The Islamic New Year is scheduled to begin at sundown on April 5 in the year 2000. I have to specify 2000 because in 1999 it began at sundown on April 16, and in 2001 it began at sundown on March 25. The Islamic New Year changes by about 11 days a year because it's based on a purely lunar calendar.

The mathematics are quite simple. If you subtract the 354+ days of a lunar year from the 365+ days of a solar year, you get approximately 11 days. Therefore a continuous series of lunar new years will begin about 11 days earlier every year on a fixed solar calendar.

Most of the ancient religions and cultures that used lunar cycles to determine the dates of their key festivals, fasts, and celebrations learned to include an extra month some years to keep their lunar calendars in synch with the solar seasons. But the early Islamic leaders wanted to break with old traditions.

They wanted to start a new calendar to mark the beginning of their new religion, and they wanted to make it purely and continuously lunar to differentiate it from the solar and lunisolar calendars that already existed.

They chose to start their new calendar on July 16, 622 A.D. — or, technically, since Islamic days begin at sunset, at sunset on the date the Gregorian calendar calls July 15 — because that was the first day of the lunar year in which the key Islamic event called the Hegira took place.

The Hegira was Mohammed’s emigration from Mecca to Medina to assert his new religion. Between July 15-16, 622 A.D. and April 5-6, 2000 A.D., the Islamic calendar worked its way through 1420 consecutive lunar years, with a few mathematical adjustments along the way.

Basically, a lunar calendar needs to alternate 29 and 30 day months to reflect the approximately 29.5 day lunar cycle. But because the moon actually takes a fraction more than 29.5 days to complete its cycle, a lunar calendar needs an occasional extra day to keep its months in synch with what the real moon is doing.

The Islamic calendar adds that day to the last month of their lunar year in 11 out of every 30 years. With these regular additions, their calendar has become as precise with respect to the moon as the Gregorian calendar is with respect to the sun.

Now all we need is a major world calendar that's precise with respect to the stars, and we modern, calendar-driven human beings might be almost as aware of the sky as our primitive, pre-calendrical ancestors were.

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