Keeping time: Holidays and the history of calendars
Different types of calendars developed throughout history illustrate humanity's long-standing quest to measure and organize time across cultures and centuries.
Some holidays, such as Christmas, fall on the same day every year. Others, like Hanukkah, move around the calendar. By sheer coincidence, the first night of Hanukkah falls on Christmas this year – the first time that's happened since 2005. Even the equinoxes – the astronomical phenomena that determine the start of fall and spring – vary slightly from year to year.
Why do some major events move while others stay in place? University experts say the answer lies in the large variety of calendars that have been created throughout history, with some aligning with lunar cycles and others aligning with solar cycles, and some using a combination of both.
Time to start keeping time
It's difficult to pinpoint when the earliest timekeeping methods developed, because they predate written records. The reason humans began keeping track of time is much more clear, said John Bauschatz, associate professor in the Department of History and the Department of Religious Studies and Classics.
John Bauschatz, Associate Professor, Department of History and the Department of Religious Studies and Classics
"The easiest answer for that is one word: agriculture," Bauschatz said. "You have to make food to survive, and people realized early on that the world changes on a predictable cycle and it was necessary to figure out how long those cycles take to plan for planting and harvesting."
Religious and cultural rituals and observances also helped shape how cultures marked time, Bauschatz said.
"Most ancient civilizations identified heavenly bodies with their gods," he explained. "They believed the positions and movements of stars and planets, which were representations of their gods, would have an impact on what was going on on Earth."
Most cultures noted two basic cycles: the daily cycle, or the spin of the Earth, and the monthly cycle, which has to do with the movement of the moon around the Earth.
"The earliest techniques for measuring time had to do with measuring the daily cycle," said Renu Malhotra, Regents Professor in the Department of Planetary Sciences. "The simplest technique is to put a stick in the ground and watch how the shadow moves as the sun moves in the sky."
Renu Malhotra, Regents Professor, Department of Planetary Sciences
As time went on, people were able to get more accurate in their measurements. Many cultures first assumed that a year was 360 days. But as the measurement has become more accurate, we now know a year is about 365 and one-quarter days, which is why we use leap days.
The variety of calendars that has been created is a reminder of humanity's millennia-long journey to organize time across cultures.
Gregorian calendar
The 12-month, 365-day Gregorian calendar, which is a solar calendar, was officially established in 1582 by Pope Gregory XIII and has become the standard through much of the world. Great Britain and its American colonies adopted the calendar in 1752.
The system is based on the Julian calendar – established by Julius Caesar in the 40s BC – with one notable difference: In the Gregorian calendar, leap days are skipped in years that are divisible by 100 but not 400.
Much of the world has adopted the Gregorian calendar for practical reasons, Malhotra said, citing the need for a common calendar to schedule global events like meetings among world leaders.
Jewish calendar
The Jewish calendar, which dates to the fourth century A.D., also has 12 months but adds a 30-day leap month seven times each 19-year lunar cycle, which is one of the reasons the dates of Jewish holidays such as Hanukkah vary on the Gregorian calendar. Because of the leap month structure, the length of a year can vary from 353-385 days. The calendar is lunisolar, meaning it is based on the positions of both the moon and the sun.
Maya calendar
In the movie "2012," a writer, played by John Cusack, fights for survival as a series of epic disasters blankets the Earth. The film, which was released in 2009, capitalized on a belief held by some that the world would end in 2012. That fear was based on the Maya Long Count calendar, which was used by the Maya civilization, although the Maya people never forecast a 2012 doomsday scenario.
Takeshi Inomata, Regents Professor, School of Anthropology
Besides the Long Count calendar, which counts the number of days from the base date in 3114 B.C., the Maya used other calendars, including a cyclical one consisting of a ritual cycle of 260 days and a solar year of 365 days. The two would intersect every 18,980 days, or 52 years, to complete what was called a "Calendar Round." The completions of various calendar cycles were important in Maya beliefs, and the largest cycle was called a "Great Cycle" of 5,125 years in the Long Count calendar.
"The latest Great Cycle was completed in 2012, so some people thought that was going to be the end of the world," said Takeshi Inomata, Regents Professor in the School of Anthropology. "Of course, Maya people didn't think that way. For the Maya people, it was just the end of one cycle, then time would continue to go on."
For those who like to plan ahead, a smaller cycle of 400 years in the Long Count calendar will end on March 26, 2407.
French Republic calendar
In 1700s France, many revolutionaries wanted religion and royalty out of their calendar. Members of the First Republic government formed a team to head up the development of a new timekeeping method. The result was a calendar built on a base 10 system: Each day was 10 hours, each hour was 100 minutes and each minute was 100 seconds. That led to 12 months consisting of three 10-day weeks.
The system, which was adopted in 1793, quickly proved unpopular with clockmakers, who had to overhaul just about every part of their craft, as well as anyone trying to do business outside of France. In addition, the calendar resulted in longer work weeks and just three days off per month for workers, which was quite unpopular. The calendar hung on until 1806, when Napoleon Bonaparte reinstated the Gregorian calendar.