Calendars
Out of all the human inventions most taken for granted, I would argue that calendars rank highly.
A calendar (derived from the Latin word calendarium, meaning "account book" or "register") is a system of time. Virtually all calendars follow the naturally occurring cycles of the moon and sun, which allows them to subsection the vastness of time into shorter periods, such as months and years.
The world's oldest known calendar is the Warren Field calendar (1), which still exists today as a curvilinear series of pits located in a field in eastern Scotland. The Warren Field calendar is thought to have been constructed by hunter-gatherers as early as 8,000 BC, which would make it around 10,000 years old. Despite its archaic origins, this calendar was only excavated very recently, from 2004 to 2006.
The Warren Field calendar is a curvilinear sequence of 12 pits, with the pits corresponding to the 12 lunar months (the time between two full moons) that occur during one solar year (revolution of the Earth around the sun). The outer pits are the smallest and the inner pits the largest, with the entire line of pits approximating the shape of an arc. Ancestral humans used the position of the moon relative to the pits to track the status of the solar year. Remarkably, rather than being constructed by sedentary farmers, this calendar was constructed by nomadic hunter-gatherers, possibly as a means of predicting the arrival times of the migrating animals that they relied on as a source of food.
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The Warren Field calendar consists of a curvilinear series of pits, with the pits corresponding to the lunar months. |
There would have been one potential problem with the Warren Field calendar. On average, a lunar month is 29.5 days, so 12 lunar months equals 354 days. However, one solar year lasts 365.2422 days. Thus, a calendar based solely on lunar months would gradually become desynchronized from the seasons of the solar year, which would have been less than ideal for a system of time that was designed to predict the arrival of migrating animals.
The builders of the Warren Field calendar solved this potential problem by taking advantage of a prominent topographic feature called the Slug Road pass. One of the pits was aligned with the pass in such a way that when the sunrise of the northern hemisphere's midwinter solstice (when the north pole is maximally distant from the sun, either December 21st or 22nd) shone through the pass, it was time to reset the solar year. Using this reset mechanism, the lunar months were able to be continuously resynchronized with the seasons from one solar year to the next.
Ancient Calendars
The Warren Field calendar is a lunisolar calendar, in which time is tracked using a combination of lunar months and solar years (2). Most ancient calendars were lunisolar calendars.
All lunisolar calendars require some sort of manual reset mechanism, or intercalation, to periodically realign the lunar months with the seasons of the solar year. The Warren Field calendar used a topographic feature, the Slug Road pass, to reset the solar year; however, the majority of lunisolar calendars performed the reset by adding an extra "leap month" to every 2second or third year, which brought the lunar months into rough agreement with the seasons of the year. The lunar cycle associated with the leap month came to be called a "blue moon" (a term that has nothing to do with the moon's colour; it may have been meant to denote something not possible, such as the moon turning blue). The relative rarity of a blue moon led to the phrase, "once in a blue moon," which is still used today to describe the occurrence of a rare event.
The world's oldest "formal" lunisolar calendar was the ancient Sumerian calendar of southern Mesopotamia, the origin of which dates back to 2100 BC. The Sumerian calendar was the precursor to the Babylonian calendar, which was the primary calendar used by the mighty empires of Assyria, Babylonia, and Persia. The Babylonian calendar's solar year consisted of 12 lunar months, which began when a new crescent moon was first sighted low on the western horizon at sunset. When it was needed, the intercalary month was inserted by royal decree; this method was later altered to incorporate a more regular intercalation cycle. Other ancient calendars, most notably the Hellenic calendar of ancient Greece, the Hindu calendar of India, and the Buddhist calendars of Southeast Asia, were also lunisolar calendars.
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The Babylonian calendar, an early lunisolar calendar. |
Despite the relative rarity of an intercalation in the lunisolar calendar, some cultures did not agree with the idea of humans tampering with systems of time. The most notable protest was issued by the Islamic prophet Muhammad, who stated around 632 AD that "intercalation is only an increase of infidelity, by which the infidels lead people astray" (3). This proclamation resulted in the Islamic calendar becoming a lunar calendar, following only lunar cycles, such that the calendar gradually became desynchronized from the seasons of the solar year. Nowadays, the Islamic calendar is mainly used to determine Islamic holidays and fasting days; it has been replaced by more modern calendars.
Modern Calendars
The majority of modern calendars are solar calendars, which do not track time by lunar months but by solar years (4). The dates of a solar calendar indicate a specific time point in the season of a solar year. There are actually two types of solar calendar, tropical calendars (which accurately track the angle of the sun) and sidereal calendars (which accurately track the location of the sun relative to the stars); the difference between them is slight, and not worth belabouring.
The first solar calendar was the Julian calendar, which Julius Caesar inserted in place of the old Roman lunisolar calendar in 46 BC (5). Like Muhammad, Julius Caesar was not a fan of manual intercalations, so he proposed a calendar that had its intercalations automatically built-in. The Julian calendar contained two types of years - a normal year of 365 days, and a "leap year" of 366 days. The Julian calendar followed a simple cycle of three normal years and one leap year, resulting in an average calendar year of 365.25 days (which is very close to the solar year of 365.2422 days). By focusing on solar cycles, the Julian calendar was the first calendar to completely dissociate the lunar cycles from the months of the calendar year.
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Julius Caesar, instigator of the Julian calendar, an early solar calendar. |
The Julian calendar was successfully implemented for over 1500 years, but it eventually ran into desynchronization problems similar to those experienced by its lunisolar predecessors. The problem was that a solar year is 365.2422 days, which is not an even number of days. Since a calendar year requires an even number of days, the slight discrepancy eventuated in the Julian calendar gradually becoming desynchronized with the solar year.
To circumvent this problem, Pope Gregory XIII replaced the Julian calendar in 1582 with the calendar that most of the world now uses today, the Gregorian calendar (6). The Gregorian calendar was very similar to the Julian calendar, except that it followed a slightly more complex rule with regards to the leap years - every year divisible by four was denoted as a leap year, except for those years divisible by 100; however, these centurial years were still leap years if they were divisible by 400. Although this rule was a tad complex, the Gregorian calendar reduced the drift between calendar year and solar year even further by shortening the average calendar year from 365.25 days to 365.24.25 days, resulting in it being very closely aligned to the solar year.
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Pope Gregory XIII, instigator of the Gregorian calendar, our current solar calendar. |