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No. 1502:
The Temptress Moon

Today, the moon deceives us. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

One chapter in David Duncan's wonderful book Calendar is titled Luna: Temptress of Time. The idea is that the moon's period of 29 ½ days has repeatedly misled calendar-makers. Twelve lunar cycles make a year of 354 days -- eleven days shy of the time it takes for us to circle the sun. Yet what more compelling measure of the passing days do we have? Our modern words measure and meter are both kin to the old Latin word mensis, which means month.

Duncan begins by looking at a ten-thousand-year-old bone. A row of vertical incisions marches across it. The marks change slightly every seven strokes. This appears to be a rudimentary lunar calendar carved by an ancient dweller beside the Baltic Sea. Every seven days the phase of the moon changes. That's probably what's being mapped out here on this old bone fragment. Other carvings, much older, offer direct sketches of the moon's phases.

As the moon rode the night sky, shaping our dreams, scripting our myths, and teasing our imaginations, it was a temptress. The twelve-month lunar year was close to right, yet maddeningly off.

The Babylonians would use a block of 13-month years and then get back on the solar cycle with a block of 12-month years. The old Jewish calendar also used lunar months, but they corrected their calendar by inserting an extra month every three years. And then they had to add yet another month every so often. The Greeks built their year out of twelve lunar months and fixed it by adding ninety days every eight years.

Only the practical Egyptians managed not to be led by the moon. They worshiped the Sun god, Ra, and gave preference to the sun. They made the year from twelve thirty-day months. Then they added five days as birthdays of Osiris, Isis, Horus, Nephthys, and Set.

Duncan thinks that Egypt's fixation on the sun sprang from its kinship with the annual cycle of the Nile river. And, as luck would have it, the so-called Dog Star, Sirus, aligns with the rising sun once each year in that region. The Egyptians had a pure benchmark to identify the new solar year. Without such clear markers, other calendars stayed wedded to the moon.

The western world took up the solar year as a direct result of Julius Caesar's affair with Cleopatra. His infatuation with her, and with Egypt itself, led him to order a solar calendar for Rome in 45 BC. At first, that Julian calendar used alternating 30- and 31-day months, except for a 29-day February. And it added an extra day every fourth year. Roman politicians eventually took a second day from February and gave it to August. That was to honor Caesar Augustus with a 31-day month.

The Julian calendar was still eleven minutes off, and that required another adjustment much later. But our solar calendar is essentially the one Caesar concocted as he held Cleopatra under the Egyptian moon, two millennia ago.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Duncan, D. E., Luna: Temptress of Time. Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year, Avon Books Inc.: 1998, Chapter 2.

I am grateful to Jim Bell, KUHF-FM, for suggesting the Duncan book and for providing me with a copy. I am also grateful to a listener, whose name I regrettably forget, who requested a program on the particular topic of the uneven length of our months.

Galileo's sketch of the moon
One of Galileo's sketches of the moon

The Egyptian symbol for the Sun god, Ra
The Egyptian symbol for the Sun god, Ra

Couple sitting by a lake watching the moon
"By the Light of the Silvery Moon"