How Long Is a Day?
The technique of dividing day and night into 12 hours each was devised by
the Babylonians, who calculated with a number system that used a base of 12 rather
than 10. Hours were introduced into the Roman calendar only rather late in Roman
history, when the seven-day week (also a Babylonian invention) was generally adopt-
ed. Originally the Romans had divided day and night into watches, each several
hours in length.In modern usage, a day is defined as being 24 hours long; an hour is defined as
60 minutes, or 3,600 seconds, long; and a second is defined as so many vibrations of a
specific line in the spectrum of a specific isotope. Naturally, this definition was
worked out in a way that makes 24 hours equal to the traditional average length of a
day. It was finding this average length that was the problem in the ancient world, for
several reasons.First, where do you measure from? The convention of starting each calendar
day at midnight was agreed upon only in modern times. In most ancient calendars,
each day began at sunset and ended at the following sunset (some ancient peoples,
such as the Egyptians, counted a day as running from one dawn to the next); this is
why the “eve” before many traditional holidays is still important and why the Jewish
Sabbath celebration begins at sunset on Friday. But exactly when is sunset? It takes
about 15 minutes for the Sun to sink completely below the horizon, which appears
higher on land than it does at sea. This ambiguity is why the Talmud prescribed that
all activities not allowed on the Sabbath should cease two hours before sunset. Some
conventional definition—such as measuring from the moment the disc of the sun first
touches the horizon—had to be introduced and adhered to.Furthermore, since the days (in the sense of hours of light) grow longer (how
much longer depends on the latitude) during half the year, shorter during the other
half, an accurate measurement needs to be correct to within less than a minute to be
useful for constructing a calendar. But there were no accurate techniques before mod-
ern times—even measuring a quarter hour accurately was difficult—and so the
ancient calendars tended to accumulate an error of a day every few years.How Many Days Are in a Month?
In most ancient calendars, a month was a lunar month, that is, one full cycle
of the Moon’s phases. We know now that the average length of a lunar month, mea-
sured from one astronomical new moon to the next, is 29.5306 days. However, an
ancient month began not at the astronomical new moon, which is an invisible event,
but at the first visible crescent. Many factors affect when the crescent of the new
moon will be visible at a particular location. Usually the interval from each first cres-
cent to the next will alternate between 29 and 30 days—and so the length of the
months will alternate likewise—but it is easily possible for two or even three intervals
of 29 days or 30 days to fall successively. Hence, it was quite late in history—long after
the length of the year was well known—before the average length of the lunar cycle
was known with usable accuracy.Calendar
[110] THEASTROLOGYBOOK