Throughout human history, celestial bodies such as the sun, moon, planets, and stars have been utilized for time measurement. Ancient civilizations like China, India, Babylon, and Greece heavily relied on observing these celestial bodies to determine seasons, months, and years. While knowledge about timekeeping in prehistoric periods is limited, evidence indicates that measuring and recording time was a common practice across all cultures. Stonehenge exemplifies this; although lacking written records, its alignments suggest it served as a means to track lunar eclipses and solstices. Calendars have undergone significant changes over time since their origin in Babylonia before culminating in the current Gregorian calendar. This journey of transformation is undeniably captivating.
The earliest known calendar based on the moon’s cycles was developed in Egypt around 4236 B.C.E. Many cultures embraced the idea of tracking time for various reasons, such as navigation for seafarers and determining planting seasons for farmers. In 2637 B.C.E., the Chinese invented another method of timekeeping. Babylon, located in modern-day Iraq, has some of the earliest surviving records of astronomical observations, which later spread to Persia and the Mediterranean. The Greeks expanded upon Babylonian theories, but when Rome became dominant, these records fell into the hands of the Christian church after Emperor Constantine declared Christianity as the official religion of the empire. During the Christian Middle Ages, Greek theories that did not align with Christian beliefs were either destroyed or sold. However, our neighbors to the East, the Arabs, found value in ancient Greek astrological records and preserved them. In the Renaissance period, church officials revisited these ancient records and discovered that some scientific data held validity. Surprisingly enough, instead of rejecting pagan influences completely, the Christian church decided to incorporate data from ancient pagan cultures into creating what is now widely used -the Gregorian calendar. Throughout history since Hellenistic Greek observations in 300 B.C.EFrom the invention of the first telescope in the seventeenth century to modern-day space probes, it is evident that astrological observations are crucial in developing a calendar. Currently, our understanding of a year is based on the earth’s orbit around the sun, where a tropical year spans from one fixed point (like a solstice or equinox) to another and measures approximately 365.242 days. Similarly, our concept of a month originates from the moon’s orbit around the earth; however, this connection has been severed in the widely used Gregorian calendar. The duration for completing a full cycle of phases by the moon is known as a synodic month and currently lasts about 29.53 days. It is important to note that these numbers represent averages since specific years can vary slightly due to gravitational forces exerted by other planets, resulting in variations of several minutes. Likewise, factors like changes in gravitational force caused by both the sun and the moon’s orbital inclination can cause differences of several hours between two new moons. Unfortunately, because a tropical year does not precisely align with multiples of a synodic month, maintaining a consistent relationship between our months and lunar phases becomes challenging with twelve months in a year. However, over nineteen tropical years (equivalent to approximately 234.997 synodic months), there is very close alignment between lunar phases and their corresponding dates (excluding leap years).The period of 19 years is referred to as a Metonic cycle, which was named after Meton, an astronomer from Athens during the 5th century B.C.E.
The Christian calendar, also known as the Gregorian calendar, is based on the earth’s orbit around the sun and does not align its months with the moon’s motions. In contrast, the Islamic calendar, or Hijri calendar, follows the moon’s movements and does not synchronize its year with the sun’s orbit. However, the Jewish calendar integrates both systems by tying its years to the earth’s motion around the sun and aligning its months with lunar cycles.
The Christian calendar evolved from the ancient Roman calendar, which was inspired by the ancient Greeks. Originally consisting of ten months totaling 304 days, it ignored 61 winter days. These ten months were Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October,
November,and December. Romulus is credited with introducing this calendar around 700 B.C.E.
However,inthe1stcenturyB.C.E.,theRomancalendarbecamechaotic.Itonly accounted for 355 days in a year based on lunar cycles and phases,fallingaround10¼daysshortofasolaryear.To remedy this misalignment,the Romans occasionally inserted an extra month in certain years,resultingin thirteen lunarmonths.This additional monthwas called Mercedonius(Evenson p.12).The issue of tracking time became even more complicated due to political strategies. The Pontifex Maximus and the College of Pontiffs had authority over altering the calendar, sometimes doing so to adjust the duration of a particular official’s term. In 45 B.C.E., Julius Caesar introduced an extensive reform that resulted in the establishment of a new dating system called the Julian calendar.
The Julian calendar, created with input from astronomers to align it with a lunar calendar, initially added eighty extra days to realign with the seasons before dividing the year into twelve months. Additionally, an intercalary day was introduced every four years to account for the average 365 days in a year. However, despite efforts to align it accurately, the Julian calendar still deviated by twelve minutes each year. Christian officials accepted this flawed system and tolerated discrepancies between equinoxes and the church’s calendar until Pope Gregory XIII made changes in 1582 C.E.
With astronomer Christopher Clavius’ assistance, Pope Gregory XIII issued a papal bull introducing a new calendar. The actual transition occurred on October 4th of the following year, but it took over 350 years for the reform to be fully completed (Richards p.216, p.246).
The modification of the rule for determining leap years means that only years divisible by 400 at the end of the century are considered leap years. This change was made in the Gregorian Calendar. As a result, there are now 303 years with 365 days and 97 years with 366 days in this calendar.
On average, a year in the Gregorian Calendar is 365.24250 days long which is equivalent to 365 days, 5 hours,49 minutes,and12 seconds.Due to this adjustment there will be slippage of less than one hour every300years between vernal equinoxes until around4000AD.
The book “Astronomy Today” was written by Chaisson and McMillan and published in 1999 by PrenticeHall.
Evenson, A.E. About the History of the Calendar. Canada: Regensteiner Publishing, 1972.
Richards, E. G. wrote a book called “Mapping Time, The Calendar and its History” which was published by Oxford University Press in 1998.
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