As US clocks go over this weekend, many people around the world will find themselves momentarily confused about that time. But scientists say a far larger temporal problem is on the horizon: With multiple missions to the moon on the clock, it’s time to measure lunar time.
“We need to define the time of the moon,” says Javier Ventura-Traveset of the European Space Agency (ESA). Without it, Ventura-Traveset warns, spaceships could crash into each other, astronauts could be lost on the surface of the moon, and of course, no one would know when to break for lunch.
When humans first traveled to the moon in the 1960s and 1970s, they weren’t overly concerned about using up time, according to Andrew Chaikin, an author and space historian. “They set their watch to Houston because that’s where the mission control was,” he says. More formally, he adds, the Apollo missions used something called “the mission’s elapsed time,” which was the clock that started the second rocket to Earth.
But in the decades since those missions, a new era has taken on a new importance here on earth.
When countries set their times according to our position on the rotating globe, the time zones are always defined globally against one universal time coordinate (UTC). UTC is established by the International Bureau of Weights and Measures in Paris, France. It was created using the types of atomic clocks all over the world: they feed their times in a central laboratory in Paris, so that each nation ticks in sync within the smallest part of the other.
In our modern world, that timing is essential for computer networks and trading markets. Perhaps most importantly, it is the cornerstone of the world’s global navigation systems.
Satellites – used like GPS – send time signals to Earth. Because those signals arrive a fraction of a second later than they do on Earth, the time difference can be used to determine a person’s position on the planet’s surface with surprising accuracy.
Ventura-Traveset is part of a European effort to create a GPS-like system for the moon. Note that the “Luna Light” system would use a few satellites to create a communications and navigation network around a port located on the lunar surface.
But for this we need both the location of the port and the satellites to know what time it is.
Currently, lunar missions use UTC like everyone else, and that seemed like the simplest solution, Ventura-Traveset says. But in reality, it will fail to use the UTC moon. Part of the problem is the sheer distance between the Earth and the moon, which means that it will take a clock signal from Earth to take a second from the earth to the moon – an eternity by the standards of today’s atomic clocks.
But the question is still more fundamental: Einstein’s theory of relativity states that time actually reaches differently in different parts of space. Specifically, the moon’s lower gravity and its motion relative to Earth cause time to pass about 56 microseconds faster than any Earth day.
This 56 microseconds is not some abstract concept: Every day, astronauts live on the moon, 56 microseconds faster than they would on Earth. The time interval is far too small to make a difference in human life, but for sailing, that kind of time difference is enormous, says Ventura-Traveset.
To get an accurate position “needs on the order of nanoseconds,” he said. In other words, different amounts of time flow on the moon means that clocks on the surface of the moon simply cannot run from Earth. But he thinks there is a need to create a set of lunar clocks that keep a special “moon” time.
Ventura-Traveset thinks that given the moon’s proximity to the Earth, some additional sensors may be used to keep the moon’s time in sync with UTC, giving the illusion of continuity between the Earth and the moon. But as humans explore further into the solar system, he believes it will be necessary to create completely separate timescales. “If you’re on Mars or even farther away, maybe you’ll have your time,” he said.
ESA is working with NASA to figure out how to create communication and navigation standards on the moon. In a statement, NASA told NPR “the subject of expertise in the international community is to discuss approaches to recommendations to the International Astronomical Union for the lunar reference frame and time system.”
Ventura-Traveset hopes that OESA’s separate lunar phase proposal will eventually be accepted by other space operators around the world. “It’s an exciting time,” he muses.
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