El Salvador is a surfing haven, but, OK, actually there is no evidence that the Mayans were surfing along El Salvador’s coast in 1000 BC. However, they were out in the surf everyday fishing from the early Mayan city of Cozumel (Chalchuapa), as well as along the Guatemalan coast. When you are out in the ocean every day you notice things like the tides coming in 50 minutes later each day and which days have the highest tides.
There are many things which affect the tides, including local geography, ocean depth, the position of the moon, and the distance between the moon and earth, since its orbit is not completely circular. In terms of time of day, the high tide repeats in just over 14 days in all oceans. Surely the Mayans figured this out early as did all early coastal peoples. Perhaps that is why we have a 7-day week. But the Mayans figured out something more important to their development.
Here in the San Francisco Bay region the tides are quite irregular. But one thing can be counted on: the highest tides always happen in the morning or early afternoon. The high and low tides which occur during late afternoon to just after midnight tend to be almost equal – very little difference between the high point and the low point.
On the other hand, the tides along El Salvador’s coast are very regular, with morning and evening high tides at about the same height. But the Mayans noticed something else, every 13 days the tide reached its highest point, during high tide, and its lowest point, during low tide. Six or seven days later there would be very little difference between the height of the high and low tides. This actually is very important to know among people who fish and gather crabs and other creatures along the shore. And to be able to predict this not just a few days, but many days, in advance allowed the Mayans to organize themselves into work parties in other activities when the fishing or crabbing would not be so good. This led to … civilization.
Why the tides have a 13 day cycle is rather complicated. But it basically follows the 26 day cycle of the moon crossing the 0 degree latitude point of the plane formed by the orbit of earth around the sun. The moon varies from 5 degrees north to 5 degrees south of that plane. Every 26 days it completes a cycle and every 13 days it crosses the 0 latitude point. Of course, when there is a full or new moon and the sun, moon, and earth are lined up, there is a stronger tide. But it turns out that along the Central American Pacific Coast that the “vertical” alignment of the sun and moon, that is being in the same 2-dimensional plane of the ecliptic, is more important than the one-dimensional horizontal alignment that occurs at the full and new moon.
Today, December 5, 2009, the difference between the high tide and low tide, the semi-diurnal tide range, is at its greatest, in El Salvador. This is the 10th day (10 Lamat) of their 13 day week (Cauac). Thirteen days ago, on Nov. 22, it was 10 Men and the high tide was at its highest. In 13 days, on Dec. 18, it will be 10 Imix and the high tide will again be at its highest. The Mayans began figuring this out about 3,800 years ago.
Then, when the Mayans figured out that there was a relationship between the tides and lunar and solar eclipses, they realized that there was something very powerful and almost magical between the interaction of sun, moon, and earth.
First posted, Dec. 5, 2009 on BlogBox
PostScript, added Jan. 23, 2010: The knowledge of the regularity of the tides does not seem to be the determining factor in the development of the 13-day calendar. The tides repeat closer to every 14 days rather than 13 days. There is evidence that the Mayans were tracking Mars much earlier than previously expected and the number 13 shows up in some of the Mars cyclical (synodic) periods. However, the relationship between 13 and the tides probably did give the number 13 special meaning among the early Mayans by about 4000 BC.
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