A time standard for Moon’s surface: what is it and why is it needed?

THE WHITE House has directed NASA to create a time standard for the Moon, which international organisations and private companies can use to coordinate activities on the lunar surface. The space agency has been asked to work with other parts of the United States government to finalise by the end of 2026 the strategy to establish a Coordinated Lunar Time (LTC), according to Reuters, which first reported about the project.

First, how does the time standard for Earth work?

Most clocks and time zones are based on Coordinated Universal Time (UTC), which is set by the International Bureau of Weights and Measures in Paris. UTC is essentially an internationally agreed standard for world time.

UTC is tracked by a weighted average of more than 400 atomic clocks placed around the globe. Atomic clocks measure time in terms of the resonant frequencies — the natural frequency of an object where it tends to vibrate at a higher amplitude — of atoms such as caesium-133.

In atomic time, a second is defined as the period in which a caesium atom vibrates 9,192,631,770 times. As the vibration rates at which atoms absorb energy are highly stable and ultra accurate, atomic clocks are excellent for gauging the passage of time.

To obtain their local time, countries subtract or add a certain number of hours to UTC, depending on how many time zones away from the 0 degree longitude meridian — also known as the Greenwich meridian — they are. If a country is to the west of the Greenwich meridian, it has to subtract from UTC; if a country is to the east of the meridian, it has to add.

So why do we need a time standard for the Moon?

Time flows differently on the Moon, and cannot be determined using UTC.

“A fundamental aspect of nature in the universe is that time is not absolute. That seems crazy to us on Earth as our experience of time is that it just constantly ticks by. But if you travelled to the Moon, your clock would be ticking slightly faster... This is a consequence of [Albert] Einstein’s Theory of General Relativity which tells us that gravity bends space and time. As there is less gravity on the Moon, time ticks slightly faster there relative to...the Earth,” Catherine Heymans, Astronomer Royal for Scotland and a professor of astrophysics at the University of Edinburgh, told The Indian Express by email.

Thus, on the Moon, an Earth-based clock will appear to lose on average 58.7 microseconds per Earth day with “additional periodic variations”, according to the memo issued by the White House Office of Science and Technology Policy. This may seem like a tiny discrepancy, but it is enough to create problems for spacecraft trying to dock on the Moon, transfer of data at a specific time, navigation, and communications.

Currently, each lunar mission uses its own timescale that is linked to UTC. “Take the example of two spacecraft, NASA’S Lunar Reconnaissance Orbiter (LRO) and ISRO’S Chandrayaan-2 Orbiter, which orbit the Moon in roughly similar polar orbits with some overlap. To ensure they do not collide — the probability of this is low, but it can happen — the two mission control teams talk and synchronise their operations standard as needed, based on differences in the way they calculate the orbits and time, etc.,” Jatan Mehta, a space exploration writer and author of the Moon Monday newsletter, told The Indian Express.

While this can work for a handful of independent lunar missions, there may be problems with multiple spacecraft working together at the same time — a situation that will soon become reality. Several countries, including India, have lunar missions lined up. NASA’S Artemis program aims to send humans back to the Moon after September 2026, and China and India intend to send their own astronauts to the lunar surface by 2030 and 2040 respectively. A long-term human outpost too, is planned on the Moon.

How will the lunar time standard be established?

The specifics are not clear yet. A White House official told Reuters that like on Earth, atomic clocks could be deployed on the Moon. A report by the journal Nature published last year said at least three atomic clocks ticking at the Moon’s natural pace would have to be put on the lunar surface, the output from which would be combined by an algorithm to generate a more accurate virtual timepiece.

“These clocks have to be placed...at different locations since the Moon’s rotation and even local lumps of mass, called mascons, beneath the crust of the Moon affect the flow of time slightly,” Mehta said. Mascons or mass concentrations are so dense that they alter the Moon’s local gravity field. The output from these clocks can be synthesised to give the Moon its own time, which can be tied back to UTC for seamless operations from Earth.

Even on Earth, atomic clocks tick at different rates because the Earth’s rotational speed varies from the equator to the poles, which also affects time. The Earth rotates faster at the equator than at the poles as it is wider at the equator.