Lunar Time

Environmental & Science Education, STEM, Solar System, Astrophysics, Earth and Space Sciences, Cosmology, Nature of Science, History of Science 

"What time is it?" "What time did X arrive or leave?" "What time is my appointment?" These and others are common questions and it is easy to assume that telling time is simple. It is one of those "not so fast" assumptions, e.g., defining time on a close neighbor: lunar time. Here, I avoid the fact that there is no such thing as time in the world of theoretical physicists and cosmologists. That is very heady stuff.

And as you will see telling lunar time requires much more precision than for that of an appointment here on earth so scientists seeking an answer to the question have something else in mind when they ask it. It has to take the physics of the cosmos into account to answer it.

The question of lunar time is the subject of an interesting essay, one with useful diagrams, including the construction of  moon clock, characterizing time and why this is an important question in the journal Nature by science writer Elizabeth Gibney. A few quotes give you a flavor and tickle, I hope, your interest in reading the full article.  

--"Although the definition of the second is the same everywhere, the special theory of relativity dictates that clocks tick slower in stronger gravitational fields. The Moon’s gravitational pull is weaker than Earth’s, meaning that, to an observer on Earth, a lunar clock would run faster than an Earth one. (Cheryl) Gramling, (head of the position, navigation and time team at NASA's Goddard Space Flight Center) estimates that a lunar clock would gain about 56 microseconds over 24 hours. 

Compared with one on Earth, a clock’s speed would also subtly change depending on its position on the lunar surface, because of the Moon’s rotation, says (Patrizia) Tavella (who leads the time department at the International Bureau of Weights and Measures in Sevres, France) 'This is a paradise for experts in relativity, because you have to take into account so many things,' she adds."

--There is a need, "the most pressing (one) comes from plans to create a dedicated global satellite navigation system (GNSS) for the Moon, similar to how GPS and other satellite navigation networks enable precise location tracking on Earth. Space agencies plan to install this lunar GNSS around 2030," necessary if all missions to the moon are to be covered." The reason an official lunar time reference is necessary is  summarized by Jorg Hahn, an engineer working on the recently funded ESA Moonlight project. "'All this has to trace to one kind of a time reference, otherwise you have chaos and things do not work together."

The story includes discussion of several open questions and contribute to an appreciation of the difficulty of the question.