2012-09-01 – What’s a blue moon? A second full moon in a month. Is there anything special about it? Not in the least.

The blue moon is nothing more than the result of trying to have an integer number of month in a year. The problem is: 12 lunar cycles is 354 days, 13 lunar cycles is 383 ½ days, and a solar year is 365 ¼ days. There is no way to make these coincide. So different societies have adopted different cheats to make it work out.

The Jews make it work out by having some years of 12 lunar months and some years of 13 lunar months. After 19 years of this it roughly comes back to the beginning. The Romans, who gave us the Julian calendar, which then morphed into the Gregorian calendar developed the cheat of stretching the month so that 12 of them equal a year. When you extend the month, you create the possibility of having two full moons within a single month.

The Islamic world gave up the effort altogether. They have 12 lunar months period. Their year is always short, so holidays like Ramadan slip back about 11 days per year. Ramadan recently ended. Next year it will be earlier in the summer. In a few years it will be in Spring. And so on.

Today is the first of September. For much of the world we are entering a period of harvest. Soon the leaves on the trees will turn red and yellow and brown and then fall off in the wind. Monday will be Labor Day. In my mind it is the end of summer. School begins in Chicago. And soon will be the Jewish New Year.

The Jewish New Year moves around in September because the Jewish calendar is based on the lunar cycle, but it doesn’t drift far because of the 13^th month that is periodically added to bring the lunar calendar in line with solar cycle.