Time of the sol

Submitted by mossy2100 on Tue, 04/25/2017 - 01:16

The convention used by NASA for marking the time of day on Mars is to use a “stretched clock”. Because a sol is 2.75% longer than a day, it can be divided into Martian hours, minutes, and seconds that are each 2.75% longer than their Terran counterparts.

However, Mars gives us an opportunity to do things differently, and decimal time is an improvement. It has been difficult on Earth to introduce decimal time for the same reason that Americans still use miles, i.e. cultural inertia. But on Mars we have the luxury of creating new and better systems from scratch, which is, after all, half the reason we’re going there in the first place.

The main advantage of having 24 hours in a day is that it is easily divided into 2, 3, or 4 equal periods. Sexagesimal minutes and seconds permit hours and minutes to be easily divided into 2, 3, 4, 5, or 6 equal periods.

The only disadvantage of decimal time in this regard is that dividing time periods by 3 or 6 is not as easy. That’s fine, however; after all, we make do with decimal calculations for every other physical measurement. Using decimal time means that calculations producing fractional sols do not then need to converted to sexagesimal in order to find the time of day. Internal representations of time, whether as sols, millisols, or microsols, can be displayed directly and understood. Decimal time is already used in some contexts on Earth; for example, some time tracking systems record durations in fractional days or hours, rather than hours, minutes, and seconds.

The decimal clock utilised in the Utopian Calendar is similar to French Revolutionary Time introduced in France during the 18th century. A day was divided into 10 decimal hours, each of which were divided into 100 decimal minutes. Each decimal minute was further subdivided into 100 decimal seconds. An earlier version of the Darian Calendar used this same division of units.

Decimal clocks for Mars have been proposed numerous times, but two that I especially liked, and that provided inspiration, are those described by the great science fiction author Edgar Rice Burroughs, and Mars colonisation expert Bruce Mackenzie.

In “The Gods of Mars” (1914), Burroughs describes the Martians’ clock as follows:

Martians divide [the day] into ten equal parts [zodes], commencing the day at about 6 A.M. Earth time. The zodes are divided into fifty shorter periods [xats], each of which in turn is composed of 200 brief periods of time [tals], about equivalent to the earthly second.

Thus, in Burroughs’ scheme:

1 sol = 10 zodes
  = 500 xats
  = 100000 tals
1 zode = 50 xats
  = 10000 tals
1 xat = 200 tals

A “zode” is thus one tenth of a sol (i.e. a decisol), and a “tal” is 10‑5 sols, or 10 microsols.

The clock described by Mackenzie is as follows:

1 sol = 25 horas
  = 100 quarters
  = 1000 mils
  = 100000 beats
1 hora = 4 quarters
  = 40 mils
  = 4000 beats
1 quarter = 10 mils
  = 1000 beats
1 mil = 100 beats
1 beat = 10 microdays

The word “hora” is Greek for “hour”, and the unit “mil” is an abbreviation of “milliday”. The unit “beat” comes from “heartbeat”, since the human heart beats around 100000 times per day or sol. A beat is equal to Burroughs’ tal. Mackenzie also offered “centiday” as an alternative unit for “quarter”.

Although it may be useful to have units of time similar in duration to Terran hours, minutes, and seconds, I have opted for a simple, fully decimal approach, with units derived using SI prefixes.

1 sol = 10 decisols (dsol)
  = 100 centisols (csol)
  = 1000 millisols (msol)
  = 106 microsols (µsol)
  = 109 nanosols (nsol)

Mackenzie notes in his paper that the French system never caught on, however, I suspect this may have been due to the cultural momentum of 24-hour time among the wider population. Martians, however, will initially be small in number, and will have the mindset that they are creating a new legacy-free culture. They will expect or even desire to do things differently and better than Terrans, and on this basis may be more willing to adopt decimal time.

Comparison with Terran units gives a sense for how long each of these units is:

1 dsol = about 2.5 hours
40 msol = about 1 hour
1 csol = about 15 minutes
1 msol = about 1.5 minutes
10 µsol = about 1 second
1 µsol = about 0.1 seconds

Rather than start the day around 6 a.m. as Burroughs describes, each sol begins at midnight, as on Earth.