Rover operators attempting to maximize productivity on the Moon would benefit from a calendar and a timekeeping system specific to the requirements on that world. This is a very different thing than the use of a Lunar calendar on Earth.   On Earth, calendars help in predicting seasons, tides, eclipses and other phenomena of interest on Earth.

What is of interest for Lunar operations  as far as a time system?  An Earth day measures 86,400.002 seconds, with a variation of plus or minus 7.9 seconds over the course of a year, mostly due to tidal influences.  On Earth, in the modern world, we measure a day’s cycle from midnight as a tradition.  In other cultures and times people have measured a day from sunrise, and from high noon.

For the Moon, things are very different.   The length of a “lunar day” is some 656 hours, plus or minus about 7 hours.  There are about 13 new moons in a year, and there is a 18 year “libration cycle” that must be incorporated in the calculations.  There is useful information in knowing which “lunar day” it is, as the days vary by plus or minus some eight hours.  There is useful information in knowing the place of the current lunar day in the 18 year libration cycle.  But there is no useful data in knowing the “year.”

Let’s enumerate the aspects of timekeeping which would be useful.  First, understand that timekeeping as we know it has no function except for humans.  Machines can figure time entirely different ways than we do.  They don’t need time based on sunrise or months or years.  Machines would be completely happy with a binary number starting at zero at the time of the Big Bang.   This would easily fit in a 16 digit hexadecimal number.  In English that means that 64 ones and zeros will encode time for that period to a millisecond precision.  Machines would be totally happy with that!

But we are humans, so if we had colonies of rovers (or humans) on the Moon, what style of timekeeping would we want?  What would be convenient for us?

FOR THE ENTIRE MOON, SOMETHING LIKE UNIVERSAL TIME – WHAT WE CALL UTC OR GREENWICH TIME

1.  The “lunation number”, which the preferred version is the NASA derived number of the lunar cycles with the first one in the year 2000 being 1.
2.   The count of the Earth hours and minutes and seconds in the current lunation.
3.   The precise moment when twilight occurs at the “prime meridian,” the longitude of ULT.   As the Moon has the same side facing the Earth continuously, the prime meridian is that longitude line which has the Earth exactly centered east to west.

Since it’s extremely important the operators of lunar equipment know when twilight occurs, so that they can have an orderly shutdown of activities, it seems reasonable to have the day clock count down, instead of up.  Zero time for a day, or the start of a new day, then is the moment of twilight.  Operators must inform themselves of the moment of sunrise, which might be something like the hour LT 400:328 or LT 400:332.  But if an operator was late starting work, that doesn’t matter much, where if he was caught at sunset in the middle of a work process, work could be lost or damaged.

FOR A SPECIFIC LOCATION ON THE LUNAR SURFACE

1. Time of sunrise and sunset.  This implies that the exact length of each lunar day is embedded in the calendar, even though they vary slightly.   As an example, the Moon’s “Day” varies plus or minus seven hours from 27.7 Earth days.  The lunar calendar may use the exact actual lengths of each lunar day, as they can be predicted accurately hundreds of years in advance there is simply no reason not to.
2. The location in the sun at Lunar Noon from the specified latitude and longitude. This will delineate the area of time around high noon when shadowing is minimal and when ground travel is inadvisable.
3. The specific times when solar eclipses will occur at the local spot.
4. The times at which the Earth has appearance equivalent to the new Moon, the quarter Moon, the Full Moon, and so forth.
5. Times of lunar day and night at the location, and times of Earthshine at night, as well as full darkness (starlight only).
6. The correspondence between the Basic Lunar Time and Greenwich mean time on Earth.

A measure of Lunar Time (LT, and ULT), where the first number is Lunation Number(LN),  might be as follows –

LT :Hour:Minute:Second

where Hour:Minute:Second countdown to last daylight.

and…

ULT(lunation number):Hour:Minute:Second

where Hour:Minute:Second countdown to last daylight at the Prime Meridian (Longitude 0)

It is very useful for someone in Central Standard Time to be able to add seven hours and get London time.  They can make their call without waking someone up at 3AM!  But this does not mean that the London time needs to be directly encoded, only that it should be easy to compute.

But this is based on cultural standards from a pre computer, pre networked era.  There is likely no need for “time zones” on the Moon.  If there is a work site at which local sunrise is 1 hour 37 minutes later than sunrise at the Prime Meridian, let’s just assume it’s okay, understandable, that the convention is simply to name the location and the offset –

Coombs Base (40 degrees west of prime meridian)

CBLT thus is offset – ULT + 656/360 hours/degree * 40 degrees

CBLT = ULT + 072:53

For convenience  we presume a perfect sphere, resulting in sunrise being seventy two hours and fifty three minutes later at Coombs Base than the Prime Meridian.

Using this convention allows for high precision, since due to the small radius of the Moon, the local horizon is only about two kilometers distant.  This means that variations in terrain can affect the moment of sunrise.  Hence a straight computation as if the Moon were a perfect sphere could be wrong considerably.  The facts of the locality must be used.

Translations between the chosen format for Lunar time and Earth time must be done via a converter tool, a calculator program.  The Lunar time may convert to UDT (Zurich) time, then any user on Earth may add his offset to get the local equivalent time.

Have other attempts at a lunar time been presented?

A group has started a website, lunarclock.org, which proposes and advocates a “Lunar Standard Time(LST).” Although interesting, this is VERY Earth-centric. For example, zero time is when the Apollo first set foot on the moon, and the 12 months are named after the 12 Apollo astronauts. This is not of interest from the point of view of the objectives of operators of Lunar rovers and equipment.

www.lunarclock.org