Why the Moon needs its own time

AT 17 MINUTES and 40 seconds past eight in the evening, Greenwich Mean Time, on July 20th it will be exactly 55 years since Neil Armstrong landed on the Moon. Those 55 years add up to 20,089 days. Of those, 20,062 were 86,400 seconds long and 27 one second longer thanks to the addition of leap seconds, a practice begun in 1972. So those 55 years will add up to 1,735,689,627 seconds.

Unless, that is, the clock by which you do the measuring is on the Moon. For a Moon clock those 55 years will have lasted 1,735,689,628 seconds: one second more. This offset is significant enough that, on April 2nd, the White House Office of Science and Technology Policy (OSTP) instructed NASA to look at the options for a global agreement defining Co-ordinated Lunar Time (LTC). Why is such a thing necessary?

Blame Einstein. His general theory of relativity conceived of gravity as a curvature imposed on “spacetime” by mass. Part of what it is for spacetime to be curved is for the “time” bit of spacetime to pass more slowly. So the curvature of spacetime caused by the mass of the Earth makes time pass more slowly close to the planet than farther away. The effect is small: as the figures above show, the difference in the rate at which time passes at the surface of the Earth and 380,000km away on the surface of the Moon is less than one part in two billion. But details matter.

The effect of gravity on time is already part of everyday life, albeit a well-hidden one, in the form of the satellite positioning systems, such as America’s GPS and Europe’s Galileo, which tell planes, cars, ships and smartphones where they are. Such systems work by putting extremely precise atomic clocks into space, from where they broadcast time signals. Because the clocks are in orbits 20,000-24,000km from the Earth and moving at a fair clip the signals they transmit have to take relativity into account.

Those adjustments are made to keep the satellite signals in step with time on Earth, which is defined by UTC, or co-ordinated universal time (in practice, much the same as Greenwich Mean Time). The ultimate guarantor of UTC is a weighted average of readings of hundreds of earthbound atomic clocks, a data product known as International Atomic Time (TAI). All those clocks count time in seconds, which are precisely defined with respect to a specific way in which caesium atoms oscillate: 9,192,631,770 oscillations to a second. To keep UTC co-ordinated with more parochial Earth-specific measures of time, such as the day and the year, the International Telecommunication Union adds occasional leap seconds to TAI to make up for variations in the Earth’s rotation and orbit (as identified by the International Earth Rotation and Reference Systems Service).

Keeping satellites used for navigating the Earth in step with UTC makes sense. If there is going to be a lot of travel to the Moon, though, a system of mission-specific hacks which allow positions and timings to be measured with respect to Earth will be cumbersome. The OSTP argues that “Precision applications such as spacecraft docking or landing will require greater accuracy than current methods allow”. What is more, some science needs highly accurate timing. Hence the attraction of a system which reflects the fact that lunar time really does pass slightly quicker.

In the long run this will probably require a network of atomic clocks on the Moon like that which defines TAI on Earth. With that in place a position, navigation and timing infrastructure like that provided by GPS on Earth could be put in place. Such a system could also help with knock-on effects that might come to matter for scientists. Because the length of a second differs on the Earth and on the Moon, so does the length of a metre and the mass of a kilogram (a metre is defined as one-299,792,458th of the distance light travels in a second in a vacuum).

If you are not involved in making exquisite cosmic measurements or piloting spacecraft, none of this is of much practical importance. But it may have a deeper significance. It is often said that the greatest impact of going to the Moon in the first place, a couple of billion seconds ago, was the opportunity it provided to gaze with wonder upon the vital complexity of the Earth. The prospect of lunar co-ordinated time allows one to look with a similar otherworldly perspective at the interaction of fundamental theory, technical wizardry and punctilious international co-ordination that allows the measurement of space and time to be as precise, and thus of such immense practical use, as it has become on Earth. ■