It’s Chinese New Year (or Lunar New Year), which for those who don’t follow it probably has you wondering how that all works–since most calendars ticked over to the next year in January. That’s because we’ve widely adopted a solar calendar as a standard, specifically the Gregorian one. But lunar calendars are very much still a thing, so what is a lunar calendar, and how do they work?
So the Gregorian calendar largely evolved as a slight modification of the Julian calendar, shortening the year by about 0.008 days on average by modifying the rules around leap years. During a leap year the Julian February 29th is the Gregorian February 24th. As a solar calendar, we operate on tropical years. Leap years are intended to correct for small imperfections in the calendar that compound over time, which is the same reason the Julian calendar was largely phased out in favor of the Gregorian one.
Anyway solar years. They’re based on the seasons; how long it takes to go from the beginning of summer to the beginning of summer again. Or any season, it’s quite arbitrary. Fun fact, this is different from how long it takes for the Earth to go around the Sun once by about 20 minutes thanks to how the precession of equinoxes works. The year in terms of “Earth orbits the Sun once” is called the sidereal year, and is measured based on fixed stars (stars that do not move relative to each other in the sky, aka most stars that are not the Sun).
If you think about it for 2 seconds you’ve probably already figured out that lunar calendars go by lunar months. They’re based on a year calculated by synodic months, which is how long it takes something to rotate one time in relation to whatever it orbits. That’s a lot of jargon so we can take it to mean “one lunar month is the one complete cycle of the Moon’s phases.” Hopefully you’re familiar with the lunar phases, but they go from a new moon, which waxes to a full moon, then wanes back to a new moon. New moons are those nights where you can’t see the Moon in the sky, full moons are when you can see the whole thing.
The longest it takes to go from full to new moon (and vice versa) is about 15.5 days, while the shortest is a little under 14 days. Which means yes, lunar calendars require you to throw in a couple extra days every now and then to offset compounding errors. These intercalculations are done to remain consistent with the solar year. The Gregorian calendar does so with the February 29th leap day, but lunar calendars need to throw in an entire month. Why? Well due to those time discrepancies there are between 12 and 13 lunar cycles in a given year. You can probably imagine how confused people were when they realized the calendar and seasons didn’t line up. Throwing in extra months is done to keep the lunar calendar in line with the solar year (and solar calendars as a result) turns purely lunar calendars into lunisolar calendars.
Long short; solar calendars are based on years, lunar calendars are based on months.
There’s also the metal draconic month, based on the belief that during an eclipse a dragon would eat the Sun or Moon. It’s in reference to orbital nodes rather than the relative positions of the Moon to the Earth. Orbital nodes are constructed based on inclines, since things don’t all orbit on a flat plane. If you tilt one plane away from another, there are still two points where an orbit on that plane would intersect. Moving between those points is a nodal period. We really just wanted an excuse to mention that dragons play a role in some lunar calendars.
Where Are Lunar Calendars Used?
Obviously, lunar and lunisolar calendars are no longer in common, legal use. Only a handful don’t use the Gregorian calendar. Afghanistan, Iran, Ethiopia, and Nepal haven’t adopted the Gregorian calendar. The Solar Hijri calendar is used in Afghanistan and Iran, Ethiopia uses the Ethiopian calendar (which is 7 years behind), and Nepal uses the Vikrami calendar. Of all of these, only one of these is non-Solar; the Vikrami calendar is lunisolar.
Some countries use modified Gregorian calendars, like the Taiwanese Minguo calendar. Others have calendars that they use with the Gregorian one, like the Hebrew calendar (lunisolar) or the Indian national calendar.
While not commonly used legally, lunar and lunisolar calendars are still used for determining religious dates or even national holidays. If you celebrate Easter you celebrate a holiday based on a lunar calendar. New years in China, Japan, Korea, and Mongolia are other examples of lunar-set festivals (based on the Chinese, Japanese, Korean, and Mongolian lunar calendars). Diwali is set to the lunar Hindu calendar; Ramadan to the Islamic calendar. This is by no means an exhaustive list, the point is many religious festivals (and national holidays where applicable) are set to the tune of lunar or lunisolar calendars even if the country in question legally uses a solar one (probably the Gregorian one).
We look at the Moon a lot, but see if you know who’s been there here.