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count). This Haab calendar is another point of resemblance between the Maya and
Egyptian civilizations.
Counting by 20 also helps to explain the rather mysterious grouping of days in a
second, 260-day calendar, known as the Tzolkin, which is said to be from the words
tzol, meaning to put in order, and kin, meaning day. One conjecture to account for
this calendar is that the Maya gave a 20-day period to each of 13 gods that they
worshipped. A modification of this conjecture is that the Maya formed from two
different groups, one naming its days after 13 gods, the other after 20 gods, and
that the Tzolkin made mutual comprehension easier. A second conjecture is that
260 days is approximately the growing period for maize from the time of planting
to harvesting. Still a third conjecture is based on the fact that 260 days is the
length of time each year in which the Sun culminates south of the zenith at a
latitude of 15° Í, where the southern portion of the Maya territory was located.
This last explanation, however, seems inconsistent with the obvious fact that the
Sun then culminates north of the zenith for the next 105 days, yet the calendar
begins another 260-day cycle immediately. The two cycles coincide after 52 Haab
years, a period called the Calendar Round.
An important aspect of Maya astronomy was a close observation of Venus. The
Maya established that the synodic period of Venus (the time between two successive
conjunctions with the Sun when Venus is moving from the evening to the morning
sky) is 584 days. By coincidence, 65 synodic periods of Venus equal two Calendar
Rounds (37,960 days), so that the Maya calendar bears a particularly close relation
to this planet.
The third Maya calendar, the Long Count, resembles the Julian day calendar,
in that it counts the days since its epoch, believed to be August 12, 3113 BCE on
the proleptic Julian calendar. This date is not certain; it is based on the dates given
in Maya inscriptions, which are presumed to be historical. Most of these dates are
five-digit numbers starting with 9. Since 9.0.0.0.0 = 9 • 20^2 • 18 • 20 = 1,296,000
days, that is, approximately 3548 years and 4 months, and this date is associated
with events believed to have occurred in 436 CE, one arrives at the stated epoch.
Even though the Long Count does not explicitly mention months or years and
counts only days, the Mayan place-value notation for numbers makes it possible
to convert any date immediately to years, months, and days since the beginning.
The digits excluding the final two represent the vigesimal (base 20) notation for
a multiple of 360, while the next-to-last represents a multiple of 20, and the final
digit is the number of units. Thus the Long Count date of December 31, 2000,
which is 1,867,664, would be written in Maya notation with the vigesimal digits
separated by commas as 12, 19, 7, 17, 4. It therefore represents 5187 Haab years
of 360 days each (5187 = 12 · 20^2 + 19 • 20 + 7), plus 17 Tzolkin months of 20 days
each, plus 4 days (1,867,664 = 5187 · 360 + 17 · 20 + 4). The Long Count may
not be a "perpetual" day calendar. That is, it may be cyclic rather than what we
have called linear. Some scholars believe that it cycles back to zero when the first
of the five vigesimal digits reaches 13. Since 13.0.0.0.0 = 1,872,000 days, or about
5125 years, the Long Count should have recycled around 1992 on the Gregorian
calendar.
4.2. Weeks. The seven-day week was laid down as a basic human labor cycle in
the Book of Genesis. If we look for human origins of this time period, we might
associate it with the waxing and waning of the Moon, since one week is the time