Establishing a Lunar Time Zone: The Challenges NASA Faces
The US government has tasked NASA (and other federal departments) with a challenge: establish "time standards" around celestial bodies (including the Moon). While this is often summarized as "coming up with a time zone for the moon", the reality is far more complex.
Initially, you might wonder why they wouldn't just arbitrarily pick some existing time zone to use on the moon. For example, perhaps US Pacific Time because of where NASA headquarters is located, or perhaps a bit more neutrally, Coordinate Universal Time (UTC). which is the approach taken on the International Space Station.
However, to understand why this solution is not sufficient, we need to examine the fundamental purpose of time zones and the unique challenges the Moon presents.
First, there's the issue of giving labels to moments in time. If I want to meet up with someone, it's very useful if I can say "Let's meet at 2PM" and we both agree on what point in time "2PM" refers to.
Note that this labeling does not need to correspond consistently to any measurement of duration. For example, the amount of time between 2AM and 3AM might not be an hour. It might actually be instantaneously, as is the case when the daylight saving time standard tells us to move our clocks forward by an hour. As a society, we agreed to label one point in time "2:00AM", and then to label a moment one minute later as "3:01AM". This is fine, as long as everyone agrees on the label and realizes that the labels are not intended to be used to measure durations.
Time zones don't exist to provide a precise measurement of duration, but rather to align our social clocks with the natural cycle of daylight and darkness. On Earth, this works well - when our clocks read "2 PM," the sun is generally high in the sky, and when they read "10 PM," it's dark outside.
Note that it's completely possible to ignore this desire, and in principle possible for the world to just use a single time zone such as UTC, allowing that in some places in the world, it might be night time during the time period labelled as 2PM.
There are some arguments about circadian rhythms, but these are easily overcome in practice, with examples including research stations in the antarctic where there is almost no correspondence between the labels of the time, and whether the sun is up in the sky.
However, there are problems with using UTC even just on Earth when measuring durations. The issue is that a day on earth is not 24 hours long — in fact, it's not any specific duration long, as the length of a day varies. We need to occasionally insert leap seconds into our labeling system to preserve the property that times with day-like labels occur when the sun is up, and night-like labels occur when the sky is dark.
Adding another complication on top of that, is that for the vast majority of computers and smart phones, rather than periodically add a leap second now and then, whenever we determine we need to adjust UTC by a second, we "smear" out that second across the next twelve hours. This was deemed to be the scheme least disruptive to everyday human societies, but it means all of our duration measurements: hours, minutes, times, milliseconds, nanoseconds, etc., are all slightly off.
If you want a labeling system of time that faithfully keeps track of duration, the thing you're looking for is International Atomic Time, or TAI.
So why don't we just use TAI as the time zone on the moon? Well, there's yet another complication: Einstein's theory of relativity.
Time actually elapses differently in different gravitational fields. The moon's gravity is sufficiently weak (compared to Earth's) that time actually elapses about about 56 microseconds slower per (Earth) day. This means that even if someone on Earth and someone on the moon agreed on a label for a particular point in time, say "2:00:00 PM", and even if they agreed that "2:00:01PM" should refer to "the point in time 1 second later", they would still disagree on what point of time that second label would refer to, because the duration of 1 second differs for the person on Earth relative to the person on the moon.
These differences are important for navigational systems. Even just on Earth, the GPS systems you use to get driving directions in your car have to take into account relativistic effects on the signals being sent from the artificial GPS satellites orbiting Earth in order to accurately determine your position and give you correct directions. These issues only become more important if we plan on using automated navigational systems to, for example, control a spacecraft docking into a space station.
Okay, so maybe we can just define an Earth-TAI and a Moon-TAI, which may tick at different rates due to the gravitational effects, and when Earthlings and Moonlings communicate with each other, they'll just have to say which time zone they're using, just as someone in Canada might have to tell their friend in Tokyo which time zone they're using when they give a time label.
Well, there's yet another complication: Relativity also shows that there is no objective sense of simultaneity. That is, two events which may appear to be simultaneous to someone on Earth might not be seen as simultaneous to someone on the moon.
So someone on Earth can say "Event A happened at 2:00:00PM Earth-TAI, and Event B also happened at 2:00:00PM Earth-TAI" (i.e. both events happened simultaneously to the Earthling). Someone on the moon might measure Event A to have happened at 2:00:37PM Moon-TAI, but for Event B to have happened at 2:00:38PM Moon-TAI (i.e. they happened one second apart to the Moonling). So do we say that 2:00:00PM Earth-TAI corresponds to 2:00:37PM Moon-TAI? Or does it correspond to 2:00:38PM Moon-TAI?
It's actually impossible, in general, to perform a conversion in this naive manner. We're going to have to come up with something more sophisticated to handle these situations. NASA's task, then, is to create a coherent time system that accounts for the unique temporal characteristics of the lunar environment. This system must be able to provide accurate timekeeping, facilitate synchronization between Earth-based and lunar-based operations, and ultimately support the safe and efficient exploration and settlement of the Moon.