The International Astronomical Union last week proposed the establishment of a timekeeping standard for the Moon. The development came after the US White House, in April this year, officially directed the National Aeronautics and Space Administration (NASA) to create a time standard for the Moon. Here is a look at why a time standard is needed for the Moon, and how it could be created. But first, how does Earth’s time standard work? Most of the clocks and time zones — a geographical region which uses the same standard time — of the world are based on Coordinated Universal Time (UTC), which is set by the International Bureau of Weights and Measures in Paris, France. UTC is essentially an internationally agreed upon standard for world time. It is tracked by a weighted average of more than 400 atomic clocks placed in different parts of 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 cesium-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 make for an excellent device for gauging the passage of time. To obtain their local time, countries need to subtract or add a certain number of hours from UTC depending on how many time zones they are away from 0 degree longitude meridian, also known as the Greenwich meridian. If a country lies on the west of the Greenwich meridian, it has to subtract from the UTC, and if a country is located on the east of the meridian, it has to add. Why do we need a time standard for the Moon? UTC, however, cannot be used to determine time on the Moon. That is because time on the Moon flows differently than it does on the Earth. “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 than if you had stayed on the Earth. 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 time on the Earth,” Catherine Heymans, the astronomer royal for Scotland and a professor of astrophysics at the University of Edinburgh, told The Indian Express over email. In other words, for someone 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 OSTP memo. The discrepancy may seem small but it can create problems for situations such as a spacecraft seeking to dock on the Moon, data transferring at a specific time, communication, and navigation. Currently, handlers of each lunar mission use their 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 the same kinds of polar orbits that have some overlap. To ensure that they do not collide with each other — the probability of this happening is quite low but it can happen — the mission control teams of the two orbiters talk to each other, and they synchronise their mission operations standard with each other as needed, based on differences in things like how they calculate the orbits and time,” Jatan Mehta, globally published independent space exploration writer and author of Moon Monday newsletter (), told The Indian Express. The approach can work for a handful of independent lunar missions, but issues will arise once there are multiple spacecraft working together at the same time — a situation which is bound to become a reality in the near future. Several countries, including India, are looking to populate the Moon in the following years. While NASA’s Artemis program aims to send astronauts back to the lunar surface no earlier than September 2026, China has announced plans to land its astronauts by 2030, and India intends to arrive by 2040. There are also proposals to build a long-term human outpost on the Moon. Therefore, there is a need for a unified lunar time standard. How will a lunar time standard be established? The specifics for creating a time standard for the Moon are not clear yet. An OSTP official, however, told Reuters that like on the Earth, atomic clocks can be deployed on the lunar surface to set a time standard. According to a 2023 report by the journal Nature, there will be a need to place at least three atomic clocks on the lunar surface that will tick at the Moon’s natural pace, and whose output will be combined by an algorithm to generate a more accurate virtual timepiece. "These clocks have to be placed on the Moon 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 ever so slightly,” according to Mehta. Mascons or mass concentrations are so dense that they alter the Moon’s local gravity field. These effects are minor but the output from these clocks can be synthesised to give the Moon its own independent time, which can be tied back to UTC for seamless operations from Earth as well. Even on Earth, atomic clocks have been placed at different locations or rather latitudes. These clocks tick at different rates due to changes in Earth’s rotational speed varying from the Equator to the poles, which also affects time. The planet rotates faster at the Equator than it does at the poles as it is wider at the Equator.
