Lunar Time: Why Clocks Run Faster on the Moon

In April 2024 the White House Office of Science and Technology Policy told NASA to deliver a plan for a new way of keeping time: Coordinated Lunar Time, or LTC, with a strategy due by the end of 2026. With crewed Artemis missions and a crowd of robotic landers heading for the Moon, everyone there will need to agree on what time it is.

The surprise is why this is hard. It is not about picking a convenient offset from London or Houston. It is that a clock sitting on the Moon runs at a genuinely different rate from a clock on Earth — and the difference is large enough to matter.

How much faster

Two physicists at the US National Institute of Standards and Technology, Neil Ashby and Bijunath Patla, worked the figure out from general relativity and published it in The Astronomical Journal (volume 168, article 112, 2024). Their result: a clock on the lunar surface gains about 56.02 microseconds per day on a clock on Earth.

That sounds tiny. Over a year it adds up to about 20 milliseconds, and left uncorrected it is ruinous for navigation. Radio and light travel about 300 metres in a single microsecond, so 56 microseconds of unmanaged clock drift corresponds to roughly 17 kilometres of ranging error — more than enough to lose a spacecraft or miss a landing site.

Why a Moon clock runs fast

Einstein's relativity gives two separate effects, and they pull in opposite directions.

• Gravity. Clocks tick faster where the gravitational field is weaker. The Moon sits far out in the Earth's gravity, where that pull is slight, and the Moon's own surface gravity is only about a sixth of Earth's (1.62 against 9.81 metres per second squared). A clock up there is, in effect, high on a hill, so it runs fast. This is the bigger effect.
• Motion. Clocks tick slower the faster they move. The Moon travels around the Earth–Moon system at roughly one kilometre per second, so special relativity drags its clock back a little. This is the smaller effect.

The weaker gravity wins, so the net result is a clock that runs fast — by those 56 microseconds a day.

Why you sometimes see 58.7

When the policy first made the news, the figure quoted was often 58.7 microseconds per day, and a 2024 analysis (arXiv preprint 2402.11150) puts it precisely at 58.72. That is not a contradiction; it is a different question.

The 58.7 figure is the rate for a clock at the Moon's location but ignores the Moon's own gravity well. A real clock does not float above the surface — it sits on it, slightly down in the Moon's own gravity, which slows it a touch. Putting that back in brings the number down to the 56.02 that Ashby and Patla report for a clock on the ground.

Figure	Clock sits in the Moon's own gravity?	Source
58.72 µs/day	No (ignores the lunar surface potential)	arXiv 2402.11150 (2024)
56.02 µs/day	Yes (a clock on the lunar surface)	Ashby & Patla, Astronomical Journal 168:112 (2024)

Both describe the same physics. For anyone standing on the Moon, 56 is the number that applies. The exact value still depends on modelling choices — different groups differ slightly on the small periodic terms — but the headline rate is settled.

A time scale, not a time zone

This is the point that trips people up. A time zone is a fixed offset that ticks at the same rate as everywhere else: when it is 12:00 in London it is 07:00 in New York, and both clocks gain one second per second. LTC cannot work that way, because a lunar clock does not gain one second per Earth second. It runs fast.

So LTC is a time scale: its own continuous count of seconds, realised by clocks on and around the Moon, with a defined relationship back to Coordinated Universal Time (UTC) rather than a simple hour offset. The US policy set four requirements for it:

• Traceable to UTC, so lunar and terrestrial systems can be related.
• Accurate enough for precision navigation and science.
• Resilient if contact with Earth is lost.
• Scalable to destinations beyond the Earth–Moon neighbourhood, Mars included.

This already happens on Earth

None of this is exotic. The satellites of the Global Positioning System carry atomic clocks that, for exactly the same two reasons, run fast by about 38 microseconds per day (roughly 45 microseconds from weaker gravity, minus about 7 from their orbital speed). GPS receivers would produce errors of several kilometres within a day if that were not corrected, so the satellites' clocks are deliberately offset before launch and trimmed in orbit. The Moon is the same problem, one order of magnitude larger, and without a ground station next door to lean on.

Worked example: turning microseconds into metres

Suppose a lunar lander's clock has drifted by just 1 microsecond against the navigation network.

1. A ranging signal travels at the speed of light, 299,792,458 metres per second.
2. In 1 microsecond that is 299,792,458 × 0.000001 = about 300 metres.
3. One day of uncorrected lunar drift (56.02 µs) is therefore 56.02 × 300 ≈ 16.8 kilometres of position error.

That single line — a microsecond is about 300 metres — is why timekeeping, not distance measurement, is the hard part of navigating anywhere in space.

Who decides what a second is

Underneath all of this sits the second itself, defined internationally through the Metre Convention and its governing body, the General Conference on Weights and Measures (CGPM). The CGPM is meeting in Versailles this very week, from 13 to 15 October 2026, to weigh the future of the second and related questions of timekeeping. A lunar time scale has to be built on whatever definition of the second the world agrees — which is why the push for LTC and the slow move towards an optical second are really the same story, seen from two ends.

Related reading

• The 28th CGPM, October 2026: leap seconds and a redefined second
• The seven SI base units and the 2019 redefinition
• Microseconds to seconds converter
• Time unit converter
• Frequency converter