Tuesday, February 28, 2012

Xochicalco's ancient observatory

A mere 45 minutes' drive south of Cuernavaca, Xochicalco's underground astronomical observatory is again open after almost three years of closure.   I was fortunate to visit it twice last week while leading study trips for four visiting North American university groups:  Carleton, McMaster, and St. Francis Xavier Universities of Canada, and the University of Minnesota.

We usually think of observatories as domed buildings where astronomers look into the center of the night sky with accurate modern telescopes allowing comparison of photos of the sky taken years ago with those taken tonight.  Lacking such precision if, with the naked eye, we look through a hole in the ceiling of a building there is no guarantee that when we return next year to confirm the observation, we’ll again look along the same sightline -- our point of reference, the ceiling, is so close that any change in our position distorts observations.  However, if we make our observations on the distant horizon from a high point, such as the doorway of a temple at the top of an ancient pyramid, it won’t much matter whether we lean against the temple’s right-hand doorjamb and next year the left-hand doorjamb.

It’s likely most ancient Mesoamerican astronomical observations were made watching for astronomical events occurring between peaks or landmarks on the horizon. Heights of pyramids at various archeological sites seem determined by how high a building must be for clear views of the horizon.  At Tikal, with 50-meter high forests, temples appear perched on treetops. At Chichen-Itza, with only 15-meter high trees, buildings also seem perched on the treetops.
Unlike other Mesoamerican astronomers, Xochicalcans were apparently not content with observing the horizon and wanted to record accurate observations in the center of the sky.  They accomplished this feat by creating underground observatories, with ceilings so thick you can only see the sky when directly beneath what I will call a "shaft". The almost vertical shaft of Xochicalco’s reopened observatory is about 7-meters long. Adding three meters from floor to ceiling of the observation chamber makes the focal distance, from the floor of the observation chamber to the outside, 10 meters.  The shaft angles slightly north to where the sun is at noon on the summer solstice, allowing direct sunlight to hit the floor of the underground room on that day.

Additionally, since the diameter of the shaft is about 45 centimeters, the sun shines directly into the underground room for 52 days before the solstice and 52 days after. Add to this the solstice itself and for 105 days a year, starting on April 30, the sun shines directly in. For 260 days it does not. Interestingly Mesoamericans had a 260-day ritual calendar!

During daylight hours a one-meter wide circle of diffuse light forms on the floor of the observatory chamber.  On days when the direct sunlight hits the floor, shortly before noon a needle point of direct sunlight appears on the west side of the diffuse circle and makes its way across the circle from west to east.  As it travels it gets larger until it resembles the size and shape of an American football in the center of the circle. Continuing east it becomes smaller until once again a needle point of light on the east side of the circle. All this in less than an hour.  Direct sunlight on the observatory floor will not reappear until the following day.
The fact that direct sunlight makes its way across the floor from west to east as the sun moves through the sky from east to west always made me think of a pinhole camera, in which everything is inverted.  I wanted to try to focus it and see if we'd be able to see an image of the sun.  In 1991 there was a total eclipse of the sun at midday in July -- the time of day when the sun shines into the observatory.  Imagine my excitement at the prospect of focusing the image of the eclipse in such an ancient observatory!
To prove the possibility of this I enlisted volunteers to go into the observation chamber and place posterboard on the floor creating a smooth white surface.  In the plaza above the observatory I covered the top of the shaft with cardboard with a ten-centimeter diameter cut-out covered with aluminum foil.   I had various diameters of wire with me -- a needle, followed by a pin, a paper clip, a coat hanger and, as a last resort, scissors.  I poked the needle through the foil and shouted down "Anything happening?"  The answer from below was "Nothing."

I enlarged the hole with the pin.  Still, "Nothing."  When I pierced it with the straightened paper clip, I heard an exultant cry, "It's in focus!"  I raced to the tunnel entrance and into the chamber.  There, on the floor of the observatory was a 12 centimeter diameter image of the sun in perfect focus; we could even see its sunspots as it moved across the floor from west to east.  Spectacular!  Imagine Xochicalco's observatory housing a focused and accurate telescope for solar observations 105 days a year, and year-long nighttime observations.  I celebrate it’s reopening.

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