As we approach the transition from one year to the next, many occupy their time by pondering the motion of Earth relative to the rest of the solar system. (I know this isn’t true—most are preoccupied with what champagne to buy and/or if not eating Doritos should be a New Year’s resolution. For the sake of this article, I ask that you humor me.) Perhaps our most direct interaction with such movement is via the Gregorian calendar. Who uses the Gregorian calendar? As you’ll see, almost everyone.
Basic Concepts
The concept of time can get remarkably complicated. I won’t delve too deeply because it makes my brain hurt—I’ll just stick to a (hopefully) simple breakdown of the types of calendars.
Lunar Calendar
A lunar calendar is based on the phases of the Moon, with each complete cycle (also known as lunation or synodic month) lasting about 29.5 days. Months on a lunar calendar therefore commonly alternate between 29 and 30 days, with 12 such months (a lunar year) amounting to somewhere around 354 days. As such, with pure lunar calendars, the lunar months end up cycling through the entire solar calendar (see below) over a long enough period of time.
Solar Calendar
A solar calendar, as you might expect, is based on the solar year (also called tropical year). Taken essentially verbatim from Wikipedia, this concept refers to the time it takes the Sun to return to the same position in the cycle of seasons, as seen from Earth. In other words, a solar year would be from one spring equinox to the next spring equinox or one summer solstice to the next summer solstice, etc.
Lunisolar Calendar
Many “lunar” calendars are actually lunisolar calendars, using something called intercalation—the intermittent insertion of an extra day, week, or month—to stay generally aligned with the solar year (and not cycle through the entire solar year as mentioned above).
More on Solar Calendars
In modern times, solar calendars are familiar to the vast majority of the world. (When the ball drops on New Year’s Eve in New York City, it’s not about the Moon.) The story of how we achieved such remarkable worldwide synchrony is worth spending the next minute to know. Let’s pick it up with the Romans.
In 46 BCE, after consulting with some very smart people, Julius Caesar proposed what is called the Julian calendar, taking effect the next year. This ended up serving as the prominent calendar in the Western world for over 1,600 years. Using the 365-day year with a leap year of 366 days every fourth year, the average year was 365.25 days. Given that the actual solar year is closer to something like 365.24219 days, the calendar overestimated each year by around 11 minutes. Over many centuries, this discrepancy put the calendar several days behind what following the true solar year would have done.
In 1582, to correct for the above, Pope Gregory XIII proclaimed a new calendar, referred to as the Gregorian calendar. After October 4, 1582, the following day was actually October 15, 1582! To prevent the same sort of drift from occurring again, the Gregorian calendar has less leap years (century years are only leap years if divisible by 400—for example 1600 but not 1700—and years divisible by 4,000 are no longer leap years). The average year is thus 365.2425 days, closer to the solar year.
The change was quickly adopted in the Roman Catholic sphere, though Protestant and Eastern Orthodox countries in Europe weren’t on board. Non-European countries, of course, also continued to do their own thing.
Over time, the list of who uses the Gregorian calendar began to grow. As an example, Great Britain and its colonies joined the party in 1752, declaring the day after September 2 of that year to be September 14. Greece held out much longer but eventually took the leap in 1923.
So Who Uses the Gregorian Calendar Now?
The Gregorian calendar is in many ways a random piece of work, maintaining essentially all of the idiosyncrasies of the Julian calendar (minus some leap years). Among other quirks, the number of days per month is inconsistent, months can begin on any day of the week, and determining where to start and end a year is an arbitrary choice. That said, it has become everyone’s shared randomness.
Well, maybe not everyone. Countries such as Afghanistan, Iran, Ethiopia, Eritrea, and Nepal have not adopted the Gregorian calendar. Others use a modified version, with the most striking example being North Korea, where the year 1912—the birth year of founder Kim Il-sung—is Juche 1, the year 2011 is Juche 100, and so on. And many Eastern Orthodox churches still opt to follow the Julian calendar.
Even among the many who have adopted the Gregorian calendar for civil purposes, other calendars are still clearly in the mix, particularly when it comes to religious festivals and holidays. The Muslim world, for example, continues to follow a pure lunar calendar, explaining why Ramadan can take place at any point during the solar year.
Lunisolar calendars and concepts also remain big players. One of the best-known examples is the methodology used to determine the date of Easter, which takes into account the spring equinox as well as what the Moon is doing. (Maintaining a consistent spring equinox is actually partly what motivated the Gregorian calendar in the first place.) Other examples include the Chinese calendar and various Hindu calendars that determine the timing of Diwali.
If you want to have some fun on New Year’s Eve, go ahead and try to devise your own solar calendar, sticking to some basic rules but embracing the unavoidably haphazard result. On second thought, just stick to the champagne.
