Moonbows Over Yosemite: Predicting Moonbows in Lower Yosemite Falls
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Most people have seen a rainbow following a spring shower or in their garden hose, but few have witnessed the spectacular beauty of a lunar rainbow, or moonbow. One location where the appearance of moonbows is a well-documented occurrence is Lower Yosemite Fall, Yosemite National Park, California. Photographers travel to see this rare event, but with the vague guidelines given by the park’s resources photographers have been disappointed by the absence of a moonbow on full-moon nights. For this reason, we created a computer program that would help us to predict the precise dates and times when moonbows would be visible in Lower Yosemite Fall. We developed six conditions that were necessary for moonbows to be visible in the waterfall for viewers at the viewing area at the base of the falls: (1)Clear Skies (2)Abundant Mist and Spray at the Base of the Fall (3)Dark Skies (4)Bright Moonlight (5)Moonlight Not Blocked by Mountains or Cliffs (6)Correct Rainbow Geometry. Precisely defining these conditions required visiting the park in order to make our own measurements, and calculations in spherical trigonometry were carried out in order to develop practical equations. We have successfully written this computer program, published the results in a variety of locations where photographers can utilize them, and predicted when moonbows would occur in Lower Yosemite Fall for 2006, 2007 and 2008.