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
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
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
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
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
Galileo's sketches of the
The Egyptian symbol for the Sun god, Ra
"By the Light of the Silvery Moon"
The Engines of Our Ingenuity is
Copyright © 1988-1999 by John H.