by Bruce Mills
Sometime in the future, Mars will be colonised. When this happens, the settlers will require time units to divide the day into more convenient portions, as we do today on Earth. The Martian mean solar day is 24 hours 39 minutes 35.24409 seconds, or 88775.24409 seconds. This day is sufficiently close to the length of an Earth day to cause confusion if the Earth system of hours, minutes and seconds was adopted. I will derive decimal time units that can be used as an alternative to hours, minutes and seconds.
Decimal time is not a new idea. However, I have not yet found a commonsense derivation of decimal time units that are based on practical human needs and human rhythms.
The metric system (SI system) of measurements is primarily based on multiples of 1000. As a first approximation, let’s devise some units based on millidays and microdays.
| Base unit | Subunit | Earth equivalent (Seconds) | Earth equivalent (other units) |
|---|---|---|---|
| 1 Martian day | 1000 millidays | 88775.24409 standard seconds | 24 hours 39 minutes 35.24409 seconds |
| 1 milliday | 1000 microdays | 88.77524409 standard seconds | 1 minute 28.77524409 seconds |
| 1 microday | 0.08877524409 standard seconds | 88.77524409 milliseconds |
The milliday shows promise as a reasonable time division, being approximately equal to 1+1/2 minutes. There is even a name for an obsolete time unit of 1+1/2 minutes - the moment. Before the meaning of the word “moment” got broadened to mean merely “a short period of time”, it was once a medieval measurement unit with a precise definition. 40 moments made one hour, so one moment had a length of 90 seconds. This very close match to the length of the Martian milliday makes the choice of the name “moment” for this length of time an obvious choice.
What about smaller time divisions?
The microday is too short to be a convenient Martian equivalent of a second. There are about 11 Martian millidays to the second. If one was to count off these periods of time, to count fast enough, one would have to count by tens. Why count by tens when one could count by ones instead? Therefore, the next convenient division of the day would be 10 microdays, or 0.8877524409 seconds.
Several names could be used for this unit, but I have chosen the name “tick” for it.
It is not natural for most people to time events to the nearest minute, so time units larger than the moment would also be required for convenience. If these time units are to be powers of 10, there are two possibilities: dividing the day into 10, and dividing the day into 100.
The division of the day into 10 parts would result in time units of about 2+1/2 hours in length. This is too large to be of any practical use, because there are few human activities where this time period would be useful. The French tried introducing time units of this length after the French revolution. It is not surprising that their endeavour failed.
The division of the day into 100 parts is more promising. These units would be about 14+3/4 minutes in length. Four of these units are only about 50 seconds short of being an hour, so it’s clear that this unit can be used in a similar way to the hour. It is also small enough to be used for such needs as medical appointments, where the hour would typically be divided into smaller units. It is not a perfect match – sometimes appointments are scheduled in 10-minute blocks instead of 15-minute blocks – but as a replacement for the hour it is quite suitable.
No obvious name suggests itself, so I will use the name “centiday” until a better name is found.
The table of time units now looks like this:
| Base unit | Subunit | Earth equivalent (Seconds) | Earth equivalent (other units) |
|---|---|---|---|
| 1 Martian day | 100 centidays | 88775.24409 standard seconds | 24 hours 39 minutes 35.24409 seconds |
| 1 centiday | 10 moments | 887.7524409 standard seconds | 14 minutes 47.7524409 seconds |
| 1 moment | 100 ticks | 88.77524409 standard seconds | 1 minute 28.77524409 seconds |
| 1 tick | 0.8877524409 standard seconds |
This results in time divisions of 100:10:100. These divisions would divide the day into 100, 1000 or 100000 parts. The traditional divisions of 24:60:60 divide the day into 24, 1440 or 86400 parts. The two smallest time divisions are both comparable in size.