The object of this visit was to confirm earlier measurements and make actual astronomical observations to provide empirical corrections to computer-based ephemeris predictions. Once the data were collected, it would be possible to define a "window of opportunity" over several days surrounding the southernmost rise of Venus to maximize chances of capturing an acceptable photographic image.
Working with a survey transit, a GPS receiver, and a short wave radio tuned to WWV for precise timings, I used known survey points to make a series of daytime measurements from the Palace doorway. Well before dawn, I made carefully timed positional measurements of celestial objects to determine possible atmospheric effects which might impact predictions by astronomical computer software.
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At the end of the sightline from the Palace of the Governor to the postulated southernmost Venus rise point is a totally undeveloped site called Cehtzuc (pronounced "ked-TZUC") where the small pyramid lies. Abandoned for approximately 1,000 years, Cehtzuc was also a destination since I needed to obtain an accurate GPS fix and a back-azimuth to confirm measurements and refine distance calculations. Moreover, another Maya scholar, Ivan Sprajc, has recently postulated that, rather than the southernmost rise of Venus as seen along the the Uxmal-Cehtzuc sightline, the northernmost set of Venus as seen along the Cehtzuc-Uxmal sightline is the more significant event. |
An employee at the Uxmal archaeological site and myself hacked our way to Cehtzuc through surprisingly dense jungle. That account is its own adventure.
"Window of Opportunity" Defined
The azimuth measured from the Palace to Cehtzuc differed less than a degree from Aveni's 1975 data (117.56 vs. 118.22 degrees) but regardless, ephemeris predictions indicated Venus would never rise quite that far south. Instead, its actual southernmost rise point lies a few degrees north (to the left of Cehtzuc, as seen from Uxmal) along the horizon. Uxmal's latitude (20.4 degrees North) results in celestial objects rising along a path tilted 20 degrees to the right. This meant Venus would have to have been in the sky for some time before reaching a point directly above Cehtzuc. Calculations by E. Myles Standish of JPL, a planetary dynamicist and keeper of ephemerides for historical eras, revealed that essentially the same rise geometry existed circa 903 A.D., the date many scholars agree the Palace was completed.
So using the azimuth from the Palace doorway, across the double-headed jaguar throne, to Cehtzuc, I sought the day when Venus crossed that sightline at its lowest point above the horizon. Ephemeris predictions produced a minimum altitude of six degrees--a value also consistent with Standish's predictions for the Tenth Century. This minimum azimuth/altitude combination--like any other of several observational extrema--can be used to benchmark the eight-year periodicity. I centered my "window of opportunity" on an interval where Venus would descend from 6.5 to 6.0 degrees, then back again. This turned out to be a seven-day period from 10-16 January 1997 (Incidentally, the 6.0 degree minimum observed altitude would occur on the morning of 12 January 1997).
Preparations and Technical Challenges
Each day within the "window" would contain a short interval approximately three minutes long when Venus would lie sufficiently close to the sightline to appear vertically above the foreground monuments. Each of these would provide my "photo op."
But complicating matters was the fact that light levels during twilight change rapidly. Moreover, twilight would begin slightly earlier on each day of the window--and, during this time of the year, this happens particularly quickly. Photographically, the net effect would be a sizeable shift in ambient lighting conditions from day to day as far as film and accompanying exposure times were concerned, i.e., each day's window would possess its own distinct photographic "identity." Worse was the fact that previous test exposures indicated the differences between twilight sky brightness and the dimly lit foreground spanned a range of illumination well beyond what most films could simultaneously capture in a single exposure.
Also influencing illumination would be the weather. The September trip featured bright, clear skies whose predawn reflections delivered ample, golden hues to assist in capturing foreground detail. The presence of clouds would block some of this light, darkening the scene dramatically. September's experience also featured radiation fog--a calm, clear-sky phenomenon where the earth's heat escapes directly into space and drops the temperature below the dew-point. My early morning celestial observations had been complicated by this dense, jungle-hugging fog obscuring the distant horizon, making it nearly impossible to see objects actually rise. I could only wait for them to emerge from the mist--usually after they had attained an altitude of at least two to three degrees--then measure a series of carefully timed positions with my surveying transit to later project the precise rise azimuth.
Distinct from September--which defines the end of the region's "rainy season"-- January would lie within the region's dry season and I could only hope for clear skies and lower relative humidities.
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