A lunisolar calendar is a calendar that marks time by tracking the movements of both the moon (Latin luna) and the sun (Latin sol). The most well-known lunisolar calendars in use right now are the Jewish calendar and the Chinese calendar, but throughout history there have been many cultures that have used different forms of lunisolar calendars.
How calendars are made
Creating useful, consistent, universal units of time is much trickier than it may seem. Though the day (sunup to sundown) seems like an intuitive basic unit at first glance, its length varies depending on where you are in the world and what time of year you find yourself in. Moreover, counting days is not very useful for measuring large units of time: most of us, especially in urban environments, need to work with larger blocks like months and years.
Following the movements of celestial bodies is a pretty good way to mark time: unlike crops, animals, and human bodies, the movements of the sun, the moon, and the stars are consistent, long-term, easy to track, and unaffected by natural disasters.
The sun and the moon each provide certain advantages and disadvantages for calendar-making. A lunisolar system tries to combine solar and lunar systems to achieve the advantages of both.
Solar calendars are used in most of the industrialized world today. The ancient Egyptians and the imperial Romans both used variations of the system that we use; in fact, many of our month's names are borrowed from the Romans. Solar calendars track the movement of the sun across the sky, in a path that astronomers call the ecliptic.
Such a calendar is keyed almost exactly to the movement of the earth around the sun in space, creating a convenient link between the calendar and the seasons. (I say "almost exactly" because the earth does not take a nice, even number of days to revolve around the sun, and systems need to be put into place to get them back in synch.)
- Advantages: Accurate with regard to the seasons: in the northern hemisphere, July will always take place in the summer. Relatively accurate over the long term, especially if your culture knows how to mess around with intercalary days.
- Disadvantages: Most people have never heard of an ecliptic and wouldn't know how to follow the sun across one. (In other words, the average person cannot simply glance up and know what day it is; a solar calendar needs to be maintained by professional priests or astronomers.) Also, a straight-up solar calendar might be accurate enough for the length of a generation or two, but elaborate systems of leap years, leap minutes, and leap seconds need to take up some of the slack over the course of centuries.
Lunar calendars, such as the Muslim calendar, count months ("moon"ths) by the phases of the moon.
- Advantages: An easy, intuitive system: a lunar month is a convenient and easily-visible way to track time.
- Disadvantages: A lunar month is almost exactly 29½ days, meaning it doesn't divide up neatly into sunup-sundown days, and thus the beginning of a new lunar month is not perfectly predictable. Thirteen lunar months make a year of a little over 354 days, which does not match the solar year of 365¼ days. Therefore the months "slide around" throughout the year. (This why Ramadan does not always take place in the same season.)
The best of both worlds?
Lunisolar calendars are an attempt to combine the best of both systems by using lunar months and inserting "leap months" (what astronomers call intercalary or embolismic months) every several years to keep the seasons in synch. The ancient Babylonians used a lunisolar calendar, which was eventually incorporated into the Jewish calendar with very similar names for the months.
Many very ancient calendars -- including that of the pre-Jewish Israelites and the ancient Chinese -- were purely lunar, but became lunisolar to make record-keeping easier.
Each culture has its own method for matching lunar months to solar movements. There are generally two goals: to make the math work out over time so that the seasons continue to match the months; and, just as importantly, to make sure that the culture's religious festivals fit a traditional pattern. For example, the Hebrew calendar adds seven intercalary months in each block of nineteen years, but the system is complicated by certain religious rules that forbid the New Year from falling on a Monday, Wednesday, or Friday. Similarly, the Chinese New Year has several traditional restrictions, including one that forces the winter solstice to take place in the eleventh month, meaning that the intercalary month is not always inserted in the same place in the calendar.
To those of us who were raised with the apparent simplicity of a solar calendar, the lunisolar calendar may seem strangely complicated. We only have to worry about a rogue day in February every four years! At least our months always have the same names!
But truth be told, no matter what system you use, you will probably need a professional to work it out for you. (Quick! On what day of the week does Christmas fall this year? What? You don't know? When do you set your clocks back for Daylight Savings Time? When's Easter next year? What date will Thanksgiving fall on in 2015? Does May have 30 or 31 days?)
Rather than working out the relationships between days, weeks, and months from scratch by ourselves, we just do what the printed calendar tells us, trusting professionals to keep everything in line. As it happens, every culture does this to some extent. Like us, they simply get used to the "exceptions."
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