Babylonia II: The Beginnings of Time
On the origins of calendars and clocks
Year 169, month XII, night of the sixth,
beginning of the night,
the Moon one cubit to the west of Alnath.
In the morning, the child was born.
~ Babylonian Horoscope
In Colorado, amid the foothills of the Rocky Mountains, we have built a clock so accurate it would not have erred by a single second in all the years since the Big Bang.
Such precise counting of time is possible only at the frontiers of modern physics. The clock measures the steady beat of electrons vibrating in atoms; a feat that required not only the use of vacuums and lattices of lasers, but also the careful elimination of every possible disturbance.
To do this, the builders of the clock isolated it from the outside world. It sits enclosed in a temperature controlled box, inside a vacuum. The mirrors in its optical systems are shielded with copper to reduce stray electrical fields, and its lasers are run at as low a power as possible.
In theory, a clock of a similar kind might someday reach almost perfect accuracy. But in reality this is impossible. The world we live in is messy, and no matter how smart we become, some disturbance will always remain. The perfect clock will never be built; just as no hand will ever draw the perfect circle or rule the perfect line.
Right now, however, there is a different question occupying the minds of the world's most accurate horologists. Since 1967, the second has officially been defined as the period necessary for a cesium atom to vibrate a precise number of times1. Sixty years ago, when atomic clocks first made measuring such things possible, this definition lay at the frontiers of physics. Yet today’s clocks are orders of magnitude better.
Proposals for changing this standard will thus be made in the coming years. The form it might take is not yet settled: one option simply suggests changing the definition to another atom, one that allows more precise measurement. Others think we should shift the base of time entirely, and define the second through the fundamental constants of nature.
Whatever they choose, our everyday lives will not feel the difference. The second will keep its length, plus or minus a few atomic vibrations, and for the past half century it has anyway been separated from the natural rhythms of the planets and the stars. Yet the influence of those cycles is still present. The length of the second is not really fixed because of an arbitrary number of atomic vibrations, but because scholars long ago tracked the stars and the Sun and the Moon, and so worked out a first accounting of time.
The origins of the second, indeed, and the reasons why we have sixty seconds in a minute or twenty-four hours in a day, go back a long time. Back to before the Romans named the months, and before the Babylonians decided how many there should be. Back, indeed, almost to the beginnings of civilization, when people first found a need to chart the rhythms of nature with precision.
Numbers
These days we count in orders of ten. The reasons for that seem obvious: we have ten fingers and ten toes, and so it is natural to base our number system on that rather than on, say, nine, or seven.
There is, however, another natural number to use: twelve. Each of our fingers has three phalanxes, or bones, and since we have four fingers on each hand, it is possible to use the thumb to count up to twelve on the first hand. The fingers and thumb on the other hand can be used to keep track of multiples of twelve. In this way it is relatively easy to count up to sixty.
Approaches like this are called sexagesimal systems, and they were widely used in the first civilizations of Egypt and Mesopotamia. It was probably in Egypt, around 1500 BC, that people first started keeping track of hours. When they did, they followed this numbering system, and divided the daylight hours into twelve sections.
Of course, the Egyptians did not have accurate clocks back then. They used sundials to approximate the time, and the twelve divisions simply followed the movement of the Sun across the sky. At night, when the sundial was useless, the Egyptians instead used the movements of the stars to track time. And again, they divided the stars, and thus the night, into twelve sections.
This did not really give the Egyptians the concept of fixed hours. The length of the day and night varies throughout the year, and so the period covered by each division would not have been constant. But it was the moment when the day was first divided into twenty-four sections.
Centuries later, about 130 BC, the Greeks worked out a way to standardise the hour at a fixed length. They kept the twenty-four hour system — such things are, after all, hard to change — and followed the sexagesimal system by dividing each hour into sixty minutes. Much later we added seconds. And so, in the end, the length of the second comes from the rotation of the Earth and the bones in our fingers.
Calendars, Months, and the Seven Gods
Counting in twelves may have seemed natural on our hands, but it was also reflected in the skies. Ancient peoples knew the length of the year — by the times of Ancient Egypt they’d worked out it had about three hundred and sixty-five days — and they also knew the Moon followed a repeating cycle twelve times over that year.
The phases of the Moon thus offered an easy way to track the passing year. Over a period of about twenty-nine days, the Moon changes in appearance from a faint crescent to a full disk, and then back again. Over a year it does this twelve full times, although a few days are always left over at the end.
In various places we have found bones scratched with either twenty-nine or thirty markings, which were probably early efforts to keep track of these lunar phases. Later, around 2000 BC, the Babylonian calendar adopted the lunar cycle as the basis of its time keeping.
That gave rise to the twelve months of the year. But it might also have given us the seven days of the week. Four sets of seven gave twenty-eight days, which is almost, but not quite, the period between two new moons. Seven was also represented in the skies: the five visible planets, the Sun, and the Moon would have stood out to the ancients, and certainly by the times of Babylon were thought of as gods.
The modern names of the week hint at such an origin. Sunday, the day of the Sun, Monday, of the Moon, Tuesday - Mars in Latin, Saturday for Saturn, and so on. Had there been another planet in the skies, or the Moon a little closer to Earth, things might have been different. But this is the way things are, and so it is that the planets dictate some of the rhythms of our modern lives.
Still, the use of the Moon as a calendar left a big problem. The Moon orbits somewhat irregularly, and its cycle is sometimes longer than twenty-nine days. Even worse, the twelve lunar cycles do not quite add up to a full year. A calendar based on the Moon alone will soon drift, and before long the summer months will fall in mid-winter.
The Cycle of Saros
The Babylonians were aware of this problem. To keep the calendar roughly constant, they would occasionally add an extra month to the year. In the beginning this was done in a somewhat arbitrary way, and by order of the King. Yet by around 700 BC, they seem to have worked out a better way.
The apparent irregularity of the Moon’s orbit is caused by three things. First is its regular orbit around the Earth. Second is the tilt of that orbit, which means it does not move directly around the equator, but is usually over some point north or south of it. And third is the shape of its orbit, which is not circular but elliptical, so that the Moon moves closer and further from Earth.
The interplay of these three things creates the irregular length of the lunar cycle. But it also creates a pattern in lunar eclipses. These occur when the Sun, the Earth, and the Moon line up, in that order. For the eclipse to occur their alignment must be pretty good — but since the Moon’s orbit is tilted, this does not always happen and so we do not get an eclipse every month.
At first glance, the dates of the lunar eclipses instead seem scattered across the calendar. But thanks to the three cycles, they actually follow a long and repeating sequence stretching over eighteen years, eleven days, and eight hours2. At the end of this period the Earth, the Moon, and the Sun will all be in more or less the same place they were at the start, and so the cycle can begin again.
This repeating pattern is called the Saros Cycle, and the evidence suggests the Babylonians figured it out around 750 BC. At the same time, they adopted a more rigorous way of watching the skies, and started compiling a long series of tablets we now called the Astronomical Diaries. In them they recorded everything from the movements of the planets to the price of wheat, and sought to link the two through omens and astrology.
From these tablets, we know the Saros Cycle gave them a tidier way to manage their calendar. Once they knew that the Moon followed a regular eighteen-year long cycle, they could manage the addition of extra months to the calendar in a more regular fashion. And later, they could even dispense of the need to look at the Moon altogether.
Eventually, we found ways to move beyond the wobbling planet we live on. The basis of modern clocks lies not in the motions of the Sun and stars, but in the innards of atoms. Priests and kings no longer track time for us: today we rely on an army of atomic clocks and satellites. Yet the ideas behind the seconds, the days, and the months stretch back into time immemorial. They are, in a very real sense, a natural consequence of the world we came from.
Precisely, one second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. Clear?
Solar eclipses follow the same cycle, but they are only visible across a narrow band on the Earth’s surface. And though the eclipses repeat every eighteen years, the eight hours at the end of the cycle means they do so with the Earth’s surface offset by a third. From any particular viewpoint, then, this pattern is almost impossible to spot. The cycle of lunar eclipses was thus easier for the Babylonians to spot.
I've always wondered why 12!!
Time is one of the greatest mysteries! Is it controlled by gravity or distance or memory? Take your pick!