HISTORY36 - Measuring Time

I was born on March 29, 1940 at 5:35 am.  I know that because of the existence of an accurate calendar that depicted the year, month, and day of my arrival, and a clock that displayed the early morning hour and minutes past the hour.  (And because someone, other than me, wrote it down on my birth certificate.)  So where did accurate calendars and timekeeping devices come from?  That is the subject of this article.

My principal resources were Britanica.com, “Calendar Chronology,” and ScientificAmerican.com, “A Chronology of Timekeeping,” along with numerous other online sources.

Prehistoric Peoples

The only measures of time available to primitive peoples were the solar day (the space between successive sunrises) and the lunar month (the space between two successive new moons).  So, they reckoned the passage of time, counting suns, or days, and moons, or months.  They counted “years” broadly, noting when leaves began sprouting on a particular tree or describing someone having lived through a certain number of harvests.

Compared to a day or a month, the length of a year, the time it takes the Earth to complete a full circuit around the Sun, was much harder to measure.  By 3,500-3,000 BC, prehistoric people in Europe were employing structures, consisting of a large mound of earth and stones, containing an underground passage, open at both ends, that was precisely oriented so that internal objects were illuminated every 365 days - illustrating their knowledge of the length of a solar year.

Early Civilizations

Calendars to measure periods of time were invented all over the world, including the Middle East, Greece, Persia, India, China, and Mesoamerica.  There were also Jewish and Islamic calendars.  My focus in this article is the development of today’s modern Western calendar, which derives primarily from Egyptian, Babylonian, and Roman calendars. 

According to archaeological evidence, the Egyptians and Babylonians began to measure time at least 5,000 years ago.

Egypt and Babylonia introduced calendars to organize and coordinate communal activities and public events, to schedule shipment of goods, and, in particular to regulate cycles of planting and harvesting.  They based their calendars on three natural cycles:  the solar day, the lunar month, and the solar year.

 

This is where the roots of the modern Western calendar were born.

Egyptians.  Ancient Egypt coalesced in the Nile Valley around 3,100 BC and lasted until 30 BC when it was conquered by Rome. 

The earliest Egyptian calendars were based on the moon’s cycles, but the lunar calendar failed to predict a critical event in their lives: the annual flooding of the Nile river.

The ancient Egyptians used the annual appearance of the star Sirius on the horizon at dawn as a measure of the solar year of 365 days.  Certain difficulties arose, however, because of the inherent incompatibility of lunar and solar years.  To solve this problem, as early as 2,800 BC, the Egyptians formulated a year of 365 days, divided into 12 months of 30 days each.  To complete the solar year, five extra days (called intercalated days) were added at its end, so that the year was equal to 360 days plus the five extra days. 

Because a solar year is actually 365 ¼ days, every four years, their calendar would fall behind the solar year by one day.  After 1,460 days, or four periods of 365 days, the calendar would once again be consistent with the solar year.

 

Illustration of an ancient Egyptian calendar.

 

The new year was established by the rising of the star Sirius at sunrise on 19 July.  This event was seen as bearing a direct causal relationship to the simultaneous start of the Nile's annual flooding.

The ancient Egyptians gave their months numbers rather than names; nor was there a given "start date" from which all subsequent years could be counted.  Each Pharaoh’s reign began with the year 1.

As was customary in early civilizations, daylight was divided into 12 parts, and the night likewise; the duration of these periods, called temporal hours, varied with the seasons and the latitude of the location.  Summer hours were long, winter ones short; only at the spring and autumn equinoxes were the hours of daylight and darkness equal.

Both sundials (for daytime use), which indicated time by the length or direction of the sun’s shadow, and water clocks (for night-time use), where falling water levels in the device indicated the passage of time, were constructed with notations to indicate the hours for the different months and seasons of the year. A standard hour of constant length was proposed in the second century BC, but was never employed in ancient Egypt.

World's oldest known sundial from Egypt's Valley of the Kings (c. 1,500 BC) - divided into 12 parts to measure work hours.

Oldest known water clock, Egyptian "hourglass of Karnak" (c. 1,400 BC)

Babylonians.  Babylonia was an ancient cultural region occupying southeastern Mesopotamia between the Tigris and Euphrates rivers (modern southern Iraq).  Because the city of Babylon was the capital of this area for so many centuries, the term Babylonia has come to refer to the entire culture that developed in the area from the time it was first settled, about 4,000 BC, until Babylonia was defeated by Persia in 539 BC. Before Babylon’s rise to political prominence (c. 1850 BC), however, the area was divided into two countries: Sumer in the southeast and Akkad in the northwest.

By about 2,100 BC, the Sumerians began using a lunisolar calendar, in which months are lunar, but years are solar.  They knew from the repetitive cycle of the moon’s phases that there were 29 ½ days in a month and about 365 days in a solar (or agricultural) year.  So, a lunar year of 12 months was 12 x 29 ½ = 354 days, 11 days short of a solar year.  The Babylonians named their months:  Nisanu, Ayaru, Simanu, Du'uzu, Abu, Ululu, Tashritu, Arakhsamna, Kislimu, Tebetu, Shabatu, and Adaru - derived from names of their gods, numerical placement within the year, or seasonal characteristics such as rain.

In order to bring lunar years into alignment with solar years (over a period of time), the Sumerians periodically added an extra intercalated month to the 12 lunar months.  But initially, they did this haphazardly; each Sumerian city inserted months at will, resulting in great confusion. In c. 380 BC, Babylonian calendar calculators, by then under Persian rule, succeeded in computing an almost perfect equivalence in a lunar-year/solar-year cycle of 19 years, with intercalations in the years 3, 6, 8, 11, 14, 17, and 19 of the cycle. 

This Babylonian calendar system came to prevail throughout the Near East. The system may seem highly complicated to a modern observer, but it worked for many centuries, and the Israelites' exposure to Babylonian culture during the Captivity ensured its lasting impact on the Hebrew calendar.  The Jews adopted not only the Babylonian calendar, but also month names.

The Captivity was a period when Jews were forcibly detained in Babylonia following Babylonia’s conquest and occupation of the kingdom of Judah in 598-538 BC.  The Captivity formally ended, when the Persian conqueror of Babylonia, Cyrus the Great, gave the Jews permission to return to Palestine.

Romans.  Rome was founded in 753 BC and ruled by kings until 510 BC, when it became a Republic.  The Roman Empire was founded in 31 BC and lasted in the West until AD 476, when the last emperor was deposed by the Germanic King, and in the East, as the Byzantine Empire, until 1453, when it fell to Ottoman Turks.

According to tradition, the early Roman calendar was drawn up in 753 BC by Romulus, the legendary founder and first King of Rome.  The year began in March and consisted of 10 months, six of 30 days and four of 31 days, making a total of 304 days.  The first Roman calendar ended in December (see below for month names), to be followed by what seems to have been an uncounted winter gap.  Around 703 BC, Numa Pompilius, according to tradition, the second King of Rome, added two extra months, January and February, to fill the gap and increase the total number of days by 51, making 355.

In about 600 BC, the so-called Roman Republican Calendar was introduced by Lucius Tarquinius Priscus, according to tradition, the fifth king of Rome.  The Republican Calendar still contained only 355 days, with February having 28 days; March, May, July, and October 31 days each; January, April, June, August, September, November, and December 29 days.   It was basically a lunar calendar and short by 10 ¼ days of a solar year, then known to be 365 ¼ days.  In order to prevent the calendar from becoming too far out of step with the seasons, an intercalary month was inserted between February 23^rd and 24^th.  It consisted of 27 or 28 days, added once every two years, with the last five days of February omitted.  The intercalation was therefore equivalent to an additional 22 or 23 days, so that in a four-year period, the total days in the calendar amounted to (4 × 355) + 22 + 23, or 1,465 days:  this gave an average of 366 ¼ days per year.

Intercalation was the duty of a council of priests. The reasons for their decisions were kept secret, but, because of some negligence and a measure of ignorance and corruption, the intercalations were irregular, and seasonal chaos resulted.  In spite of this, and the fact that on average the calendar was a day too long compared with the solar year, the Republican Calendar was used for over 500 years.

The years were not counted; instead, they were named after the Republic’s consul in power at the start of a year.

In the mid-first century BC, Julius Caesar invited astronomer Sosigenes of Alexandria to advise him about the reform of the Republican Calendar, and Sosigenes decided that the only practical step was to abandon the lunar calendar altogether.   Months must be arranged on a seasonal basis, and a solar year used, as in the Egyptian calendar, but with its length taken as 365 ¼ days.  Caesar directed that a calendar year of 365 days be adopted and that an extra day be intercalated every fourth year (our familiar leap year) to bring the calendar into alignment with the solar year.

Julius Caesar led the Romans from 59-44 BC.  This portrait is possibly the only surviving sculpture of Caesar made during his lifetime.

The new calendar was named the Julian Calendar, with the months taken over from the Roman Republican Calendar, but the number of days in each month were slightly modified to provide a more even pattern of numbering.  After some manipulations in Julius Caesar’s day, and further manipulations by Augustus Caesar in 8 BC, the Julian Calendar was set with 12 months in the order, and with the number of days per month, that we have today.  January became the first month of the year.  February was the shortest month, containing only 28 days, a carryover from the Republican Calendar, and the month that Romans honored the dead and performed rites of purification.  In “leap” years, the extra day was added to February, as February 29^th.

 

Months in the Julian Calendar.

Month	Number of Days	Origin of Months Name
January	31	Roman god Janus, protector of gates and doorways.  In ancient Rome, the gates of the temple of Janus were open in times of war and closed in time of peace.  Inserted as first month when the Romans went to a 12-month calendar.
February	28                       (29 in leap year)	From Latin word februa, “to cleanse.”  Named for Februalia, a festival of purification and atonement that took place during this period.  Inserted as second month, when the Romans went to a 12-month calendar.
March	31	Roman god of war, Mars.  This was the time of year to resume military campaigns after being interrupted by winter.
April	30	From Latin word aperio, “to open (bud),” because plants begin to grow in this month.
May	31	Roman goddess Maia, who oversaw the growth of plants.
June	30	Roman goddess Juno, patroness of marriage and well-being of women.
July	31	To honor Julius Caesar.
August	31	To honor first Roman emperor (and grandnephew of Julius Caesar), Augustus Caesar.
September	30	From Latin word septem, meaning “seven” because it was the seventh month of the early Roman calendar.  When the Romans converted to a 12-month calendar, they tried to rename this month (and succeeding months), but the original names stuck.
October	31	From Latin word octo, meaning “eight,” because it was the eighth month of the early Roman calendar.
November	30	From Latin word novem, meaning “nine,” because it was the ninth month of the early Roman calendar.
December	31	From Latin word decem, meaning “ten,” because it was the tenth month of the early Roman calendar.

 

There were no weeks in the original Julian Calendar.  The days were designated business days, or days on which the courts were open, as had been the practice in the Roman Republican Calendar.

The seven-day week may owe its origin partly to the four (approximately) seven-day phases of the Moon and partly to the Babylonian belief in the sacredness of the number seven, which was probably related to the seven planets.  Moreover, by the first century BC, the Jewish seven-day week (seven days of creation) seems to have been adopted throughout the Roman world, and this influenced Christendom.  The Roman Empire, through the action of Byzantine Emperor Constantine, officially adopted the seven-day week in AD 321. 

After the fall of the Western Roman Empire in AD 476, the German language started to influence the names of Julian Calendar months, originally derived from Roman gods and ruling planets:  Saturn, the Sun, the Moon, Mars, Mercury, Jupiter, and Venus.  Today’s weekday names derive from a combination of Roman and Norse terms.  Thus Tiu, Woden, Thor, and Freya replaced Mars, Mercury, Jupiter, and Venus to lend their names to Tuesday, Wednesday, Thursday, and Friday respectively.

The names in English of the days of the week were derived from Latin or Anglo-Saxon names of gods.

Day of Week	Origin of Days Name
Sunday	From the Babylonians, after the planetary body, the Sun.
Monday	From the Babylonians, after the planetary body, the Moon.
Tuesday	From Tiu, or Tiw, the Anglo-Saxon name for Tyr, the Norse god of war.
Wednesday	From Woden, the chief Anglo-Saxon/Teutonic god, the leader of the Wild Hunt.
Thursday	From Thor, the Norse god of thunder.
Friday	From Freya, the Teutonic god of love, beauty, and procreation.
Saturday	From the ancient Roman god, Saturn, god of agriculture.

 

Following the Egyptians, the Roman day and night were each divided into 12 temporal hours whose length varied with the seasons and the latitude of the location.   Again, as the Egyptians did, the Romans used sundials and water clocks to measure time.  Sundials were used to calibrate water clocks.

 

2,000-year-old Roman sundial.

Middle Ages

For the purpose of this discussion, I’m considering that the Middle Ages spanned from the fall of the Western Roman Empire in fifth century through the late 15^th century.

Year Numbering.  Through the sixth century AD, Julian calendar years were identified by naming the consuls who held office (first in Rome, later in the Ostrogothic Kingdom) e.g., Bob Ring, nth year of rule.  In AD 525, Dionysius Exiguus, a monk in today’s southeastern Europe, invented Anno Domini (in the year of the Lord) dating, abbreviated as “AD,” that counts years forward from the birth “our Lord Jesus Christ,” which he stated was 525 ago.  Years before AD 1 were to be abbreviated as “BC” (before Christ) and counted backward into the past; there was no “year zero.”  This dating system was slow to be accepted and was not widely used until after AD 800.

European monk, Dionysius Exiguus, invented AD/BC calendar dating in AD 525.

Most theologians today assume a year for the birth for Christ between 6 and 4 BC.  The historical evidence is too incomplete to allow a definitive dating.

Since the later 20^th century, CE (common era) and BCE (before common era) have been popular in academic and scientific circles as culturally neutral terms to be used instead of AD and BC.  These terms are also used by others who wish to be sensitive to non-Christians.  The two notation systems are numerically equivalent.

Mechanical Clocks.  The concept of fixed-length hours within a 24-hour day originated in the Hellenistic period, when Greek astronomers began using such a system for their theoretical calculations.  Hipparchus, whose work primarily took place between 147-127 BC, proposed dividing the day into 24 equal-time hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days.  Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries until mechanical clocks first appeared.

The earliest recorded weight-driven mechanical clock was installed in 1283 at Dunstable Priory in Bedfordshire, England.  The Roman Catholic Church played a major role in the invention and development of clock technology; the strict observance of prayer times by monastic orders occasioned the need for a more reliable instrument of time measurement.  Additionally, the growth of urban mercantile populations in Europe during the second half of the 13th century created demand for improved timekeeping devices.  By 1300, artisans were building clocks for churches and cathedrals in France and Italy.  Because the initial mechanical clocks indicated the time by striking a bell (thereby alerting the surrounding community to its daily duties), the name for this new machine was adopted from the Latin word for bell, clocca.

These early mechanical clocks employed a mechanism to allow a gear train to advance at regular intervals or “ticks,” and a balance wheel for accurate timekeeping.  The first examples were truly huge devices and relied on the use of heavy-weights to drive the clock's hands. They were often constructed in tall towers and were able to keep relatively good time, losing about two hours a day.  While that might sound very inaccurate today, they were cutting edge at the time.

 

Medieval mechanical clock from Salisbury Cathedral in England, operated in a bell tower.  Supposedly dating from about 1336, restored in 1956.

Although the mechanical clock could be adjusted to maintain temporal hours, it was naturally suited to keeping equal ones.  Soon clocks split the day, as we currently do, into two 12-hour periods commencing at midnight.

During the 1580s, clockmakers received commissions for timekeepers to show minutes and seconds, but their mechanisms were not sufficiently accurate for these fractions to be included on dials until the 1660s, when the pendulum clock was developed.  Minutes and seconds derive from Babylonians who used a sexagesimal (counting in 60s) system for mathematics and astronomy.  The word “minute” has its origins in the Latin prima minuta, the first small division; “second” comes from secunda minuta, the second small division.  The sectioning of the day into 24 hours, and of hours and minutes into 60 parts, soon was well established in Western culture.

For centuries after the invention of the mechanical clock, the periodic tolling of the bell in the town church or clock tower was enough to demarcate the day for most people.  But by the 15th century, a growing number of clocks were being made for domestic use.  Those who could afford the luxury of owning a clock found it convenient to have one that could be moved from place to place.  Innovators accomplished portability by replacing the weight with a coiled spring.

Hourglasses. The first documented use of an hourglass dates from the eighth century, crafted by a Frankish monk named Liutprand, who served at the cathedral in Chartres, France.  But it was not until the 14th century that hourglasses were seen commonly.  Sandglasses were very popular on ships as they were the most dependable measurement of time while at sea.  The motion of the ship while sailing did not affect the hourglass.  

The hourglass also found popularity on land. As the use of mechanical clocks to indicate the times of events like church services became more common, the demand for time-measuring devices increased.  Hourglasses were inexpensive, as they required no rare technology to make, and their contents were not hard to come by, and as the manufacturing of these instruments became more common, their uses became more practical.   Hourglasses were commonly seen in use in churches, homes, and work places to measure sermons, cooking time, and time spent on breaks from labor. 

Because they were being used for more everyday tasks, the size of the hourglass began to shrink. Typically, hourglasses were used to measure times of an hour or so, but 12-hour, and even 24-hour hourglasses have been built.  Hourglasses normally vary from a few inches to a few feet in height.

After 1500, the use of hourglasses was not as widespread as it had been. This was due to the continued development of the mechanical clock, which became more accurate, smaller, and cheaper, and made keeping time easier

The sandglass is still widely used as a kitchen timer for cooking eggs, a three-minute timer is typical, hence the name "egg timer" for three-minute hourglasses.  Egg timers are sold widely as souvenirs.  Sand timers are also sometimes used in games such as Pictionary and Boggle to implement a time constraint on rounds of play.

 

German half-hour sand glass from the early 16th century.

Gregorian Calendar.  By 1545, astronomers realized that the Julian calendar year of 365.25 days was too long.  The vernal equinox, which was used in determining Easter, had moved 10 days from its proper date of March 21^st.   This error of 11 minutes 14 seconds per year amounted to almost one and a half days in two centuries, and seven days in 1,000 years.  Once again, the calendar became increasingly out of phase with the seasons. 

After almost four decades of study by astronomers, Pope Gregory XIII introduced a modification to the Julian calendar in October 1582.   First, in order to bring the vernal equinox back to March 21^st, October 5^th was to become October 15^th, thus omitting 10 days from the calendar.  Second, to bring the calendar year closer to the true solar year, a revised value of 365.2425 days per was calculated.  It was further directed that three out of every four centennial years should be common years, that is, not leap years; and this practice led to the rule that no centennial years should be leap years unless exactly divisible by 400.  Thus, 1700, 1800, and 1900 were not leap years, as they would have been in the Julian calendar, but the years 1600 and 2000 were. 

 

Pope Gregory XIII introduced the Gregorian Calendar in 1582, and it is still used today.

 

In the switchover from the Julian to Gregorian calendar, the month of October, 1582 lost 10 days.

This reform, which established what became known as the Gregorian Calendar and laid down rules for calculating the date of Easter, was well received by Catholics in Europe.  Many Protestants, however, saw it as the work of the Antichrist and refused to adopt it.  Eventually all of Europe, as late as 1918 in the case of Russia, adopted the Gregorian calendar, and it is in use in much of the world today.  

The current estimate for the length of a solar year is 365.2422 days, making the error in the Gregorian calendar one day in 3,236 years.

To the Modern Age

The last historical period to discuss, in term of measuring time, is from the end of Middle Ages to today.  Let’s talk about calendars first.

Calendars.  We will continue to analyze the length of a solar year and the astronomical factors that may perturb it.  But, for those of us who live only one lifetime, surely the current Gregorian calendar is sufficiently accurate, off only one day in 3,236 years.

Even if astronomically, the calendar year really calls for no improvement, the seven-day week and the different lengths of months are unsatisfactory to some.  Clearly, if the calendar could have all holidays and special event days fixed on the same dates every year, this arrangement would be more convenient, and two general schemes have been put forward:  The International Fixed Calendar and the World Calendar.

The International Fixed Calendar is essentially a perpetual Gregorian calendar, in which the year is divided into 13 months, each of 28 days for a total of 364 days, with an additional day at the end.  Present month names are retained, but the new month named Sol is intercalated between June and July.  The additional day follows December 28th and bears no designation of month, date, or weekday name, while the same would be true of the day intercalated in a leap year after June 28.  In this calendar, every month begins on a Sunday and ends on a Saturday.

It is claimed that the proposed International Fixed Calendar does not conveniently divide into quarters for business reckoning; and the World Calendar is designed to remedy this deficiency, being divided into four quarters of 91 days each, with an additional day at the end of the year.  In each quarter, the first month is of 31 days and the second and third of 30 days each.  The extra day comes after December 30^th and bears no month or weekday designation, nor does the intercalated leap year day that follows June 30th.  In the World Calendar, January 1, April 1, July 1, and October 1 are all Sundays.  Critics point out that each month extends over part of five weeks, and each month within a given quarter, begins on a different day.

Some “food for thought” I guess.

We're already having trouble remembering what day it is.

Clocks.  Although mechanical clocks satisfied the requirements of monastic and urban communities, it was too inaccurate and unreliable for scientific application until the pendulum was employed to govern its operation.   Dutch astronomer and mathematician Christiaan Huygens devised the first pendulum clock on Christmas Day in 1656. Pendulum clocks were much more accurate than mechanical clocks, and with improvements over the next 20 years, reduced a typical gain or loss of 15 minutes a day in the mechanical clock to about a few seconds a week in a pendulum clock.  The more accurate timekeepers that were subsequently developed went on to play key roles in the industrial revolution and the advance of Western civilization.

 

Sketch of the world's first pendulum clock invented by Christiaan Huygens in 1656.

The first wristwatch was made by Prussian Abraham-Louis Breguet for the Queen of Naples in 1810.  That first watch was a mechanical device, powered by winding a mainspring which turned gears and then moved the hands; it kept time with a rotating balance wheel.  Self-winding mechanical wristwatches made their appearance during the 1920s.

At the turn of the 19^th century, clocks and watches were relatively accurate, but they remained expensive.  Mass production soon drove prices down so that average citizens could afford accurate timekeepers.

High precision clocks were developed in the 20^th century.  In 1928, Warren A. Marrison, an engineer at Bell Laboratories in New York, discovered an extremely uniform and reliable frequency source that was as revolutionary for timekeeping as the pendulum had been 272 years earlier.  Developed originally for use in radio broadcasting, the quartz crystal vibrates at a highly regular rate when excited by an electric current.  The first quartz clocks installed at the Royal Observatory in 1939 varied by only two thousandths of a second a day.  By the end of World War II, this accuracy had improved to the equivalent of a second every 30 years.

Quartz-crystal technology did not remain the premier frequency standard for long, however.  By 1948, Harold Lyons and his associates at the National Bureau of Standards in Washington, D.C., had based the first atomic clock on a far more precise and stable source of timekeeping; an atom's natural resonant frequency, the periodic oscillation between two of its energy states.  Subsequent experiments in both the U.S. and England in the 1950s, led to the development of the cesium-beam atomic clock. Today the averaged times of cesium clocks in various parts of the world provide an accuracy of better than one nanosecond a day.

Today, highly accurate timekeeping instruments set the beat for most of our electronic devices.  Nearly all computers, for example, contain a quartz-crystal clock to regulate their operation.  Moreover, not only do time signals beamed down from Global Positioning System satellites calibrate the functions of precision navigation equipment, they do so as well for cellular telephones, instant stock-trading systems and nationwide power-distribution grids.  So integral have these time-based technologies become to our day-to-day lives that we recognize our dependency on them only when they fail to work.  And of course, today, we’re used to digital clocks and watches.

The precise measurement of time is of such fundamental importance to science that the search for even greater accuracy continues.  

I will close with a quote from my Scientific American source:

“Although our ability to measure time will surely improve in the future, nothing will change the fact that it is the one thing of which we will never have enough.”