Astronauts, set your watches. Time on the surface of the moon has been calculated to tick at a rate of 57 millionths of a second a day faster than it does on Earth, a difference that could be crucial as lunar exploration ramps up.
Later this decade, NASA hopes to return humans to the moon for the first time in more than 50 years, while in the past two years, six uncrewed spacecraft have attempted lunar landings. But increasingly, our lunar timekeeping isn鈥檛 up to scratch.
鈥淲e鈥檙e looking at a sustained presence on the moon,鈥� says at NASA鈥檚 Goddard Space Flight Center in Maryland. 鈥淚nfrastructure on Earth such as GPS provides time down to the nanosecond level. If you鈥檙e trying to navigate or land on the moon, and avoid dangerous areas, then that precision matters.鈥�
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To tackle this issue, NASA was recently to create a Coordinated Lunar Time (LTC) for the moon by the end of 2026. at NASA鈥檚 Jet Propulsion Laboratory in California and his colleagues were already working on the problem, and now have an answer. 鈥淪omebody needed to sit down and work out the maths,鈥� says Turyshev.
Time ticks faster on the moon because of its gravity is one-sixth that of Earth, a result of time dilation, as postulated by Albert Einstein鈥檚 theory of general relativity.
Turyshev and his team have calculated the flow of time on Earth and the moon in reference to the centre of the solar system, known as its barycentre, which moves depending on the position of the planets relative to the sun. Their calculations show that time on the moon鈥檚 surface ticks 57.5 microseconds per Earth day (0.0000575 seconds) faster than it does on Earth鈥檚 surface, meaning that over the course of 50 years, an astronaut on the moon would be around a second older than if they had stayed on Earth.
鈥淭his approach may now be used to synchronise all the assets on the moon,鈥� says Turyshev. 鈥淭he issue of lunar time from a theoretical standpoint is successfully addressed.鈥�
Other previous results for calculating lunar time have arrived at a similar number. In February, another study calculated that time on the moon鈥檚 surface than on Earth, based on the orbits of the two bodies. But having multiple results will be useful in order to create an accurate LTC, says Gramling. 鈥淭he mathematics behind this will have to be synthesised together to make sure all the equations match up,鈥� she says, noting it is likely that the moon will have just a single time zone at first for any point on its surface.
A definition of lunar time will come from a number of bodies, including the International Bureau of Weights and Measures and the International Astronomical Union, with some discussions set to take place in August. It will be up to individual countries also interested in lunar exploration, such as China, whether they decide to follow this recommendation, though.
Ultimately, defining moon time will also require us to set a future start date on which we begin tracking lunar seconds compared with Earth, in the same way that international atomic time, as measured by atomic clocks, was agreed to begin on Earth on 1 January 1977.
鈥淎t some point, we have to agree on what is the 鈥榸ero鈥� day,鈥� says Turyshev. The ideal situation would then be to place atomic clocks on the moon to monitor the effect of time dilation compared with Earth from that date. 鈥淭here鈥檚 nothing like the real thing,鈥� says Gramling.
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