Time KeepersCelestial bodies – the sun, moon, planets, and stars – have provided us a reference for measuring the passage of time throughout human existence. Ancient civilizations like: China, India, Babylon, and Greece relied upon the apparent motion of these bodies through the sky to record and determine seasons, months, and years. We know little about the details of timekeeping in prehistoric eras. However, records and artifacts usually uncover that in every culture, people were preoccupied with measuring and recording the passage of time.
Stonehenge, built over 4000 years ago in England has no written records, but its alignments show its purposes apparently included the determination of seasonal or celestial events, such as lunar eclipses, solstices and so on. As time has passed so has the evolution of the calendar, a device created to track our time and seasons from the earliest recordings in Babylonia to the Gregorian calendar the history of this transformation is and interesting journey.
The earliest know calendar to keep track of the cycles of the celestial bodies was an Egyptian calendar that was based on the moon’s cycles and is thought to have been created in 4236 B.
C.E. Many cultures and societies have embraced the idea of tracking time and seasons as they pass for a myriad of reasons, Seafarers needed to navigate their vessels, and farmers had to know when to plant their crops. (Chaisson / McMillan p.30) The Chinese are credited with having invented the second oldest method of time keeping; Emperor Huangdi implemented the Chinese legend in 2637 B.C.E. Babylonia (where modern day Iraq can be found) is attributed with having some of the earliest surviving records of astronomical observations. It is believed, Babylonian astronomical knowledge spread far and wide to the East, to Persia, and to the Mediterranean. (Richards p. 38) However, the knowledge that was disbursed was not treasured by all that received it, in the Mediterranean the Greeks improved upon the theories of the Babylonians. The Greeks theories were recorded; however, when Rome over-took most of Europe the records fell into the hands of the Christian church. When Constantine was Emperor of Rome he declared Christianity to be the official religion of the empire; thus, giving the church officials the power to decide the validity of the recordings. Given that the Greeks theories were of a secular nature and they did not fit into Christian dogma, the documents were destroyed or sold during the Christian Middle Ages. Luckily, our neighbors to the East, the Arabs, found the documents to be of great interest and kept old Greek astrological records. In the Renisannce period of Christianity the church officials decided to re-examine the ancient records, and actually found some validity in some of the scientific data. Amazingly enough the Christian church decided to implement data from ancient pagan cultures to help create the most widely used calendar to date, the Gregorian calendar. Ancient Greek astronomers made some amazing mathematical and philosophical discovers about our universe. From the Hellenistic Greek observations in approximately 300 B.C.E., to the invention of the first telescope in the seventeenth century, to the launching of todays space probes, one thing is evident: astrological observations are imperative to creating a calendar. Currently, the concept of a year is based on the earth’s motion around the sun. The time from one fixed point, such as a solstice or equinox, to the next is called a tropical year; its length is currently 365.242. Our concept of a month is based on the moon’s motion around the earth, although this connection has been broken in the calendar commonly used now, the Gregorian calendar. The time for the moon to complete a full cycle of phases is called a synodic month, and its length is currently 29.53 days. Note that these numbers are averages. The actual length of a particular year may vary by several minutes due to the influence of the gravitational force from other planets. Similarly, the time between two new moons may vary by several hours due to a number of factors, including changes in the gravitational force from the sun, and the moon’s orbital inclination. It is unfortunate that the length of the tropical year is not a multiple of the length of the synodic month. This means that with 12 months per year, the relationship between our month and the moon cannot be maintained. However, 19 tropical years is 234.997 synodic months, which is very close to an integer. So every 19 years the phases of the moon fall on the same dates (if it were not for the skewness introduced by leap years). 19 years is called a Metonic cycle (after Meton, an astronomer from Athens in the 5th century B.C.E.).
The Christian calendar (Gregorian calendar) is based on the motion of the earth around the sun, while the months have no connection with the motion of the moon. On the other hand, the Islamic calendar (Hijri calendar) is based on the motion of the moon, while the year has no connection with the motion of the earth around the sun. Finally, the Jewish calendar combines both, in that its years are linked to the motion of the earth around the sun, and its months are linked to the motion of the moon. The Christian calendar developed from the ancient Roman calendar, who borrowed its design from the ancient Greeks. The ancient Roman calendar consisted of 10 months in a year of 304 days. The Romans ignored the remaining 61 days, which fell in the middle of winter. The 10 months were named Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. Romulus, a legendary first ruler of Rome, was supposed to have introduced this calendar in approximately 700 B.C.E. By the 1st century B.C.E., the Roman calendar had become hopelessly confused. The year, based on cycles and phases of the moon, totaled 355 days, about 10 1/4 days shorter than the solar year.Because this calendar did not agree with the seasons, an extra month was added in some years. Those years had thirteen lunar months. Some called this extra month Mercedonius. (Evenson p. 12) This confusion of tracking time was compounded by political maneuvers. The Pontifex Maximus and the College of Pontiffs had the authority to alter the calendar, and they sometimes did so to reduce or extend the term of a particular magistrate or other public official. Finally, in 45 B.C.E., Julius Caesar initiated a thorough reform that resulted in the establishment of a new dating system, the Julian calendar.
The Julian calendar was created with advice from contemporary astronomers and the pretence to a lunar calendar was abandoned, but before he completely abandoned the current calendar he added eighty extra days to the calendar, so it would be maneuvered back in time with earths seasons. Then, the tropical year was divided into twelve months and the introduction of an intercalary day was put into effect. This intercalary day would occur once every four years to offset a total 365 average days in a year; however, unrenowned to the astronomers of that time the Julian calendar would still be off by twelve minutes a year. He must have been well acquainted with Hipparchus measurement of the length of the tropical year and that 365 days was significantly different form its true length. (Richards p.216) Although the Julian calendar lacked complete unity the Christian officials implemented its use and tolerated the sluggish and inaccurate legend. From the fourteenth to the early sixteenth centuries the Papacy of the Christian faith tolerated the inaccurate days and watched the equinoxes occur in contradiction with the churchs calendar, and the date of Easter was going awry. Due the politics of the church officials no major changes were implemented until Pope Gregory XIII. Finally, in 1582 C.E. with the aide of astronomer Christopher Clavius (1537-1612), he signed a papal bull and that was followed by, The actual change over to the new calendar took place the following year on 4 October. After 350 years or more the reform had at last been accomplished. (Richards p.246)Furthermore the rule for leap years (which said that years divisible with 4 should be leap years) was changed so that years, at the end of the century, should be leap years only if they were divisible with 400 (e.g. 1600, 2000, 2400 etc.)In the Gregorian Calendar there is then 303 years with 365 days and 97 years with 366 days, which gives a mean year of 365.24250 days: 365 days, 5 hours, 49 minutes and 12 seconds. Related to the mean interval between vernal equinoxes this corresponds to a slippage of less than one hour in every 300 years for the foreseeable future – until circa 4000 AD.
Chaisson, Eric and Steve McMillan. Astronomy Today. New Jersey: PrenticeHall, 1999.
Evenson, A.E. About the History of the Calendar. Canada: Regensteiner Publishing, 1972.
Richards, E. G. Mapping Time, The Calendar and its History. New York: Oxford University Press, 1998.
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