Options for Martian Timekeeping

Copyright © 1997 by William Woods

wwoods@ix.netcom.com

Introduction

Mars is unique in the Solar System.

The natural conditions of the moons and asteroids are so harsh that humans will have to surround themselves with artificial environments. They will not need to change the systems of timekeeping we use on Earth. When they care about the position of the sun in the sky, they will find it out the same way that we find out the state of the tide: table look-up.

By contrast, the environment of Mars is sufficiently hospitable that explorers and colonists will surely choose to adjust their lives to fit its pattern of days and seasons. This article examines the choices they (we!) can and should make.

The primary, immovable constraints are set by nature: the length of the day, the length of the year, and the climatic cycle. The secondary constraints are set by human nature. In the first draft of this article, the phrase "for convenience, something should be thus-and-so" kept appearing. I've excised most of these; instead I'll say up front that this is an very important consideration. For instance, The day could be divided into 24 regular hours plus a short 25th hour, but this is needlessly inconvenient - therefore it's wrong.

Days

Studies have shown that humans operate on a ~25 hour internal clock, which is reset every morning, so adjusting to Mars' 24 2/3 hour day will pose no problem.

The SI second must remain the fundamental unit of time. Unfortunately, there is no reasonable way to use it as a subunit of the 88775.244 sec Martian day. For shift work, the Mday{1} must be divisible by 2, 3, and 4; hence by 12. Twenty-four Mhours would obviously work very well, but 60 might work even better.

The Mhour need not be divided into 60x60 Msecs{2}. Instead we could decimalize, dividing the Mhour into 1000 or 100x100 Msomethings.

Seasons

Since the Myear is so long, it could plausibly be divided into six or eight seasons. Or even into five, if the long slow trip past aphelion seemed to justify having two northern-summer seasons.

Choosing the number of seasons restricts the choice for the calendar, since each season should be divided into the same number of whole months, and each year should be a set of whole seasons.

The Martian year should be divided into seasons which divide the climatic cycle into coherent chunks, never mind the Martian Celestial Latitude of the sun. Getting this wrong forces people to talk about, for instance, the dust storms of the southern summer as happening in the late Spring & early Summer. The dust storm period should be inside one big season, or two small ones.

On the other hand, there's no point in being too fussy, since the climatic cycle varies with location, especially with latitude. In parts of California, summer weather lasts more than half the year, while in Minnesota, "last year, summer fell on a Wednesday".

On Earth, the old convention made summer and winter the quarters of the year with the most and least insolation. Summer ran from May Day through the solstice (Midsummer Day) to Lammas. Winter ran from Halloween to Groundhog Day. For some reason we shifted to the modern convention, in which these seasons run from solstice to equinox.

Because of thermal inertia, the warmest and coldest quarters lag about a month behind the brightest and darkest quarters.

On Mars, with a long year and no oceans (yet), the temperature must follow the insolation pretty closely. Except at the poles of course, where the seasonal dry-ice caps keep the temperature from falling below the sublimation point in the winter, and then keep it from rising well into the spring, after which it leaps up. Between the Tropics of Virgo and Pisces the variation of insolation is driven by the orbit's eccentricity, which takes Mars from 207 to 250 gigametres out from the sun.

Years

A 668-day year can be divided 20 months of 33 or 34 days, grouped into four or five seasons. A 672-day year can be divided 24 four-week months (or 12 awfully long ones), grouped into 4, 6, or 8 seasons.

The excess days can be subtracted from the last month of four seasons, or all from the last month of the year.

With 668.592 days per year, the rule for intercalary days is pretty simple: years divisible by two and/or five have 669 days, years ending in 1, 3, 7, or 9 have 668 days (or alternately, years ending in 2, 4, 6, and 8 are short, the rest are long). For additional accuracy, centennial years are short, except for years divisible by 500. The 669th day should be added to the last month of long years.

Month Names

The Earth names for months could be adopted and extended: ... , November, December, Undecember, Duodecember, ... , perhaps as far as Quattourvigintember - but that would be awfully clunky. The names of months should be acceptable universally, preferably not even needing translation. For mnemonic convenience, they should be ordered in some way - alphabetical, numerical, chronological, ...

Naming them for constellations would do nicely. Perizodiacal constellations can be used to pad the list of the canonical dozen.

Year Numbers

Putting the year 1 of the Martian Era in the recent past puts most of history on minus time. For convenient conversion, 0.0 ME should be close to AD 0.0 (actually, 1.0 BC). Then

ME xxxx = AD yyyy/1.880868 + <a small constant>.

However, the fiducial date should be modern. Mars had a southward (autumnal) equinox {3} at about

AD 1975.1 ~= 1050.1 ME.

This implies that the southward equinox should be in the first season of the Myear. This one could be defined as ME 10../dd, hh.mmss Airy Mean Time. Martian Celestial Longitude could be measured from the First Point of Taurus, a nice match for Earth's Aries.

Options:

Mday = 24 hours (3699 secs)

= 24 x 60 minutes (61.65 secs)

    = 24 x 60 x 60 Somethings (1.0275 secs)

    = 24 x 60 x 100 Somethings (0.6165 secs)

= 24 x 100 minutes (36.99 secs)

    = 24 x 100 x 100 Somethings ( 0.3699 secs)

Mday = 60 1st units (24.67 Emins = 1480 secs)

    = 60 x 60 2nd units (24.66 secs)

      = 60 x 60 x 60 3rd units (0.4110 secs)

    = 60 x 100 2nd units (14.8 secs)

      = 60 x 100 x 10 3rd units (1.48 secs)

      = 60 x 100 x 100 3rd units (0.148 secs)

Myear = 4 seasons (167 or 168 days)

      = 4 x 5 months (33 + 34 + 33 + 34 + [33 or 34] days)

      = 4 x 6 months (28 +..+ 28 + [27 or 28] days)

    = 5 seasons (133 or 134 days)

      = 5 x 4 months (33 + 34 + 33 + [33 or 34] days)

    = 6 seasons (111 or 112 days)

      = 6 x 4 months (28 + 28 + 28 + [27 or 28] days)

    = 8 seasons (83 or 84 days)

      = 8 x 3 months (28 + 28 + [27 or 28] days)

Mars Data

Mday = 1.02749125 Edays (Mday = Martian mean solar day)

    = 88775.244 SI sec

    = 24.659790 Ehours = 24h 39m 35.244s

Myear = 686.9725 Edays (Myear = Martian tropical year)

    = 1.880868 Eyears

    = 7.5 Eseasons

    = 59354420. SI sec = 668.592 Mdays

For example:

Supposing a 24-month, four-season year, with solstices ~ in middle of 2nd and 4th seasons ( MDate 1050.151 = EDate 1975.124/Myear + 0.038 ).

MDates are +/- a day or so, since an Mday overlaps 2 or 3 Edays.


EDate           Event                      Lsun   MDate 
-------------------------------------------------------------- 
Feb 16, 1975    Southward Equinox           180   1050/4/17
Aug 26, 1996    Northward Equinox             0   10../8
Jan 29, 1997    Aphelion                     70   10../21
March 13, 1997  Northern Solstice            90   10../7
Sept 12, 1997   Southward Equinox           180   1062/4/18
Jan 7, 1998     Perihelion                  250   1062/8/21
Feb 6, 1998     Southern Solstice           270   1062/9/22
July 14, 1998   Northward Equinox             0   10../9
Dec 17, 1998    Aphelion                     70   10../22
Jan 29, 1999    Northern Solstice            90   10../8
Nov 7, 1996     Mars Global Surveyor Launch       10../23
Dec 4, 1996     Mars Pathfinder Launch            10../22
July 4, 1997    Mars Pathfinder Landing           1062../PRE>

Sept 12, 1997   Mars Global Surveyor Capture      1062/4/18

EDate           Mday#   MDate 
-------------------------------------------------------------- 
May 30, 1997        1   1062/1/1 
Jun 28, 1997       29   1062/2/1 
Jul 27, 1997       57   1062/3/1 
Aug 25, 1997       85   1062/4/1 
Sep 23, 1997      113   1062/5/1 
Oct 21, 1997      141   1062/6/1 
Nov 18, 1997      168   1062/7/1 
Dec 17, 1997      196   1062/8/1 
Jan 15, 1998      224   1062/9/1 
Feb 12, 1998      252   10../1 
Mar 13, 1998      280   10../1 
Apr 11, 1998      308   10../1 
May 9, 1998       335   10../1 
Jun 7, 1998       363   10../1 
Jul 5, 1998       391   10../1 
Aug 3, 1998       419   10../1 
Aug 31, 1998      447   10../1 
Sep 29, 1998      475   10../1 
Oct 28, 1998      502   10../1 
Nov 25, 1998      530   10../1 
Dec 24, 1998      558   10../1 
Jan 22, 1999      586   10../1 
Feb 19, 1999      614   10../1 
Mar 20, 1999      642   10../1 
Apr 17, 1999        1   1063/1/1 



Footnote 1: The term "sol" is sometimes used for the Martian day, but this is usually unnecessary. When the context is ambiguous, I use E- and M- to distinguish between Earth and Mars units.

Footnote 2: In fact, to avoid confusion, the smallest unit must NOT be called the "second", or anything which would be shortened into "sec" or "s". In this case a new name really is needed.

Footnote 3: On Earth, it is natural to describe the equinoxes and solstices by the seasons in the northern hemisphere, since only a very small fraction of the land and population is in the south temperate zone. On Mars the situation is quite different so I suggest using southward, southern, northward, and northern in place of autumnal, winter, vernal, and summer.