Perhaps the greatest, mind-bending quirk of our universe is the inherent trouble with timekeeping: Seconds tick by ever so slightly faster atop a mountain than they do in the valleys of Earth.
For practical purposes, most people don’t have to worry about those differences.
But a renewed space race has the United States and its allies, as well as China, dashing to create permanent settlements on the moon, and that has brought the idiosyncrasies of time, once again, to the forefront.
On the lunar surface, a single Earth day would be roughly 56 microseconds shorter than on our home planet — a tiny number that can lead to significant inconsistencies over time.
NASA and its international partners are currently working on this mystery.
According to Cheryl Gramling, manager of lunar positioning, navigation, timing and standards at NASA’s Goddard Space Flight Center in Maryland, scientists don’t simply want to create a new “time zone” on the moon, as some headlines suggest. Rather, Gramling said, the space agency and its partners want to create an entirely new “time scale,” or measurement system, that takes into account the fact that seconds pass faster on the lunar surface.
The agency’s goal, working with international partners, is to develop a new way to measure time, especially on the moon, and spacefaring nations will strive to adhere to this method.
A recent White House memo directed NASA to outline a plan for this new time scale by December 31, calling it “fundamental” to renewing U.S. lunar exploration efforts. The memo also calls on NASA to have such a system in place by the end of 2026, the same year the space agency plans to send astronauts back to the moon for the first time in 50 years.
The coming months could be crucial for timekeepers around the world in figuring out how to accurately measure time on the moon, and in reaching an agreement on when, where and how to place clocks on the lunar surface.
Such a framework will be crucial for people visiting our closest celestial neighbor.
Lunar astronauts, for example, will leave their habitations to explore the surface and conduct scientific research, she said. They may also communicate with each other and drive lunar buggies while on the moon.
“If they navigate by the moon, then time must be by the moon.”
A Brief History of Earth Time
The simple sundial, or shadow-tracking rock formation, marks the passage of a day just as the phases of the moon can mark the passage of a month on Earth. These natural clocks have helped humans stay on schedule for millennia.
But perhaps since the advent of mechanical clocks in the early 14th century, watchmakers have paid ever-increasing attention to precision.
The precise measurement of a second also became more complicated in the early 20th century, thanks to Albert Einstein, the German-born physicist who shook up the scientific world with his theories of special and general relativity.
“That Einstein, he invented the theory of general relativity, and a lot of weird stuff came out of that,” said Dr. Bruce Betts, chief scientist at the Planetary Society, a nonprofit space advocacy group. “One is that gravity slows down time.”
General relativity is complex, but broadly speaking, it’s a framework that explains how gravity affects space and time.
Imagine that our solar system is a piece of fabric suspended in the air. That fabric is space and time itself, which — under Einstein’s theories — are inextricably linked. And every celestial body within the solar system, from the sun to the planets, is like a heavy ball sitting atop the fabric. The heavier the ball, the deeper the divot it creates, warping space and time.
Even the idea of an earthly “second” is a humanmade concept that’s tricky to measure. And it was Einstein’s theory of general relativity that explained why time passes slightly more slowly at lower elevations — because gravity has a stronger effect closer to a massive object (such as our home planet).
Scientists have found a modern solution to measure all the complexities of the theory of relativity on Earth. To compensate for imperceptible differences, they have installed hundreds of atomic clocks at different locations on Earth. Atomic clocks are ultra-precise instruments that use the vibrations of atoms to measure the passage of time, and according to Einstein’s theory, these clocks tick more slowly the closer they are to the Earth’s surface.
Readings from atomic clocks around the world can be averaged to give a general and most accurate sense of time for the entire planet, resulting in Coordinated Universal Time (UTC). However, “leap seconds” are sometimes taken into account to adjust UTC for slight changes in the Earth’s rotation speed.
This systematic timekeeping helps keep the modern world running – figuratively speaking, says Kevin Coggins, associate administrator and program manager for NASA’s Space Communications and Navigation Programs.
“Anyone who has studied time on Earth knows that it is a crucial prerequisite for everything: economy, food security, trade, finance, even oil production. Accurate clocks are used,” Coggins said. “Everything that matters in modern society is included.”
Space, time: eternal questions
You can imagine if time flows differently on the top of a mountain than on the shore of the ocean. The further away you get from Earth, the stranger things get.
To complicate things even further: Einstein’s theory of special relativity says that the faster a person or a spacecraft travels, the slower time passes.
For example, astronauts on the International Space Station are lucky, Dr. Vijnath Patra, a theoretical physicist at the National Institute of Standards and Technology, said in a telephone interview. Because the space station orbits at an altitude of about 200 miles (322 kilometers) above Earth’s surface but also moves at high speed, orbiting the planet 16 times a day, the effects of relativity partially cancel each other out, Patra said.
That’s why astronauts in the orbiting laboratory can easily use Earth time to stay on schedule. It’s not so easy on other missions. Fortunately, scientists have decades of experience dealing with these complexities. For example, the spacecraft is equipped with its own clock, called an oscillator, Gramling said.
“They keep their own time,” Gramling says, “and most of our spacecraft operations, even the ones in the Kuiper Belt like Pluto and New Horizons, depend on ground stations back on Earth. So everything they do has to correlate with UTC.
But those spacecraft also rely on their own kept time, Gramling said. Vehicles exploring deep into the solar system, for example, have to know — based on their own time scale — when they are approaching a planet in case the spacecraft needs to use that planetary body for navigational purposes, she added.
For 50 years, scientists have also been able to observe atomic clocks that are tucked aboard GPS satellites, which orbit Earth about 12,550 miles (20,200 kilometers) away — or about one-nineteenth the distance between our planet and the moon.
Studying those clocks has given scientists a great starting point to begin extrapolating further as they set out to establish a new time scale for the moon, Patla said.
“We can easily compare (GPS) clocks to clocks on the ground,” Patla said, adding that scientists have found a way to gently slow GPS clocks down, making them tick more in-line with Earth-bound clocks. “Obviously, it’s not as easy as it sounds, but it’s easier than making a mess.”
For the moon, however, scientists likely won’t seek to slow clocks down. They hope to accurately measure lunar time as it is — while also ensuring it can be related back to Earth time, according to Patla, who recently co-authored a paper detailing a framework for lunar time.
The study, for the record, also attempted to pinpoint exactly how far apart moon and Earth time are, as estimates have wavered between 56 and 59 microseconds per day.
Clocks on the moon’s equator would tick 56.02 microseconds faster per day than clocks at the Earth’s equator, according to the paper.
Lunar clockwork
What scientists know for certain is that they need to get precision timekeeping instruments to the moon.
Exactly who pays for lunar clocks, which type of clocks will go, and where they’ll be positioned are all questions that remain up in the air, Gramling said.
“We have to work all of this out,” she said. “I don’t think we know yet. I think it will be an amalgamation of several different things.”
Atomic clocks, Gramling noted, are great for long-term stability, and crystal oscillators have an advantage for short-term stability.
“You never trust one clock,” Gramling added. “And you never trust two clocks.”
Clocks of various types could be placed inside satellites that orbit the moon or perhaps at the precise locations on the lunar surface that astronauts will one day visit.
As for price, an atomic clock worthy of space travel could cost around a few million dollars, according Gramling, with crystal oscillators coming in substantially cheaper.
But, Patla said, you get what you pay for.
“The very cheap oscillators may be off by milliseconds or even 10s of milliseconds,” he added. “And that is important because for navigation purposes — we need to have the clocks synchronized to 10s of nanoseconds.”
A network of clocks on the moon could work in concert to inform the new lunar time scale, just as atomic clocks do for UTC on Earth.
(There will not, Gramling added, be different time zones on the moon. “There have been conversations about creating different zones, with the answer: ‘No,’” she said. “But that could change in the future.”)
The new time scale would underpin an entire lunar network, which NASA and its allies have dubbed LunaNet.
“You can think of LunaNet like the internet — or the internet and a global navigation satellite system all combined,” Gramling said. It’s “a framework of standards that contributors to LunaNet (such as NASA or the European Space Agency) would follow.”
“And you can think of the contributors maybe as your internet service provider,” Gramling added.
Creating such a framework means bringing a lot of people across the world to the table. So far, Gramling said, conversations with US partners have been “very, very positive.”
It’s not clear whether NASA and its partners on this effort, which include the European Space Agency, will get a buy-in from nations that aren’t among US allies, such as China. Gramling noted those conversations would be held through international standard-setting bodies, such as the International Astronomical Union.
‘A whole different mindset’
Accurate clockwork is one matter. But how future astronauts living and working on the lunar surface will experience time is a different question entirely.
On Earth, our sense of one day is governed by the fact that the planet completes one rotation every 24 hours, giving most locations a consistent cycle of daylight and darkened nights. On the moon, however, the equator receives roughly 14 days of sunlight followed by 14 days of darkness.
“It’s just a very, very different concept” on the moon, Betts said. “And (NASA is) talking about landing astronauts in the very interesting south polar region (of the moon), where you have permanently lit and permanently shadowed areas. So, that’s a whole other set of confusion.”
“It’ll be challenging” for those astronauts, Betts added. “It’s so different than Earth, and it’s just a whole different mindset.”
That will be true no matter what time is displayed on the astronauts’ watches.
Still, precision timekeeping matters — not just for the sake of scientifically understanding the passage of time on the moon but also for setting up all the infrastructure necessary to carry out missions.
The beauty of creating a time scale from scratch, Gramling said, is that scientists can take everything they have learned about timekeeping on Earth and apply it to a new system on the moon.
And if scientists can get it right on the moon, she added, they can get it right later down the road if NASA fulfills its goal of sending astronauts deeper into the solar system.