How Many Days Are in a Year?
We know now that the average length of the year is 365.2422 days, but this
precise value was taken into account only by the Gregorian reform of the calendar in
- The Julian calendar (devised by Julius Caesar), which the Gregorian calendar
replaced, assumed the year to be 365.25 days long (as we all do for ordinary purposes),
and the earlier Roman calendar that Julius replaced apparently assumed the year to be
366.25 days. The Egyptian calendar, which Julius borrowed as the basis for his,
assumed the year to be exactly 365 days.
The problem in the ancient world again was finding a fixed point from which
to measure the length of (or to begin) the year. The most popular choices were the
winter solstice, when the days begin growing longer again, and the spring equinox,
when the hours of sunlight and darkness are equal, but many others were also used.
Measuring the moment of winter solstice would seem a difficult task for ancient peo-
ples, but it now seems clear that the people who built Stonehenge about 3000 B.C.E.
could do so quite accurately. They could also predict all eclipses of the Sun and the
Moon. The Egyptians seem to have solved the problem by observing the heliacal ris-
ing of Sirius each year: The fixed stars, including Sirius, appear to rotate about the
Earth each sidereal day, which is always the same length. Which stars are visible in
the night sky depends on where the Earth is in its annual orbit around the Sun. Sirius
(and any other star) will always first become visible after sunset (weather and local
conditions allowing) on the same day each year relative to the solstices and equinox-
es; this is its heliacal rising. In classical times, the Mesopotamians claimed that they
had also solved the problem in another way, as early as the Egyptians had, but it is not
certain that they had done so before the seventh century B.C.E.
How Many Months Are in a Year?
This is the most difficult of the four questions (and the one that causes the
most differences between calendars), because the length of the solar year is not a sim-
ple multiple of the length of the lunar month. Hence, if the months are to stay in
phase with the Moon, there are many problems.
Twelve months that alternate between 29 and 30 days produce a year of 29.5 x
12 = 354 days, which is 11.2422 days short of an average solar year. Every three years
this difference will add up to 33.7266 days, allowing an extra lunar month—of, say, 30
days—to be added. This still leaves a difference of 3.7266 days, which will add up to
33+ days after 27 years, allowing an extra lunar month to be inserted, and so on. It
seems clear, however, that people generally would not like to have a feature in their
calendars that appears only once in 27 years; for example, what would this extra
month be called? Would it contain any holidays?
Only two basic kinds of calendars have succeeded in dealing adequately with
the various problems of timekeeping: (1) the lunisolar calendar of the Mesopotami-
ans, which added lunar months during years three, five, and eight of eight-year
cycles, and (2) the purely solar calendar, devised by the Egyptians. Despite the reten-
tion of 30- to 31-day periods that are still termed months, the Western calendar is a
solar calendar.
THEASTROLOGYBOOK [111]
Calendar