating that we're actually calculating extraterrestrial passages of time! Truly the stuff of scifi gone real.
posted by revmitcz at 10:24 PM on January 26, 2023

If the state of the art for atomic-clock-altimetry is now millimeters, that may mean that the 2005 mountain camping trip version

The clock in that 2005 trip is still pretty much the state of the art for clocks accessible to entities that are not national laboratories.
posted by Dr. Twist at 10:57 PM on January 26, 2023

What's really annoying if you're trying to keep accurate time on Earth is that it's not just altitude that matters. The Earth is lumpy, so the exact force of gravity is different in different places.
posted by wierdo at 11:06 PM on January 26, 2023 [4 favorites]

Obligatory "Time is an illusion. Lunchtime doubly so" comment.
posted by Pyrogenesis at 11:38 PM on January 26, 2023 [6 favorites]

The Earth is lumpy, so the exact force of gravity is different in different places

Same
posted by aubilenon at 12:00 AM on January 27, 2023 [17 favorites]

There are some strange statements in that article. One is the comment about special relativity being responsible for time dilation in a gravitational well. I expect much better from Nature of all places!

The other is the odd "paradise for experts in relativity" which, well, not really. This isn't a hard technical problem but a graduate student exercise. GPS satellites are a much trickier problem, they have a hugely eccentric orbit and in order to have them synchronized we need to take into account both general relativity since they are subject to less gravity and also special relativity since they are moving quickly with respect to us.

In the case of GPS, because of SR, clocks appear to go slower on the satellite. But, because of GR, clocks appear to go faster! The two effects don't cancel out. The GR effect is greater but in any case it is all just calculations, like the complex orbital calculations we have to do when we launch satellites. It is known science and computers do all the work.

The moon problem is about new conventions or standards, not about breaking new technical ground. The moon is so close that it makes sense to strap everything to UTC and let the spacecrafts figure out the corrections. Local Lunar time can be its own time which can subtract a backleap second every once in a while to correct with UTC if thats what lunar residents want.
posted by vacapinta at 12:08 AM on January 27, 2023 [11 favorites]

It is exceedingly weird to realize that the GR and SR effects for orbital bodies mean that there is an altitude at which the effects cancel each other out. It happens to be pretty close to the orbital altitude of the ISS.
posted by wierdo at 12:32 AM on January 27, 2023 [3 favorites]

Ahh c'mon. That's a unit of length not time.

Oh I know (B.S. in Physics). I just thought it was wonderful you mentioned Mos Eisley in a discussion of units of time when it contains that scene. I'm also dimly aware that it has been retconned, but the original Star Wars was like a hit of space heroin that I've given up ever re-experiencing so I haven't really followed the franchise. All I know is: Luke better have those units on the south ridge repaired by midday, or there'll be hell to pay.

Also, interestingly, No One Has Measured The Speed Of Light
posted by LURK at 12:43 AM on January 27, 2023 [4 favorites]

Luke better have those units on the south ridge repaired by midday, or there'll be hell to pay.

Relax, Uncle Owen. I'll have it done by second midday.
posted by GeckoDundee at 2:31 AM on January 27, 2023 [7 favorites]

Lurk : you definitely know more than me in this regard, and I'm genuinely delighted we had this exchange.
posted by revmitcz at 2:37 AM on January 27, 2023 [1 favorite]

Joan Vinge's _The Outcasts of Heaven's Belt_ (and some other stories, I think) had people in the future measuring time in large quantities of seconds. A good science fictional idea, but it doesn't do justice to the complexity of the real world.
posted by Nancy Lebovitz at 4:00 AM on January 27, 2023

I mean, sure, it really would be impossible to compare, say, what day/time it is on Endor when you're on Tattooine. But surely if you're meeting a friend at the Mos Eisley Cantina Bar, you'd reference some version of time, yeah?

I needed to try to figure this out for the SW TTRPG campaign I'm GMing right now. Because characters in The Clone Wars (and probably other sources) will sometimes reference "rotations"—and always on or next to a planet AFAICT—my interpretation is that when a SW character says "days," they're referring to the galactic standard calendar, and when they say "rotations," they're referring to the local day/night cycle. Thus, the only planet where "day" and "rotation" actually mean the same thing is Coruscant, upon which the standard calendar is based (according to pre-Mouse EU canon and possibly also nu-"EU").

And re: the article, I found even the above interpretation to get a little strained when I foolishly sent my TTRPG adventuring party to an inhabited moon of an also-inhabited planet! Lucky for me, before they started getting bogged down in details like local bus schedules, they got into too much trouble to stay in the system.

I have to admit I was amused that they found a way to explain that after presumable decades of ridicule.

Well, maybe a decade and a half or so: Solo's explanation was largely borrowed from one of the old EU stories. (Very possibly the above-average young Han trilogy of novels.)
posted by CheesesOfBrazil at 4:06 AM on January 27, 2023

So how much faster are we talking? How long would I have to be on the moon to move 1 day ahead of earth time?

If I tried to stream Netflix the whole trip, would I have buffering problems?

We I get back to earth, is it similar to jet lag where everything is the wrong time and I just have to get used to it?

When traveling from earth to another planet where there is no strong local gravety, will that trip go "faster"?
posted by rebent at 5:19 AM on January 27, 2023

Time is an illusion
posted by Eddie Mars at 5:20 AM on January 27, 2023

The early "GPS" satellites (NTS satellites) were designed so that they could switch from an Earth-calibrated clock frequency to the correct one based on the combined effects of GR and SR on timekeeping.

In the internet age there's a persistent claim that this post-launch adjustment was done because "there were some who doubted that relativistic effects were real".

However, as far as I've been able to tell by going back to see actual 60s and 70s papers, nobody I found specifically doubted the effect. Rather, the mode seems fully explained by the need to calibrate the system on earth with an Earth-correct rate before launch, then switch to the correct orbital rate post-launch.

I gathered some of the references I could find on my blog a couple of years ago.
posted by the antecedent of that pronoun at 5:46 AM on January 27, 2023 [1 favorite]

Ahh c'mon. That's a unit of length not time.

Oh I know (B.S. in Physics)

Well actually (PhD physics) it's all just spacetime; c is just a conversion factor between the same sorts of units, and there's nothing ambiguous about saying my height is just under 6 nanoseconds, it just confuses people who aren't used to dealing with the aforementioned concepts.

Also, time isn't the illusion. Simultaneity is the illusion. Which I suppose means time, like wristwatch time is an illusion. But not time time: the rhythms and cycles and diffusions and chaotic spirals of existence all with their own rates. Those are still very much real, if a bit more complex and weird at large scales and high speeds than we are quite used to living at the human scales of spacetime.
posted by Zalzidrax at 5:49 AM on January 27, 2023 [6 favorites]

Time is what keeps everything from happening all at once.
Space is what keeps everything from happening to me.
Neither has been working well lately.

But they got Michelle Yeoh to play you in that movie, that must feel pretty good!
posted by solotoro at 5:57 AM on January 27, 2023 [4 favorites]

Voltron time units. Spare a couple of doboshes to check it out. (And, yes, it's complete rubbish, particularly in a series where "time dilation" sometimes occurs.)
posted by SPrintF at 6:37 AM on January 27, 2023

Local Lunar time can be its own time which can subtract a backleap second every once in a while to correct with UTC if thats what lunar residents want.

Yeah, but I’m going to hold off on emigrating until the lunar residents make their position on Daylight Saving Time clear.
posted by nickmark at 6:47 AM on January 27, 2023 [2 favorites]

Voltron time units.

You know, Seahawk claims to have run the 50 klick Galebreath Gauntlet in only 20 klicks. That has to be impressive.
posted by RonButNotStupid at 6:51 AM on January 27, 2023

What about moonlight saving time?
posted by Jon_Evil at 7:12 AM on January 27, 2023 [1 favorite]

Luke better have those units on the south ridge repaired by midday, or there'll be hell to pay.

Relax, Uncle Owen. I'll have it done by second midday.

Uncle Owen, I know I'm on probation
I cleaned the droids, can I go to Tosche Station?
posted by slogger at 7:26 AM on January 27, 2023 [1 favorite]

Time is what keeps everything from happening all at once.
Space is what keeps everything from happening to me.

Time isn't holding up. Time isn't after us.

Rabbis have been working out how you keep the sabbath in space for decades now, so I'm sure we can science something out when the time comes. Or we can just stop all the clocks and then twice a day we'll...
posted by Mchelly at 7:27 AM on January 27, 2023 [3 favorites]

I like that we use all our extra earth time created by the slower clock to talk about Star Wars.
posted by srboisvert at 8:29 AM on January 27, 2023 [1 favorite]

Veritasium isn't wrong exactly, but any measurement of anything is going to entail at least some assumptions about the way the universe works. The one-way speed of light isn't special in this regard. The other thing is that "the one-way speed of light" sounds like it should unambiguously mean something, but translating it into the mathematical language of physics is going to rely on conventions. And if you use different conventions you're going to get different answers: that's not going to be surprising to physicists. There are interesting questions of the philosophy of science for which the one-way speed of light would be a good example, but I felt the Veritasium video did not explore them very well.
posted by mscibing at 9:31 AM on January 27, 2023 [2 favorites]

The Kessel run has come up enough here that maybe this isn't a derail, just more creaking along one of many tracks: my Doylist theory about those much-mocked parsecs.

[mild spoiler for 50-year-old fantasy novel] In Madeleine L'Engle's A Wind in the Door, there's a long sequence in "Yadah," a mystical/trans-dimensional setting where time and space don't work the way they do for us earthlings. Meg, the human protagonist, keeps asking "where" someone is, and has to keep being told "where doesn't matter"--along with size. Meg is there with a grumpy, excitable mouse-creature, who--well, here:

The mouse-creature's whiskers twingled. "It can't have been much of an ordeal. Can we please get going, Blajeny? We only have a parsec before I make my preliminary report. And I can see I have a great deal to teach whomever I'm unfortunate enough to have as a partner...."

My read is that L'Engle treats "parsec" here as a unit of time in an intentional and playful mashup of time & space, the way she does through the whole section; that Lucas lifted this usage and forgot or missed its context; that when Han Solo's misuse of it was pointed out, he doubled down and retconned an explanation in a now familiar handwavey way because of sexism, arrogance, or both--if he remembered where he got it from at all.

A Wind in the Door, the sequel to A Wrinkle in Time, came out in January 1973. Lucas started work on the New Hope screenplay in 1973, and I think completed drafts of it were all 1974-76. I don't have any evidence that he read it, but he did know about Wrinkle in Time. (Unrelatedly, L'Engle gets cited as a source of another Lucas retcon moment: midichlorians look an awful lot like L'Engle's mitochondrial "farandolae"... but that is a swiftly tilting derail.)
posted by miles per flower at 10:45 AM on January 27, 2023 [5 favorites]

Metafilter: A swiftly tilting derail
posted by Greg_Ace at 11:42 AM on January 27, 2023 [4 favorites]

- As a physics undergraduate in the early 1980s, I was wiring some circuits that processed the signals coming from the particle detectors at an accelerator. The signals going from box to box in different parts of a house-sized building had to arrive simultaneously. We had lots of coax cables whose lengths were marked in nanoseconds.

- About 15 years ago I was part of a group of amateur radio astronomers who were brainstorming on building a very long baseline interferometer in which radio telescopes anywhere on Earth could record observations simultaneously and the signals combined later. A very good electronics engineer devised a small box that could decode the time broadcasts in GPS signals and maintain an accuracy of about one nanosecond. (The project was never developed.)
posted by neuron at 1:37 PM on January 27, 2023 [2 favorites]

To add to the Star Wars derail, they do say "minutes" as the death star approaches the rebel base. "Death Star will be in range in five minutes."
posted by Billy Rubin at 3:05 PM on January 27, 2023 [1 favorite]

Not only do cables in timing-sensitive applications come labeled in nanoseconds, but a device I used in a similar experiment was called a "delay module." Rack-mounted like many other modules, two coaxial "in" and "out" connectors next to each other at the bottom, and switches marked "1ns," "2ns," "4ns," and other powers of two, so you could dial in a delay by counting in binary. At some point I was debugging a problem and opened the delay box up: no transistors or circuitry, just coils of cable.

For what it's worth, there is a clock speed where an entire microchip can't be synchronized, because it takes longer for the clock signal to propagate across the chip than the duration of the clock. For a centimeter-scale chip like a CPU, that frequency is around 10 GHz. If you have two modern computers nest to each other on your desk, they clock signals may be spacelike-separated from one another, and whether a certain instruction was carried out first by one chip or the other might friend on your velocity relative to the desk.
posted by fantabulous timewaster at 3:09 PM on January 27, 2023 [4 favorites]

That's why I always try to drive as fast as possible everywhere I go, so I'll experience time more slowly and my life will last longer.
posted by Greg_Ace at 4:41 PM on January 27, 2023

nothing much to contribute on the subject of Moon Math, but

If I recall correctly the OG Star Trek navigation console prop had a chronometer on the pedestal between Sulu and Checkov. I always assumed that was meant for elapsed time, from launch, rather than some attempt to sync with Starfleet HQ time.
So your 'five year mission' star-date logs are complete when you've reached 43,830 hours as measured from within the ship's local spacetime warp bubble. Then you do the math to readjust when you get home.

But I suppose that only covers FTL travel, and doesn't account for travel at relativistic speeds? If your warp coil goes out, and you're stuck at sublight impulse engine speeds? "18 months limping back home at 0.93c to a starbase for repairs didn't seem that long to us; but when we left, a young ensign named Picard waved us off. What? He's a Captain now? - and what happened to his hair?"
posted by bartleby at 4:49 PM on January 27, 2023

Not only is a parsec a distance but it’s an earth-bound unit since it’s the distance of a star that has a parallax of one second when viewed from opposite sides of the earth’s orbit. But then again the kilometer was originally defined as a ten thousandth of the distance from the pole to the equator, curiously a feat that was hardly practical at all though iirc they did try to measure a degree of l’attitude at one point.
posted by sjswitzer at 5:37 AM on January 28, 2023

It's often said that in general relativity gravity causes time to slow. but it's rather the reverse. Mass distorts spacetime and distorted spacetime causes gravity. At low speeds the whole of gravity is due to slower time.

A nice way to visualize this is with an image stolen from descriptions of refraction. Imagine you're hurling through time with your arms held out. Now imagine that your right hand is travelling slower than your left. You'd turn in the direction of the slower hand. That's how slower time causes gravitational attraction: we are drawn toward slower time.
posted by sjswitzer at 6:07 AM on January 28, 2023 [1 favorite]

The camping trip experiment was in 2005, and involved lots of expensive and unwieldy equipment.

In 2023, has the technology advanced enough for a regular person to easily reproduce the experiment with miniaturized, off-the-shelf and cheap atomic clocks?

I did a quick search, and it looks like most things sold to consumers as "atomic clocks" are just regular clocks that automatically adjust themselves with radio signals from a real atomic clock.
posted by The genius who rejected Anno's budget proposal. at 6:26 AM on January 28, 2023

we are drawn toward slower time.

Especially on summer weekends.
posted by Greg_Ace at 9:02 AM on January 28, 2023 [2 favorites]

whuppy: At accuracies beyond 1 part in 1016, the gravitational redshift or, more precisely, the gravitational blueshift, predicted by general relativity, scrambles time with Earth’s gravity in a rather unmanageable fashion that ultimately upsets what we mean by “keeping time.”

Something seems strange about this quote. 1 part in 1016 is almost a minute and a half per day. I'm looking at a dollar store quartz clock in front of me that keeps time at least a hundred times more accurately than that. Would I really be able to detect relativistic effects with my dollar store clock?
posted by clawsoon at 4:36 PM on January 29, 2023

Looking back at the original article it’s 10¹⁶, but the superscript formatting on the exponent got lost while quoting. So it’s a bit less.
posted by aubilenon at 5:25 PM on January 29, 2023 [1 favorite]

Would I really be able to detect relativistic effects with my dollar store clock?

You could drop it and see if it falls.
posted by sjswitzer at 2:04 AM on January 30, 2023 [1 favorite]

hippybear: you might enjoy a book called Relativity Visualized.

In the book, the author introduces the idea that we are all traveling at the speed of light, all the time. There is only one speed through space-time we can travel. This is best visualized from your own perspective (reference frame)

How can you be moving at the speed of light if you are standing still? All your speed is focused in the Time direction. Another person or object moving away from you is moving at an angle in spacetime, "converting" their time direction into space direction. In order to move through Space, you have to move slower through Time in order to maintain your constant speed through Space-time. Light, of course, is only travelling through Space and not through Time at all.

There's a nice explanation and some diagrams from the book here.
posted by vacapinta at 2:33 AM on January 30, 2023 [3 favorites]

aubilenon: Looking back at the original article it’s 10¹⁶, but the superscript formatting on the exponent got lost while quoting. So it’s a bit less.

Ah, that smells more likely, thanks.
posted by clawsoon at 3:25 AM on January 30, 2023

...although I am a little disappointed that I won't be doing general relativity experiments with dollar store supplies.
posted by clawsoon at 3:48 AM on January 30, 2023 [1 favorite]

Yes, most "atomic clocks" in the US are really radios that synchronize with the signal from NIST's broadcast from WWVB in Colorado. If your laboratory actually needs its own independent frequency standard, an "inexpensive" atomic clock starts at a few thousand dollars.

This is partly because our other technologies for frequency standards are also really good. I had an experience as a postdoc where I needed to send the output from two function generators into the same experiment, and then later wound up setting the two function generators to the same frequency. When people talk about a quartz oscillator having a stability of better than 10^-7, they usually talk about a quartz watch gaining or losing less than a second in a year. But if you are using your quartz oscillators to generate some kilohertz square waves, that same precision means it will take days for those millisecond-long signals to drift out of sync by one cycle.

If you're the kind of person who would like to take some atomic clocks camping on a mountain, a nice "gateway drug" is to set two function generators to be different by a millihertz and see whether their beat frequency really has a period of a thousand seconds. You can just barely see this move on an oscilloscope: compare to the minute hand of a clock, with a period of 3600 seconds, whose motion is also just barely visible. But in terms of cost, two function generators and an oscilloscope are already several thousand dollars. An off-the-shelf atomic clock is going to look a lot more like a function generator than it is going to look like a wall clock displaying the current time to sixteen significant figures.
posted by fantabulous timewaster at 5:41 AM on January 31, 2023 [1 favorite]