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Marking Time on Earth and Mars

(from information supplied by Woody Sullivan, professor of astronomy at the University of Washington in Seattle)

Did you ever wonder where our time system came from? Why is a year 365 days? Why are there seven days in a week? 60 seconds in a minute? Why do our calendars equal 12 months? And what about keeping track of time on another world? What should be the units of time on our neighboring planet Mars?

Time has always been central to the development of civilizations and of science. The first accurate measurements of time were the result of careful observations of the heavens. Units of time based on these astronomical observations are the day, the month, and the year. They are defined by the motions of the Earth, the Moon and the Sun and form the basis for the calendar we use today.

The Day — is the rotation period of the Earth. The rising and setting of the Sun defines the familiar alternation of the day and night and controls our biology and our civil affairs. The Earth spins or rotates on its axis once per day, creating an ever-changing view of the sky for humans living on this moving platform. The axis of rotation is the line extending through the South and North Poles. The time-honored way to define the day is to track the Sun as it rises in the eastern sky and moves steadily to the west. This motion in fact defines the directions we call East and West.

The Month — is the orbital period of the Moon about the Earth. The changing shape of the Moon has long been one of the most fascinating phenomena in the sky. The Moon regularly cycles through its phases changing from a thin crescent to a half-lit disk to a fully lit disk, and then shrinking back down again. This cycle turns out to be extremely regular in its length and in all cultures it has been used as a natural unit of time, the “moon-th” or month.

The Year — is the orbital period of the Earth about the Sun. The Sun does not rise or set in the same spot on the horizon, but is constantly changing. On the spring or vernal equinox (March 20th most years) the Sun rises due east and sets due west. In addition, the length of daylight is the same as that of nighttime, hence the name equi-nox (equal night).

But on each succeeding day it rises a bit more to the north of east, passes a bit higher through the southern sky, and sets a bit more to the north of west. The hours of daylight continually increase until a maximum is reached on the summer solstice (June 21st) and the Sun stops its northern march and heads back towards the south (sol-stice = sun-stopping).

The next three months see a similar, but opposite daily movement of the Sun back toward rising due east and setting due west. This benchmark is eventually reached on the autumnal equinox (Sept. 23). In succeeding weeks the Sun continues heading south, mirroring its springtime northern motion, so that each day brings sunrise and sunset farther south. Finally, the Sun stops again at an extreme value on the winter solstice (Dec. 21).

The Calendar — Faced with the harsh reality of changing seasons, early humans needed a method to estimate the near future as accurately as possible. A reliable and predictable method was the annual cycle of the Sun. As discussed earlier, careful observation would reveal the four benchmarks of the year that we know as the solstices and equinoxes. When one of these points was reached by the Sun, the beginning of a new season could be declared. It would be only natural to count how many days occurred between these benchmarks and, basically, start on the road to finding the number of days in a year.

But the Moon also demanded inclusion in the calendar and so lunar phases would also be recorded to determine the number of days in a month and the number of months in a year. Here is where the first major problem of calendrics was encountered: the number of months in a year was 12, with about 11 days “left over.” What to do with the extra 11 days? Furthermore, the number of days in a month was about 29 — what to do with the extra half day?

Julius Caesar abandoned the Moon as a basis for the calendar and declared the year to have 365 days. Every fourth year would have 366 days — a leap day on Feb. 29. But with a long-term average of 365.25 days and the actual length of the year equaling 365.2422 days, the Julian Calendar was slipping one day for every 125 years. This was corrected with a slight modification by Pope Gregory XIII in the 16th century. His Gregorian Calendar is now the international standard. It differs from the Julian in that a few years that normally would be leap years are skipped. Only the century years divisible by 400 would be leap years. The Gregorian mean year of 365.2425 days keeps the calendar to within a day for another 2900 years!

There are other units of time that are basic to our culture that have nothing to do with the motions of the heavenly bodies. The week, the hour, minutes and seconds are non-astronomical units of time.

The Week — The basic origin of a set cycle of many days duration had to do with marketing, a common time for people to come together and exchange goods. In various cultures at various times, periods ranging from 3 to 16 days have been used for this purpose. The seven-day week that is now used is thought to have primarily originated with the ancient Hebrews. One theory for the birth of this seven day cycle is that it relates to the account in the Bibles’ Genesis where God took seven days to create the World. God rested on the seventh day.

Object English French
Sun Sunday Dimanche
Moon Monday Lundi
Mars Tuesday Mardi
MercuryWednesday Mercredi
JupiterThursday Jeudi
Venus Friday Vendredi
Saturn Saturday Samedi

But the origin of the seven days in Genesis may come from other reasons for preferring the number seven. For example, the ancients recognized seven planets in the sky. They were named Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. The present names of the days of the week directly relate to the seven ancient planets: In English there are Sun-day, Moon-Day, and Saturn-day. In French, the word for Tuesday is mardi (Mars’ Day), Wednesday is mercredi (Mercury’s Day), Thursday is jeudi (Jupiter’s Day), and Friday is vendredi (Venus’ Day).

The Hour — The Egyptians were apparently the first to divide the day into 24 hours, although actually it was into two 12-hour periods, one for the period of daylight and one for the nighttime. It is not exactly clear why the period of daylight was broken into 12 units, although it may be related to the fact that there are approximately 12 lunar months in a year.

The Minute and Second — Babylonians first introduce the sexagesimal system (a system based on the number 60), probably primarily because there are approximately 360 days in a year and the number 360 (and 60) is evenly divisible by a variety of factors (2, 3, 4, 5, 6, 12). The Babylonian number system was thus based on 60 just the same way that ours is based on 10. This made it natural to divide things into 1/60 parts. Eventually, one-sixtieth of an hour became known as a pars minuta (small part) in Latin, and a minute in English. One-sixtieth of a minute was a pars secunda (second part) our second.

Now what about Mars? What should be the units of time and the calendar system that would be natural for a denizen of Mars?

Think like a Martian when considering the following facts: The martian solar day, which has had the name sol since the Viking lander days in the 1970s, is 24 (Earth) hours, 39 minutes, 35.2 seconds long, or 1.02749 Earth days.

What should be the subunits of the sol?

An unimaginative answer would be to decide that 24 Martian hours (perhaps called “mhours”?) make up a sol. The peculiar division of our day into 24 units goes back at least to the ancient Egyptians and has nothing to do with Mars, so why choose 24? Is 10 mhours in a sol better? As a Martian, you’d better first count the number of fingers on your hands! And how would you break the mhour up into smaller units?

A Martian calendar is even trickier.

Mars’ axis of rotation has a similar tilt to that of Earth (25.2 degrees versus 23.4 degrees), which means that it similarly goes through four seasons (which need characteristically Martian names, please). But because Mars is much farther from the Sun, its year is a good bit longer, at 668.60 sols (686.98 Earth days).

What should be the name of the Martian year? And how should it be divided?

Here on Earth, we have 12 months in our year basically because our Moon happens to go around us roughly 12 times every time we orbit the Sun once. But Mars has two moons, Phobos and Deimos, that zip around every 7.6 and 30.3 Earth hours, respectively. You’re a Martian standing on a surface that is itself rotating once every 24.6 hours. The upshot is that speedy Phobos actually laps you about twice a day, rising in the west and setting in the east with an apparent period of 0.45 sols, while Deimos behaves more “normally” by rising in the east every 5.38 sols. Should there be a length of time called a phobe (or would a milliphobe be more useful?), or maybe a conjie for the interval of time between conjunctions of the two moons, when they pass each other in the sky and surely create an auspicious occasion for any Martian skygazer?

Whatever the answers, Mars surely deserves a set of unique and distinctly martian units of time!