|MadSci Network: Astronomy|
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 celebration (http://emuseum.mnsu.edu/prehistory/egypt/dailylife/calendar.html). 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. Erika
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