(from information supplied by
Sullivan, professor of astronomy at the University of Washington in
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
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
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
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
|Object ||English ||French|
|Sun ||Sunday ||Dimanche|
|Moon ||Monday ||Lundi|
|Mars ||Tuesday ||Mardi|
|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
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
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!