MadSci Network: Astronomy


Date: Tue Jul 31 12:52:30 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Astronomy
ID: 996029982.As

Hi Paul!

You are correct in that if the length of the year in the past were 360
days, the Earth would have had to have been slightly closer to the Sun. 
Either that, or the Earth's rotation about its axis would have been
slower.  Both of these are possible if the Earth had interacted
gravitationally with a large body that passed very close.  However, that
would also cause other changes (such as earthquakes and volcanic eruptions)
that certainly would have been recorded around the 8th century when most
civilizations changed over to a 365-day calendar. There is no evidence that
the Earth's orbit or rotation about its axis has changed significantly in
the historical past.  In fact, other than the very gradual slowing of the
Earth's rotation due to tidal interactions with the moon (see the answer to
"Why isn't earth's moon farther away?"), the orbital period of Earth and
its revolution about its axis have been remarkably constant through most of
the solar system's lifetime.  To understand where the 360 day calendar came
from, we have to look at the cultures that used them.

It is true that many ancient civilizations made use of a 360 day calendar,
including the ancient Egyptians, Celts, Hebrews, Greeks, Mayans, and
Sumerians.  Usually the calendar was based on lunar rather than solar
observations and was divided into 12 30-day months.  (Some months are
actually 29 days and these were usually considered to be bad or unlucky.)
I'll discuss two of the best known examples of 360 day calendars.

The Maya calendar had 18 20-day months in a year for a total of 360 days. 
They then had an additional 5 day month that was unnamed and considered
unlucky, bringing the total up to 365 days.

The Egyptians also had a 360 day calendar, but they too were aware that
this was not adequate for agricultural usage.  They added an additional 5
days to the beginning of the year which were a time of great feasting and
(  In
their religion, the goddess Nut was pregnant, but the god Ra declared that
she could not give birth during any month of the year.  Thoth, the god of
learning, decided to help her by adding an extra 5 days to the year, thus
bringing the calendar in line with the agricultural year (as determined by
observations of the star Sirius).

This should illustrate that even though many societies used a 360 day
calendar, this does not indicate that the year was actually a different
length than it currently is or that those societies were unaware of the
variation between the solar (agricultural) year and that determined from
lunar observations.


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