Testing the Azimuth-Gnomon at Izapa
John Major Jenkins
December 5, 2006
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In this article I will describe the design, operation, and field testing of a very simple astronomical measuring instrument. This device was designed for a very specific purpose, measuring the December solstice sunrise azimuth at a site located at 15° N latitude. Simplicity and compactness was essential. It is also important to be able to use the device at any time of the year, even though it is measuring the December solstice sunrise horizon. How could this be done?
By applying a working knowledge of astronomy, I reasoned as follows. As the sun rises into the sky, it arcs through the southern sky (to viewers in the northern hemisphere). At precise noon, the sun will be crossing the southern meridian. A shadow cast from a pole at this moment will point to the northern meridian which is to say, the north celestial pole, located by definition at 0° azimuth. If one could measure the perpendicular to this shadow, one would know the direction of true east, the sun's rise azimuth at the equinoxes, 90° azimuth.
A simple schematic will show how this works. Looking down from above, we can see how a vertical pole (dot on the right) casts a shadow to the left. If the observation of the shadow is made at precise noon, the shadow will point to 0° azimuth. Then, the perpendicular azimuth of sunrise on the equinox can be known.
The object of the device to be designed, however, is to be able to know where the sun will rise during the December solstice. A simple adjustment allows us to make a preset positioning of a target pole, so the viewer can have a sight line to where the sun will rise on the December solstice. One can look up the solstice sunrise azimuth for the 15° latitude and build into the device an angular sight line between the two vertical poles the gnomon pole and the target pole. The December solstice sunrise azimuth at 15° N latitude is 114°:
With this adaptation, we have the basic parameters of how we can construct a three-dimensional device of great simplicity that will allow us to measure the December solstice sunrise azimuth at latitude 15° North:
The Azimuth Gnomon. A = arm A of horizontal square; B = arm B of horizontal square.
Several indispensable features are labelled in the photograph of the device as follows. A tripod for stabilizing. A bull's eye level can be used so that the horizontal arms of the horizontal square can be made level. The bull's eye level will not be fixed, so it can be moved to various places to make sure the tripod is adjusted for a level reading. The vertical gnomon on arm A will cast the shadow. A target rod will be placed on arm B at a 114° angle from the "north shadow line", using the gnomon as the fulcrum, so the viewer can sight from the gnomon to the target rod and observe the December solstice sunrise horizon. A GPS clock or watch is essential for being able to track precise Greenwich Mean Time. A chart will provide the precise Greenwich time of the sun's south meridian passage (precise astronomical noon). It may be dozens of minutes on either side of local clock noon.
This device will be tested at the site of Izapa, in southern Chiapas near the Guatemala border, in late December 2006. The ball court at this site, according to archaeological surveys done in 1960s, is aligned to the December solstice sunrise horizon. This test will provide additional data and help others understand that the solstice alignment in the Izapan ball court is real, thus providing support for the iconographic interpretations I've offerred. See http://alignment2012.com/Izapa.html.
Elevation of the eastern horizon (very minor), and changes in the obliquity of the ecliptic (very minor) since the site's heyday 2,100 years ago will be taken into consideration.
This device was conceived and designed in the Spring of 2006, and built in November and early December of 2006 at a cost of $104 ($79 of which was for the GPS clock). I will fly into Tapachula (near Izapa) on December 27, and be at the site for two days. Then off to Palenque, Yaxchilan, and Yucatan. Return on January 9, 2007.
Results of the solstice observation at Izapa.