A Brief History of Time Measurement : nrich.maths.org A Brief History of Time Measurement Stage: 2, 3, 4 and 5 Article by Leo Rogers Published May 2008,February 2011. Ever since man first noticed the regular movement of the Sun and the stars, we have wondered about the passage of time. Prehistoric people first recorded the phases of the Moon some 30,000 years ago, and recording time has been a way by which humanity has observed the heavens and represented the progress of civilization. Natural Events The earliest natural events to be recognised were in the heavens, but during the course of the year there were many other events that indicated significant changes in the environment. Seasonal winds and rains, the flooding of rivers, the flowering of trees and plants, and the breeding cycles or migration of animals and birds, all led to natural divisions of the year, and further observation and local customs led to the recognition of the seasons. Measuring time by the Sun, the Moon and the Stars As the sun moves across the sky, shadows change in direction and length, so a simple sundial can measure the length of a day.

It was quickly noticed that the length of the day varies at different times of the year. The reasons for this difference were not discovered until after astronomers accepted the fact that the earth travels round the sun in an elliptic orbit, and that the earth's axis is tilted at about 26 degrees. This variation from a circular orbit leads to the Equation of Time (see 'Note 2' below) which allows us to work out the difference between 'clock' time and 'sundial time'. Another discovery was that sundials had to be specially made for different latitudes because the Sun's altitude in the sky decreases at higher latitudes, producing longer shadows than at lower latitudes. Today, artists and astronomers find many ways of creating modern sundials. A sundial with roman numerals. As you look at this dial, which direction are you facing? The progress of the sun can be recorded using the four faces of this cube. Can you discover the orientation of these faces? Wall Sundial Prehistoric carving said to represent the Orion constellation The oldest image of a star pattern, the constellation of Orion, has been recognised on a piece of mammoth tusk some 32,500 years old.

The constellation Orion is symbolized by a man standing with his right arm raised and a sword at his belt and can be seen throughout the world at different times of the year. Orion was the sun god of the Egyptians and Phonecians and called the 'strong one' by the Arabs. In parts of Africa, his belt and sword are known as 'three dogs chasing three pigs' and the Borana people of East Africa based a sophisticated calendar on observations of star clusters near Orion's belt. Orion contains some of the brightest stars in the southern part of the winter sky in the northern hemisphere and can be seen later in the southern hemisphere. The three stars of Orion's belt and the red star of his right arm can be easily recognised The earliest Egyptian Star Map is about 3,500 years old and shows the most unusual conjunction of the planets (Venus, Mercury, Saturn and Jupiter) in the constellation of Orion and the occurrence of a solar eclipse that happened in 1534 BCE. Babylonian records of observations of heavenly events date back to 1,600 BCE.

The reason for adopting their arithmetic system is probably because 60 has many divisors, and their decision to adopt 360 days as the length of the year and 3600 in a circle was based on their existing mathematics and the convenience that the sun moves through the sky relative to fixed stars at about 1degree each day. The constellation Taurus, the bull, a symbol of strength and fertility, figures prominently in the mythology of nearly all early civilizations, from Babylon and India to northern Europe. The Assyrian winged man-headed bull had the strength of a bull, the swiftness of a bird and human intelligence. From about 700 BCE the Babylonians began to develop a mathematical theory of astronomy, but the equally divided 12-constellation zodiac appears later about 500 BCE to correspond to their year of 12 months of 30 days each. Their base 60 fraction system which we still use today (degrees / hours, minutes and seconds) was much easier to calculate with than the fractions used in Egypt or Greece, and remained the main calculation tool for astronomers until after the 16th century, when decimal notation began to take over.

The earliest archaeological evidence of Chinese calendars appears about 2,000 BCE. They show a 12 month year with the occasional occurrence of a 13th month. However, traditional Chinese records suggest the origin of a calendar of 366 days depending on the movements of the Sun and the Moon as early as 3,000 BCE. Over such a long period of observation, Chinese astronomers became aware that their calendar was not accurate, and by the second century CE it was recognised that the calendar became unreliable every 300 years. This problem is called Precession and was recorded by Chinese historians in the fourth and fifth centuries CE. In the fifth century CE the scholar Zu Chongzi created the first calendar which took precession into account, and the most comprehensive calendar was the Dayan Calendar compiled in the Tang Dynasty (616-907 CE) well ahead of any such development in Europe. Precession is due to the gradual movement of the Earth's rotational axis in a circle with respect to the fixed stars. This movement produces a slow 'wobble' which means that the positions of the stars complete a cycle of about 26,000 years.

The Earth's axis completes a circuit about once every 26,000 years In the Mediterranean, Hipparchus made the earliest calculations of precession in about 160 BCE. The problem was taken up by astronomers in the Middle East and India who recognized that precession gradually altered the length of the year. Calendars have had to be altered regularly. In 325 CE the spring (vernal) equinox had moved to March 21. The Emperor Constantine established dates for the Christian holidays, but Easter is based on the date of the vernal equinox which varies every year because the equinox is an astronomical event. By 1582 the vernal equinox had moved another ten days and Pope Gregory established a new calendar, and this change is the reason for having an extra day in every leap year. However, there are still small changes accumulating, and one day we shall have to adopt a new calendar! Inventions for measuring and regulating time The early inventions were made to divide the day or the night into different periods in order to regulate work or ritual, so the lengths of the time periods varied greatly from place to place and from one culture to another.

Oil Lamps There is archaeological evidence of oil lamps about 4,000 BCE, and the Chinese were using oil for heating and lighting by 2,000 BCE. Oil lamps are still significant in religious practices, symbolic of the journey from darkness and ignorance to light and knowledge. The shape of the lamp gradually evolved into the typical pottery style shown. It was possible to devise a way of measuring the level in the oil reservoir to measure the passing of time. Candle Clocks Marked candles were used for telling the time in China from the sixth century CE. There is a popular story that King Alfred the Great invented the candle clock, but we know they were in use in England from the tenth century CE. However, the rate of burning is subject to draughts, and the variable quality of the wax. Like oil lamps, candles were used to mark the passage of time from one event to another, rather than tell the time of day. Water Clocks The water clock, or clepsydra, appears to have been invented about 1,500 BCE and was a device which relied on the steady flow of water from or into a container. Measurements could be marked on the container or on a receptacle for the water.

In comparison with the candle or the oil lamp, the clepsydra was more reliable, but the water flow still depended on the variation of pressure from the head of water in the container. Astronomical and astrological clock making was developed in China from 200 to 1300 CE. Early Chinese clepsydras drove various mechanisms illustrating astronomical phenomena. The astronomer Su Sung and his associates built an elaborate clepsydra in 1088 CE. This device incorporated a water-driven bucket system originally invented about 725 CE. Among the displays were a bronze power-driven rotating celestial globe, and manikins that rang gongs, and indicated special times of the day. Improvements were made to regulate the flow by maintaining a constant head of water Su Sung's astronomical water clock Sandglass Hour Glasses or Sandglasses As the technology of glass-blowing developed, from some time in the 14th century it became possible to make sandglasses.

Originally, sandglasses were used as a measure for periods of time like the lamps or candles, but as clocks became more accurate they were used to calibrate sandglasses to measure specific periods of time, and to determine the duration of sermons, university lectures, and even periods of torture. The Division of the Day and the Length of the 'Hour' An Egyptian sundial from about 1,500 BCE is the earliest evidence of the division of the day into equal parts, but the sundial was no use at night. The passage of time was extremely important for astronomers and priests who were responsible for determining the exact hour for the daily rituals and for the important religious festivals, so a water clock was invented. The Merkhet The Egyptians improved upon the sundial with a 'merkhet', one of the oldest known astronomical instruments. It was developed around 600 BCE and uses a string with a weight as a plumb line to obtain a true vertical line, as in the picture. The other object is the rib of a palm leaf, stripped of its fronds and split at one end, making a thin slit for a sight.

A pair of merkhets were used to establish a North-South direction by lining them up one behind the other with the Pole Star. Viewing the plumb lines through the sight made sure the two merkhets and the sight were in the same straight line with the Pole Star. This allowed for the measurement of night-time events with a water clock when certain stars crossed the vertical plumb line (a 'transit line'), and these events could then be recorded by 'night-time lines' drawn on a sundial. See also 'Note 1' below. An Egyptian Merkhet. The wooden upright has a notch to use as a sight when using two plumb lines There are various theories about how the 24 hour day developed. The fact that the day was divided into 12 hours might be because 12 is a factor of 60, and both the Babylonian and Egyptian civilisations recognised a zodiac cycle of 12 constellations. On the other hand, (excuse the pun) finger-counting with base 12 was a possibility. The fingers each have 3 joints, and so counting on the joints gives one 'full hand' of 12.

In classical Greek and Roman times they used twelve hours from sunrise to sunset; but since summer days and winter nights are longer than winter days and summer nights, the lengths of the hours varied throughout the year. In about 50 BCE Andronikos of Kyrrhestes, built the Tower of Winds in Athens. This was a water clock combined with Sundials positioned in the eight principal wind directions. By then it was the most accurate device built for keeping time. The Tower of the Winds in Athens contained a clepsydra and shows the North-East, North and North-West deities in this picture Hours did not have a fixed length until the Greeks decided they needed such a system for theoretical calculations. Hipparchus proposed dividing the day equally into 24 hours which came to be known as equinoctial hours. They are based on 12 hours of daylight and 12 hours of darkness on the days of the Equinoxes. However, ordinary people continued to use seasonally varying hours for a long time. Only with the advent of mechanical clocks in Europe in the 14th Century, did the system we use today become commonly accepted.

Earliest mechanical clock Mechanical clocks replaced the old water clocks, and the first clock escapement mechanism appears to have been invented in 1275. The first drawing of an escapement was given by Jacopo di Dondi in 1364. In the early-to-mid-14th century, large mechanical clocks began to appear in the towers of several cities. There is no evidence or record of the working models of these public clocks that were weight-driven. All had the same basic problem: the period of oscillation of the mechanism depended heavily on the driving force of the weights and the friction in the drive. In later Mediaeval times elaborate clocks were built in public places. This is the Astronomical clock in Prague, parts of which date from about 1410. This mechanism illustrates a basic escapement. The weight rotates the drum which drives the toothed wheel which gives the mechnism its "tick-tock" movement Prague Astronomical Clock Showing the Zodiac Circles and early versions of the digits 2, 3, 4 and 7 The earliest surviving spring driven clock can be found in the science museum in London and dates from about 1450.

Replacing the heavy drive weights with a spring permitted smaller and portable clocks and watches. More Accurate Mechanical Clocks Christiaan Huygens made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation in 1656. Galileo studied pendulum motion as early as 1582, but his design for a clock was not built before his death. Huygens' pendulum clock had an error of less than 1 minute a day, and his later refinements reduced his clock's errors to less than 10 seconds a day. There was no device for keeping accurate time at sea until John Harrison, a carpenter and instrument maker, refined techniques for temperature compensation and found new ways of reducing friction. By 1761, he had built a marine chronometer with a spring and balance wheel escapement that kept very accurate time. With the final version of his chronometer, which looked like a large pocket watch, he achieved a means of determining longitude to within one-half a degree. It was not until 1884 that a conference at Greenwich reached agreement on global time measurement and adopted Greenwich Mean Time as the international standard.

Today we rely on atomic clocks for our most accurate time measurements. The pendulum moves the lever which creates the rocking movement of the escapement N.B. Pedagogical notes related to measurement and time can be found by clicking on the " Notes " tab at the top of this article. Supporting notes Note 1 When you think about the problem - we can find due South easily from the sun at midday. Looking at the night sky, we eventually deduce that there is a fixed point in the heavens around which all the stars rotate once every day (24 hours). This is where we find the 'Pole Star' (from the Great Bear or Ursa Major, measure the distance of about four lengths of the two stars at the end, 'the pointers' to find Polaris). This is the Celestial Pole - which was different for the Egyptians from today because of the phenomenon of Precession. Up to about 1,900 BCE the Celestial Pole was Thuban a star in the 'tail' of the constellation Draco. By 1,000 BCE it was Thuban in the constellation Ursa Minor.

Today Polaris is the last star in the 'tail' of Ursa Minor Note 2 'Sun time' and 'clock time' are different. Sun time is based on the fact that the sun reaches its highest point (the meridian), in the middle of the day, and on the next day at its highest point, it will have completed a full cycle. However, the time between the sun reaching successive meridians is often different from clock time. According to clock time, from May to August, the day is close to 24 hours, but in late October the days are about 15 minutes shorter, while in mid February the days are about 14 minutes longer. For our daily routines, it is important to have a constant 'clock time' of 24 hours. This variation is called the 'Equation of Time' and shows the relationship between sun time and clock time. The variation has two causes; the plane of the Earth's equator is inclined to the Earth's orbit around the Sun, and the orbit of the Earth around the sun is an ellipse and not a circle. The National Maritime Museum website shows two separate graphs for these causes, and a third graph where they are combined to give the full correction.

See National Maritime Museum - link below. Some Useful Links This site gives very good examples of the development of time measurement with pictures and brief explanations Make your own sundial at Many interesting sundial designs Sundials on the internet has many examples from all over the world Precession of the Equinoxes explains the way the Earth'??s rotation changes. This site has a good explanation and a useful animation: For Galileo, Huygens and Harrison go to the MacTutor website: Other Ancient Calendars | Calendars → See detailed information on the early Roman calendar . When did ancient months start? In the eighth century B.C.E., civilizations all over the world either discarded or modified their old 360 day calendars. The 360 day calendars had been in use for the greater part of a millennium. In many places, month lengths immediately after that change were not fixed, but were based instead upon observation of the sky. Priest-astronomers were assigned the duty of declaring when a new month began  it was usually said to have started at the first sighting of a new moon. Month length at that time was simply the number of days that passed from one new lunar crescent to the next.

During those years in Rome, for example, a Pontifex (priest) observed the sky and announced a new moon and therefore the new month to the king. For centuries afterward Romans referred to the first day of each new month as Kalends or Kalends from their word calare (to announce solemnly, to call out). The word calendar derived from this custom. This practice of starting a month at the first sighting of a new moon was observed not only by Romans but by Celts and Germans in Europe and by Babylonians and Hebrews in the Lavant. All of these peoples began their month when a young crescent was first seen in the sky. This is still done for the Islamic Calendar, but a new moon’s date is calculated for traditional lunar calendars that are currently used in China and India. During the period when month lengths were not fixed, new moons were usually sighted after either 29 or 30 days. If clouds obscured vision on the thirtieth day, a new month was declared to have begun. When month lengths were identical with lunations, only those that lasted 30 days were considered to be normal.

This was probably because all months had previously been 30 days for such a long period of time. During this period in Greece, for example, months that consisted of 30 days were considered to be "full;" those that lasted only 29 days were said to be "hollow." Months containing 30 days were also called "full" in Babylon, but those containing 29 were deemed to be "defective." After month lengths in the Celtic Calendar became fixed, those that had been given 30 days were termed "matos" (lucky) and those given 29 days "anmatos" (unlucky). This notion still exists today, months of 30 days in the Hebrew Calendar are called "full" and those with 29 are deemed to be "deficient." When was the ancient new year? In addition to their declaring the beginning of each month based upon a sighting of the new moon, priest-astronomers were also charged with pinpointing the start of a year. By observing the movement of Sirius, Egyptians came to grips with the fact that the year was more than five days longer than their venerable 360-day calendar.

This resulted in a change to their method of approximating year length that had been in use for nearly a millennium. But it also caused them to wonder where the additional days came from. In order to account for these additional days, Egyptians created a myth about their sky-god, Nut. During the reign of the Babylonian king Nabonasser (traditionally dated between 747 and 734 B.C.E.) priest/ astronomers in that country discontinued their practice of looking for the new moon in order to name the beginning of a month. Instead, they returned to a fixed-length calendar that had 12 months of 30 days each, but with five days added at the end. 10 Usually at a date later than the mid-eighth century B.C.E., many other peoples who had previously considered the year to be 360 days in length reluctantly returned to a calendar of twelve 30-day months, but added five days to the end of their year. These additional days were considered to be very unlucky or unpropitious. Two eastern Mediterranean peoples who did not embrace Islam were early Christians in upper Egypt, whom we now call Copts, and their neighbors to the south, the Ethiopians.

Probably because they were surrounded by Islamic peoples, Coptic and Ethiopian churches never adopted the Western calendar. Instead, these two isolated pockets of Christianity continued to use the old 360-day calendar. These two calendars are identical except for year number. Copts date their calendar from C.E. 284 but Ethiopians date theirs from C.E. 7. Both of them observe three 365 day years followed by one 366 day year. Their years are divided into 12 months of 30 days each, and the extra five or six days are added after the twelfth month. Zoroastrians, who began their calendar in 389 B.C.E. with the birth of their founder, the prophet Zoroaster, use a calendar of 365 days. It consists of twelve 30-day months with five "gatha days" added at the end of the year. Each of the thirty days as well as each of the gatha days has its own name. They are referred to by that name just as we speak of a day by its number in the month.

Beginning in 1906 of the Common Era, some modern Zoroastrians adopted the practice of adding an additional day every four years. One of Alexander the Great’s generals, Seleucus Nicator, founded (late 4th Century BCE and early 3rd century BCE) an empire that stretched from Asia Minor to India. He established a new calendar that was essentially the same as one that had been used for some time in Syria. It contained twelve months of 30 days each and an extra five days at the year’s end. Every fourth year an additional day for a total of six days were added at the end of the year. In Persia under the Sassanids, and in Armenia and Cappadocia the official system of time-reckoning was twelve months of 30 days followed by five more days at the end of the year. However, Arabian astronomers said the Sassanian year of twelve 30-day months was adjusted to the seasons by intercalating a month every 120 years. Following are details about several ancient calendars...

The Hanging Gardens of Babylon East bank of the River Euphrates, about 50 km south of Baghdad, Iraq Palace with legendary gardens built on the banks of the Euphrates river by King Nebuchadnezzar II (which might have never existed except in the minds of Greek poets and historians, although recent archaeological excavations uncovered the foundation of the palace). Greek historian Diodorus Siculus wrote: "The approach to the Garden sloped like a hillside and the several parts of the structure rose from one another tier on tier... On all this, the earth had been piled... and was thickly planted with trees of every kind that, by their great size and other charm, gave pleasure to the beholder... The water machines (raised) the water in great abundance from the river, although no one outside could see it." (Painting by Mario Larrinaga) Babylonian calendar Overview The ancient Babylonians used a calendar with alternating 29- and 30-day months. This system required the addition of an extra month three times every eight years, and as a further adjustment the king would periodically order the insertion of an additional extra month into the calendar.

The Babylonians, who lived in what is now Iraq, added an extra month to their years at irregular intervals. Their calendar, composed of alternate 29-day and 30-day months, kept roughly in step with the lunar year. To balance the calendar with the solar year, the early Babylonians calculated that they needed to add an extra month three times every eight years. But this system still did not accurately make up for the accumulated differences between the solar year and the lunar year. Whenever the king felt that the calendar had slipped too far out of step with the seasons, he ordered another extra month. However, the Babylonian calendar was quite confused until the 300’s B.C.E., when the Babylonians began to use a more reliable system. Details Babylonia was the ancient cultural region occupying southeastern Mesopotamia between the Tigris and Euphrates rivers (modern southern Iraq from around Baghdad to the Persian Gulf). 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 4000 B.C.E. Before Babylon’s rise to political prominence (c.

1850 B.C.E.), however, the area was divided into two countries: Sumer in the southeast and Akkad in the northwest. The Babylonian kingdom flourished under the rule of the famous King, Hammurabi (1792-1750 BC). It was not until the reign of Naboplashar (625-605 BC) of the Neo-Babylonian dynasty that the Mesopotamian civilization reached its ultimate glory. His son, Nebuchadnezzar II (604-562 BC) is credited for building the legendary Hanging Gardens. It is said that the Gardens were built by Nebuchadnezzar to please his wife or concubine who had been "brought up in Media and had a passion for mountain surroundings." Five thousand years ago, Sumerians had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes).

In Mesopotamia, the solar year was divided into two seasons, the "summer," which included the barley harvest in the second half of May or in the beginning of June, and the "winter," which roughly corresponded to today’s fall-winter. Three seasons (Assyria) and four seasons (Anatolia) were counted in northerly countries, but in Mesopotamia the bipartition of the year seemed natural. As late as c. 1800 B.C.E. the prognoses for the welfare of the city of Mari, on the middle Euphrates, were taken for six months. The months began at the first visibility of the New Moon, and in the 8th century B.C.E. court astronomers still reported this important observation to the Assyrian kings. The names of the months differed from city to city, and within the same Sumerian city of Babylonia a month could have several names, derived from festivals, from tasks (e.g. sheepshearing) usually performed in the given month, and so on, according to local needs.

On the other hand, as early as the 27th century B.C.E., the Sumerians had used artificial time units in referring to the tenure of some high official  e.g., on N-day of the turn of office of PN, governor. The Sumerian administration also needed a time unit comprising the whole agricultural cycle; for example, from the delivery of new barley and the settling of pertinent accounts to the next crop. This financial year began about two months after barley cutting. For other purposes, a year began before or with the harvest. This fluctuating and discontinuous year was not precise enough for the meticulous accounting of Sumerian scribes, who by 2400 B.C.E. already used the schematic year of 30 x 12 = 360 days. At about the same time, the idea of a royal year took precise shape, beginning probably at the time of barley harvest, when the king celebrated the new (agricultural) year by offering first fruits to gods in expectation of their blessings for the year.

When, in the course of this year, some royal exploit (conquest, temple building, and so on) demonstrated that the fates had been fixed favorably by the celestial powers, the year was named accordingly; for example, as the year in which "the temple of Ningirsu was built." Until the naming, a year was described as that "following the year named (after such and such event)." The use of the date formulas was supplanted in Babylonia by the counting of regnal years in the 17th century B.C.E. The use of lunar reckoning began to prevail in the 21st century B.C.E. The lunar year probably owed its success to economic progress. A barley loan could be measured out to the lender at the next year’s threshing floor. The wider use of silver as the standard of value demanded more flexible payment terms. A man hiring a servant in the lunar month of Kislimu for a year knew that the engagement would end at the return of the same month, without counting days or periods of office between two dates.

At the city of Mari in about 1800 B.C.E., the allocations were already reckoned on the basis of 29- and 30-day lunar months. In the 18th century B.C.E., the Babylonian Empire standardized the year by adopting the lunar calendar of the Sumerian sacred city of Nippur. The power and the cultural prestige of Babylon assured the success of the lunar year, which began on Nisanu 1, in the spring. When, in the 17th century B.C.E., the dating by regnal years became usual, the period between the accession day and the next Nisanu 1 was described as "the beginning of the kingship of PN," and the regnal years were counted from this Nisanu 1. It was necessary for the lunar year of about 354 days to be brought into line with the solar (agricultural) year of approximately 365 days. This was accomplished by the use of an intercalated month. Thus, in the 21st century B.C.E., a special name for the intercalated month iti dirig appears in the sources.

The intercalation was operated haphazardly, according to real or imagined needs, and each Sumerian city inserted months at will; e.g., 11 months in 18 years or two months in the same year. Later, the empires centralized the intercalation, and as late as 541 B.C.E. it was proclaimed by royal fiat. Improvements in astronomical knowledge eventually made possible the regularization of intercalation; and, under the Persian kings (c. 380 B.C.E.), Babylonian calendar calculators succeeded in computing an almost perfect equivalence in a lunisolar cycle of 19 years and 235 months with intercalations in the years 3, 6, 8, 11, 14, 17, and 19 of the cycle. The new year’s day (Nisanu 1) now oscillated around the spring equinox within a period of 27 days. The Babylonian month names were Nisanu, Ayaru, Simanu, Du'uzu, Abu, Ululu, Tashritu, Arakhsamna, Kislimu, Tebetu, Shabatu, Adaru.

The month Adaru II was intercalated six times within the 19-year cycle but never in the year that was 17th of the cycle, when Ululu II was inserted. Thus, the Babylonian calendar until the end preserved a vestige of the original bipartition of the natural year into two seasons, just as the Babylonian months to the end remained truly lunar and began when the New Moon was first visible in the evening. The day began at sunset. Sundials and water clocks served to count hours. The influence of the Babylonian calendar was seen in many continued customs and usages of its neighbor and vassal states long after the Babylonian Empire had been succeeded by others. In particular, the Jewish calendar in use at relatively late dates employed similar systems of intercalation of months, month names, and other details (see below The Jewish calendar). The Jewish adoption of Babylonian calendar customs dates from the period of the Babylonian Exile in the 6th century B.C.E. The Egyptian calendar Overview The earliest Egyptian calendar was 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 Egyptians soon noticed that the first day the "Dog Star," which we call Sirius, in Canis Major was visible right before sunrise was special. The Egyptians were probably the first to adopt a mainly solar calendar. This so-called ‘heliacal rising’ always preceded the flood by a few days. Based on this knowledge, they devised a 365-day calendar that seems to have begun in 4236 B.C.E., the earliest recorded year in history. They eventually had a system of 36 stars to mark out the year and in the end had three different calendars working concurrently for over 2000 years: a stellar calendar for agriculture, a solar year of 365 days (12 months x 30 + 5 extra) and a quasi-lunar calendar for festivals. The later Egyptian calendars developed sophisticated Zodiac systems, as in the stone calendar at right. According to the famed Egyptologist J. H. Breasted, the earliest date known in the Egyptian calendar corresponds to 4236 B.C.E. in terms of the Gregorian calendar.

Details The ancient Egyptians originally employed a calendar based upon the Moon, and, like many peoples throughout the world, they regulated their lunar calendar by means of the guidance of a sidereal calendar. They used the seasonal appearance of the star Sirius (Sothis); this corresponded closely to the true solar year, being only 12 minutes shorter. Certain difficulties arose, however, because of the inherent incompatibility of lunar and solar years. To solve this problem the Egyptians invented a schematized civil year of 365 days divided into three seasons, each of which consisted of four months of 30 days each. To complete the year, five intercalary days were added at its end, so that the 12 months were equal to 360 days plus five extra days. This civil calendar was derived from the lunar calendar (using months) and the agricultural, or Nile, fluctuations (using seasons); it was, however, no longer directly connected to either and thus was not controlled by them. The civil calendar served government and administration, while the lunar calendar continued to regulate religious affairs and everyday life. In time, the discrepancy between the civil calendar and the older lunar structure became obvious.

Because the lunar calendar was controlled by the rising of Sirius, its months would correspond to the same season each year, while the civil calendar would move through the seasons because the civil year was about one-fourth day shorter than the solar year. Hence, every four years it would fall behind the solar year by one day, and after 1,460 years it would again agree with the lunisolar calendar. Such a period of time is called a Sothic cycle. Because of the discrepancy between these two calendars, the Egyptians established a second lunar calendar based upon the civil year and not, as the older one had been, upon the sighting of Sirius. It was schematic and artificial, and its purpose was to determine religious celebrations and duties. In order to keep it in general agreement with the civil year, a month was intercalated every time the first day of the lunar year came before the first day of the civil year; later, a 25-year cycle of intercalation was introduced. The original lunar calendar, however, was not abandoned but was retained primarily for agriculture because of its agreement with the seasons. Thus, the ancient Egyptians operated with three calendars, each for a different purpose.

The only unit of time that was larger than a year was the reign of a king. The usual custom of dating by reign was: "year 1, 2, 3 . . . , etc., of King So-and-So," and with each new king the counting reverted back to year One. King lists recorded consecutive rulers and the total years of their respective reigns. The civil year was divided into three seasons, commonly translated: Inundation, when the Nile overflowed the agricultural land; Going Forth, the time of planting when the Nile returned to its bed; and Deficiency, the time of low water and harvest. The months of the civil calendar were numbered according to their respective seasons and were not listed by any particular namee.g. third month of Inundationbut for religious purposes the months had names. How early these names were employed in the later lunar calendar is obscure. The days in the civil calendar were also indicated by number and listed according to their respective months.

Thus a full civil date would be: "Regnal year 1, fourth month of Inundation, day 5, under the majesty of King So-and-So." In the lunar calendar, however, each day had a specific name, and from some of these names it can be seen that the four quarters or chief phases of the Moon were recognized, although the Egyptians did not use these quarters to divide the month into smaller segments, such as weeks. Unlike most people who used a lunar calendar, the Egyptians began their day with sunrise instead of sunset because they began their month, and consequently their day, by the disappearance of the old Moon just before dawn. As was customary in early civilizations, the hours were unequal, daylight being divided into 12 parts, and the night likewise; the duration of these parts varied with the seasons. Both water clocks and sundials were constructed with notations to indicate the hours for the different months and seasons of the year. The standard hour of constant length was never employed in ancient Egypt. Sirius: the 'Dog Star’ Early Egyptians depended on the Nile’s annual rising and flooding. Each year as that great river flooded it brought down mountain soil to the Egyptian plain.

This enriched the fields and enabled creation of an agricultural system that supported a large civilization. In the eighth century B.C.E., the Egyptian Pharoh’s primary advisor, the Vizier, was charged with reporting the first appearance of the bright star we call Sirius after it had been missing from the sky for (depending upon the observer’s latitude) approximately two weeks. This first appearance of Sirius in the pre-dawn sky was used to start the so-called Egyptian "lunar" calendar year, which was used for purposes of regulating religious affairs and everyday life. Shortly after Sirius first reappeared in the east, the Nile would have its annual life-giving flood. Because of the Nile’s flooding at this time, the fixing of the new year could well be said to have been based on a geophysical as well as an astronomical event. Although many other stars may be used to fix the beginning of a sidereal year, the Egyptians made an excellent choice for this purpose. Sirius  Egyptians called it Sothis  not only signaled the approaching Nile flood, but is the brightest "fixed" star in the heavens. In Egypt at the present time, Sirius rises just before the sun late in July, but usually can’t be seen until early August.

This is because as sunrise approaches, stars fade from view and the light of dawn obliterates starlight. At the time Sirius is about to reappear, the constellation Orion is fully visible in the lower eastern sky. With the bright star Betelguese on his shoulder, anyone familiar with constellations would find Orion hard to miss. Sirius can be seen in the next constellation to rise (Canis Major). Because of this close relationship, Sirius was sometimes referred to as the "dog star" by early Greeks who thought of Canis Major as one of Orion’s hunting hounds. Other calendars used in the ancient Near East Of the calendars of other peoples of the ancient Near East, very little is known. Thus, though the names of all or of some months are known, their order is not. The months were probably everywhere lunar, but evidence for intercalation is often lacking; for instance, in Assyria. The Assyrians Assyria was a kingdom of northern Mesopotamia that became the center of one of the great empires of the ancient Middle East. It was located in what is now northern Iraq and southeastern Turkey.

For accounting, the Assyrians also used a kind of week, of five days, as it seems, identified by the name of an eponymous official. Thus, a loan could be made and interest calculated for a number of weeks in advance and independently of the vagaries of the civil year. In the city of Ashur, the years bore the name of the official elected for the year; his eponym was known as the limmu. As late as about 1070 B.C.E., his installation date was not fixed in the calendar. From about 1100 B.C.E., however, Babylonian month names began to supplant Assyrian names, and, when Assyria became a world power, it used the Babylonian lunisolar calendar. Assyria was a dependency of Babylonia and later of the Mitanni kingdom during most of the 2nd millennium B.C.E. It emerged as an independent state in the 14th century B.C.E., and in the subsequent period it became a major power in Mesopotamia, Armenia, and sometimes in northern Syria. Assyrian power declined after the death of Tukulti-Ninurta I (c.

1208 B.C.E.). It was restored briefly in the 11th century B.C.E. by Tiglath-pileser I, but during the following period both Assyria and its rivals were preoccupied with the incursions of the seminomadic Aramaeans. The Assyrian kings began a new period of expansion in the 9th century B.C.E., and from the mid-8th to the late 7th century B.C.E., a series of strong Assyrian kingsamong them Tiglath-pileser III, Sargon II, Sennacherib, and Esarhaddonunited most of the Middle East, from Egypt to the Persian Gulf, under Assyrian rule. The last great Assyrian ruler was Ashurbanipal, but his last years and the period following his death, in 627 B.C.E., are obscure. The state was finally destroyed by a Chaldean-Median coalition in 612-609 B.C.E. Famous for their cruelty and fighting prowess, the Assyrians were also monumental builders, as shown by archaeological sites at Nineveh, Ashur, and Nimrud.

The Hittites The calendar of the Hittite Empire is known even less well. As in Babylonia, the first Hittite month was that of first fruits, and, on its beginning, the gods determined the fates. Hittites were a member of an ancient Indo-European people who appeared in Anatolia (modern day Turkey) at the beginning of the 2nd millennium B.C.E.; by 1340 B.C.E. they had become one of the dominant powers of the Middle East. Probably originating from the area beyond the Black Sea, the Hittites first occupied central Anatolia, making their capital at Hattusa (modern Bogazköy). Early kings of the Hittite Old Kingdom, such as Hattusilis I (reigned c. 1650-c. 1620 B.C.E.), consolidated and extended Hittite control over much of Anatolia and northern Syria. Hattusilis’ grandson Mursilis I raided down the Euphrates River to Babylon, putting an end (c. 1590 B.C.E.) to the Amorite dynasty there.

After the death of Mursilis, a dynastic power struggle ensued, with Telipinus finally gaining control about 1530 B.C.E. In the noted Edict of Telipinus, long upheld by succeeding generations, he attempted to end lawlessness and to regulate the royal succession. The fall of the Hittite empire (c. 1193 B.C.E.) was sudden and may be attributed to large-scale migrations that included the Sea Peoples. Hittite cuneiform tablets discovered at Bogazköy (in modern Turkey) have yielded important information about their political organization, social structure, economy, and religion. The Hittite king was not only the chief ruler, military leader, and supreme judge but also the earthly deputy of the storm god; upon dying, he himself became a god. Hittite society was essentially feudal and agrarian, the common people being either freemen, "artisans," or slaves. Anatolia was rich in metals, especially silver and iron. In the empire period the Hittites developed iron-working technology, helping to initiate the Iron Age.

The religion of the Hittites is only incompletely known, though it can be characterized as a tolerant polytheism that included not only indigenous Anatolian deities but also Syrian and Hurrian divinities. Iran At about the time of the conquest of Babylonia in 539 B.C.E., Persian kings made the Babylonian cyclic calendar standard throughout the Persian Empire, from the Indus to the Nile. Aramaic documents from Persian Egypt, for instance, bear Babylonian dates besides the Egyptian. Similarly, the royal years were reckoned in Babylonian style, from Nisanu 1. It is probable, however, that at the court itself the counting of regnal years began with the accession day. The Seleucids and, afterward, the Parthian rulers of Iran maintained the Babylonian calendar. The fiscal administration in northern Iran, from the 1st century B.C.E., at least, used Zoroastrian month and day names in documents in Pahlavi (the Iranian language of Sasanian Persia). The origin and history of the Zoroastrian calendar year of 12 months of 30 days, plus five days (that is, 365 days), remain unknown.

It became official under the Sasanian dynasty, from about C.E. 226 until the Arab conquest in 621. The Arabs introduced the Muslim lunar year, but the Persians continued to use the Sasanian solar year, which in 1079 was made equal to the Julian year by the introduction of the leap year. Read more about the current use of this calculator (the Persian calculator) in Iran. The rainy season The Himba people in Ekambu, Namibia, are some of the last peoples in the world living in relative isolation from modernity. "When the thunderstorms start and the leaves grow from the ground, that’s how we know it’s the new year," said Maverihepisa Koruhama, one of the villagers in Ekambu. They measure time by the shifting sun and mark the coming of the new year with the arrival of seasonal rains that transform the parched red soil into a carpet of green.

In their Herero language, the word for "day" is the same as the word for "sun," and the word for "year" means "rain." (Above left, children watch as a Himba woman, senior wife of Waitavira Tjambiru, anoints her arm with butter fat mixed with red ochre outside her hut in Etengwa, Namibia.) Synergy with the earth James Lynch, an American scientist who has spent the past two decades helping Costa Rican farmers, said he has learned from them the importance of timing. A tree cut down during a new moon, he said, will quickly be ravaged by the insects, while one felled several days before a full moon will stay free of termites for years. Lynch now follows the practice. "But I’ve never seen any scientific study to back it up," he said. Indigenous knowledge can be faulty. "Traditional people sometimes get things right, and sometimes get them wrong," said Alan Fiske, a psychological anthropologist at the University of California at Los Angeles. "Some things people do are bad for them." The problem, Fiske noted, is that verifying traditional knowledge is not easy. The scientific method can be expensive, and data can be difficult to obtain.

A dappled, brown horse and a lunar calendar Lascaux caves, France (about 15,000 years old) Half the cycle: 13 dots and an empty square The Pleiades star cluster sits above the bull’s shoulder Cro-magnon man (Lascaux caves in France) What could be the oldest lunar calendar ever created was been identified on the walls of the famous, prehistoric caves at Lascaux in France. The interpretation that symbolic paintings, dating back 15,000 years, show the Moon going through its different phases comes from Dr. Michael Rappenglueck, of the University of Munich. The German researcher has previously associated patterns left in the caves with familiar stars and constellations. He now says groups of dots and squares painted among representations of bulls, antelope and horses depict the 29-day cycle of the Earth’s satellite. Works of art Visiting the Lascaux caves is an opportunity most people would never get to protect the historic site from unnecessary wear and tear, all visitors now tour a mock-up of the caves, the so-called Lascaux II.

Visiting the caves, once one’s eyes adjust to the half-light, visitors are struck with amazement. Anyone who has seen the paintings on the walls can be left in no doubt that they represent some of the greatest works of art every created. "The secret of understanding these caves," Dr Rappenglueck says, "is to understand the people who painted these walls. They painted the sky, but not all of it. Just the parts that were specially important to them." Cro-magnon man The animals were painted on to the walls of the chamber by Cro-magnon man, one of our close relations, 15,000 years ago. He thrived in a temperate valley in the Dordogne while the rest of Europe was held in the grip of an ice age. Dr. Rappenglueck gave a tour to David Whitehouse of the BBC. "Here it is," he said, as he headed down a passage. He was pointing to a line of dots painted half way up the wall. "Count them. Count them." Below a stunning painting of a deer was a row of 13 dots, ending in a square.

"Why 13?" "It’s half of the Moon’s monthly cycle," Dr. Rappenglueck said. "One dot for each day the Moon is in the sky. At the new Moon, when it vanishes from the sky, we see an empty square, perhaps symbolically representing the absent Moon. "But there’s more, further along." The Munich researcher gestured to me to move along the passageway. Beneath a dappled, brown horse with a dark mane was another row of dots. This time there were more of them. "There are 29 of them - one for each day of the Moon’s 29-day cycle when it runs through its phases in the sky. It was a rhythm of nature that was important to these people." Dr. Rappenglueck looked around at the bulls, antelope and horses painted on the walls with such obvious admiration. "They were aware of all the rhythms of nature. Their survival depended on them, they were a part of them." But there is another puzzle. The series of dots that curve away from the main row.

"Why do they do that?" "I think that indicates the time of the new Moon, when it disappears from the sky for several days," said Dr. Rappenglueck. There is definitely astronomy on the walls of Lascaux. Earlier this year, Dr Rappenglueck identified a series of constellations painted on the wall of a shaft off the main chamber at Lascaux. The tiny pattern of the Pleiades star cluster can also be seen hanging above the shoulder of a bull near the entrance to the main passageway. We will probably never understand completely what Cro-magnon man had in mind when he painted the Lascaux caves. The images of the animals seem obvious but what are we to make of the geometrical shapes and patterns scattered in between these creatures? Calendars in Iceland (before literacy) Viking origins Traditionally, the Vikings originating in Scandinavia in the early Middle Ages are associated with violence and brutal force. However, the views of modern scholars paint a less mono-chromatic picture. Many of the activities of the Vikings required and produced knowledge of time-reckoning and of what we would nowadays classify as astronomy. For example, their extensive travelling and trade must have involved some knowledge of astronomy.

The necessity of such knowledge is generally recognized in the case of coastal navigation, but also holds for inland travel through previously unknown areas, such as the vast lands of Eastern Europe. Inland travel and coastal navigation is one thing, but regular trans-oceanic traffic is quite another. Yet such traffic was required to support the Scandinavian settlement of Iceland and Greenland, around the years 900 and 1000 respectively, at a time when the people of Europe knew nothing of the compass or the sextant. Even with good luck the oceanic voyage would take about a week, and without it land might not be sighted for several weeks. The navigational methods used included both terrestrial and celestial observations. There is hardly any doubt that the knowledge written down on vellum in Iceland in the twelfth and thirteenth centuries derives to a high degree from these observations and this experience. Why did they need a calendar? In 930, the Icelanders decided to establish the Althingi, a kind of parliament where an important part of the population gathered once a year for purposes of legislation and justice. Those who went there would spend two to five weeks away from home at a precious time of the year.

The farms were scattered at long distances and the landscape often barely passable. Therefore the traditional Scandinavian method of summoning meetings by message was not viable  they needed a simple and reliable calendar to help people know when to start from home so as to arrive at the same time as the others. Moreover, since the Icelandic summer is short, it was a matter of primary concern to utilize summer time as well as possible, and date the parliament at the time of summer when the loss of domestic labor was least harmful. To understand the need for a calendar we may also look at the agriculture itself and its annual cycle. Certainly, the caprices of Icelandic weather and nature are such that the calendar may often be a bad guide for action. In deciding when to let cattle and sheep out on grass or when to start hay-making it is better to observe the actual signs of nature than the calendar. But there are certain kinds of annual operation where the calendar proves superior: for example, in determining when to sow the grain, something which people had tried with little success in the first centuries of settlement in Iceland. Another good example is that of deciding when to let the ram to the ewes.

It is important to do this at the right time in the winter so that the lambs have the best possible prospect of growing in the short summer, without too much risk of interludes of bad weather in the spring just after they are born. When the individual farmer makes his decision on this at some point around Christmas time, he has no clear natural signs of a terrestrial nature to go by. How similar was it to the Julian calendar? In the brief history of Iceland called Íslendingabók (The Book of the Icelanders, Libellum Islandorum), written by Ari the Learned in the period 1122-33, we have a report on a calendar reform around 955: This was when the wisest men of the country had counted in two semesters 364 days or 52 weeks-then they observed from the motion of the sun that the summer moved back towards the spring; but there was nobody to tell them that there is one day more in two semesters than you can measure by whole weeks, and that was the reason. There was a man called Thorsteinn the black, a very wise man.

When they came to the Althing he sought the remedy that they should add a week to every seventh summer and try how that would work. By a correct count there are 365 days in a year if it is not a leap year, but then one more; but by our count there are 364. But when in our count a week is added to every seventh year, seven years together will be equally long on both counts. But if there are 2 leap years between the ones to be augmented, you need to add to the sixth. How did Thorsteinn the Black determine his intercalation? His farm was favorably located in the country to utilize the so-called mountain circle method, that is, to follow the annual motion of sunrise and sunset near the horizon where he would have suitably distant mountains and other reference points in the landscape to make fairly exact observations possible. At high latitudes the points of sunrise and sunset move so fast that this method could easily be used to determine the length of the year to within a day. According to this, people started by counting 52 weeks or 364 days in the year.

When they realized the insufficiency of this they tried the remedy of intercalating one week every seventh year (sumarauki), thus making the average year 365 days. The method chosen may seem strange to us but it is a natural consequence of the important role of the week in the original calendar. So far the interpretation of the text seems straightforward. However, the text continues to describe the relation and adaptation of the Icelandic calendar to the Julian one, which must have been gradually introduced in Iceland in the eleventh and twelfth centuries, following formal Christianisation of the country in the year 1000. The text says that if there are two leap years between the years to be increased by a week, then the sixth year (instead of the seventh) should be increased. This is plainly wrong and would yield a worse approximation than the more simple rule of intercalating a week every sixth year. Scholars find this confusing, except by assuming the Latin meaning of the numerals. Thus ‘septimo quoque anno’ actually means ‘every sixth year’ by our count. In this way Ari’s text can be interpreted so as to coincide with practice in his time, as seen from almost contemporary Easter tables.

Also, he would escape Occam’s razor, since his formula would otherwise be more complicated than necessary for its accuracy.

Question: Who were the first people to measure the year?
Answer: Babylonians