A Lesson on Luni-solar Calendars for Losar


[A Tibetan mandala, representing the orbits of the moon, sun, and planets]

February 27, 2017 marks Losar, the beginning of the Year 2144 on the Tibetan Calendar, a year of the Fire Bird. You might be saying, hey, wasn’t the Chinese New Year last month, and you would be correct. The Chinese New Year indeed fell on January 28 this year. Both the Han Chinese and Tibetans traditionally follow what we call luni-solar calendars, but despite their geographical proximity, these two cultures have very distinct rules for calculating their calendars.

Unlike our solar calendar of 365.25 days (called solar because it is based on the orbit of the earth around the sun), luni-solar calendars use the 29.5 day cycle of the moon’s phases to define 12 lunar months (normally 6 each of 29 and 30 day lengths) in their year. This means that a luni-solar year is too short, by about 11 days, and in response the holidays will drift earlier and earlier each year. To prevent this, every few years, an extra lunar month (called an intercalary month) is added, and the holidays move back to their original starting point relative to the seasons. You might have noticed that the Jewish holidays can drift somewhat relative to our solar calendar, but you never celebrate Hanukkah in July, for example. This is because the traditional Jewish calendar is also a luni-solar one. In contrast, the Islamic calendar is strictly lunar, so Islamic holidays (such as Ramadan) continually shift earlier and earlier relative to the solar year, occurring in different seasons as the decades pass.

But if both the Chinese and Tibetan calendars are luni-solar, why don’t their New Years occur on the same day relative to each other? This is because they have different calendars with very different rules. For example, the Chinese calendar adds an intercalary month about every 3 years. The lunar month begins at the astronomical New Moon, and there are specific rules for which months have 29 or 30 days based on the exact time of the New Moon relative to a specific longitude line (120 East). Chinese New Year falls on the day after the second New Moon after the Winter solstice (so it is often in February but sometimes in January, like this year). In contrast, in Tibetan Buddhism the phases of the moon are calculated by different rules, and the Full Moon must fall on the 15th day of any month, with the New Moon on the 30th day. So the Tibetan calendar uses skip days (tsi chad-pa) and doubled days (tsi lhag-pa) to accomplish this. Therefore a particular month might have two 19ths and no 23rd (roll that around in your head for a while!). Intercalary months are added about every thirty lunar months, and which months have 29 versus 30 day months are decided by the phase of moon at sunrise. New Year (Losar) is traditionally the first day of the first lunar month of the year. It does not always fall on Chinese New Year because the rules for calculating intercalary months, phases, and defining which months have 29 and 30 days differ. It will not surprise you to learn that the rules for calculating the Tibetan calendar are complex, and passed down from master to student.

So Tashi Losar to all my Tibetan friends, and if you want to learn more about Tibetan cultural astronomy, peruse the following page.

-Kris Larsen



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