Lunar Year

Lunar Year[1]

Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

A lunar year or lunar calendar is one that is based on the cycles of the moon phases. The problem with a lunar calendar is that it drifts away from the seasons. Each year, the start and end dates of each month drift by 11 days. In order to stay correct, every lunar calendar has to deal with this drift away from the calendar year.

Lunar New Year, was called Chinese New Year, Tet (in Vietnam), and in other countries they would have another name for it. I will stick with Lunar New Year, so we could include all the countries who celebrate it. In China, Chinese New Year is also known as “Spring Festival”.

Let’s examine a year. A lunar month lasts 29.53 days. So after 12 lunar months, you’re about 354 days. This is short of the 365 days that it takes the Earth to orbit the Sun. This is a problem since after about 3 years, the lunar months are out of cycle with the solar year by about a month. And this problem would just continue.

To make the lunar calendar work in China, farmers would add in a leap month every 3 years. This would mostly get the lunar month to line up with the solar year, but they still drifted apart somewhat. For some calendars used for religious purposes, such as the Islamic Hirji calendar, they never bothered to sync up the calendars and let them drift. It takes 33 years for the cycle of lunar years to get back to the original position.

A lunar calendar was used in England up until Tudor times.

The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Lunar year—12 days (and some minutes and seconds.)

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Sun, Moon and Stars (February 26, 2013) and Astronomy (January 2016, p. 12)


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