Spring 2007 - Yosemite Online

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MOONBOW CONDITIONS1. Clear sky2. Abundant mist and spray at the base of the fallThe best moonbows at Yosemite occur during peaksnowmelt runoff, from April to June and sometimesearly July.3. Dark skyThe Sun should be more than 9 degrees below thehorizon, ending bright twilight.4. Bright moonlightThe brightness of the Moon depends on its phase,distance from Earth, and altitude above the horizon.The minimum brightness conditions considered by theprogram correspond to a Moon at an altitude of 25degrees, at its mean distance, and with an illuminatedfraction of 95%.5. Unobstructed moonlightFor moonlight to strike the spray at the base of LowerYosemite Fall, the Moon must be above the nearbymountains, domes, and cliffs.DONALD OLSONJohn Muir, commemorated in this 1964 postage stamp, was anenthusiastic Yosemite moonbow observer.6. Correct rainbow geometryThe Lower Yosemite Fall viewing area is a paved terraceat the west end of a wooden bridge over YosemiteCreek. From here, a moonbow will appear only whenthe angle between the antilunar point and the directiontoward the base of the fall is near the rainbow angle of42 degrees.he could view smaller intermediate falls on the plateaubetween Upper and Lower Yosemite Falls:…the moonbeams were pouring through…I saw a welldefinedspray-bow, beautifully distinct in colors…whilepure white foam-waves beneath the beautiful bow wereconstantly springing up out of the dark into the moonlightlike dancing ghosts.By the time he published this book, Muir had beenobserving moonbows for more than four decades. In anApril 1871 letter to Mrs. Jeanne S. Carr, he wrote:Silver from the moon illumines this glorious creationwhich we term “falls,” and has laid a magnificent doubleprismatic bow at its base. The tissue of the fall is delicatelyfilmed on the outside like the substance of spentclouds, and the stars shine dimly through it.COMPUTING MOONBOW VISIBILITYInspired by Muir’s dramatic accounts, we decided to writea computer program to predict the dates and times whenmoonbows should appear. Six conditions are requiredfor a moonbow to be readily visible. The first two areweather-dependent, but the remainder require astronomicalconditions that can be modeled by computer. Ourprogram forecasts optimal moonbow viewing times forthe area near the base of Lower Yosemite Fall.TRIP TO YOSEMITEAscertaining the profile of the local horizon and the geometryof the viewing area relative to the falls required a visitto the site. Fortunately, our research group had alreadyscheduled a trip toYosemite. In an article in Sky & Telescopemagazine, we predicted that on September 15, 2005, theMoon’s position would recreate the scene in AutumnMoon, a famous Ansel Adams photograph from GlacierPoint. The moonrise event was a great success, withhundreds of photographers in attendance, but we spentmost of our time in the park near Lower Yosemite Fall.That September, the flow of water was little morethan a trickle. This worked to our advantage. Wecould stand right at the base of the fall, somethingthat would be impossible in the thundering torrentsof the spring runoff season. We took night photographsof the nearby mountains and cliffs silhouetted4 YOSEMITE ASSOCIATION, SPRING 2007
against brilliant star fields. The images allowed usto accurately trace the profile of the horizon.By day we did conventional surveying with rulers,plumb bobs, and a laser level. Websites indicated thatLower Yosemite Fall would be about 100 yards fromthe viewing area, but our survey found this distanceto be 180 yards. We also determined the precise angles(azimuth and altitude) of the direction of view from theterrace to the densest part of the spray near the base ofthe fall. With information in hand, we returned home tofinish writing our moonbow program.MOONBOW FORECASTDuring the snowmelt runoff season of 2006, we circulated the program’s predictions to interested photographers. Thephotographic results verify the accuracy of our method.The accompanying table gives our moonbow predictions for 2007. The calculations of dates and precise times canhelp visitors avoid long waits in a location that can be cold and wet with blowing mist. Our website, http://uweb.txstate.edu/~do01/, contains more detailed descriptions of the position of the Moon and appearance of the moonbows.MOONBOW PREDICTIONS FOR LOWER YOSEMITE FALLCalculations are for the terrace at the west end of the wooden bridge near the base of Lower Yosemite Fall.DATE IN 2007 TIMES (Pacific Daylight Time) LUNAR PHASEApril 29 (Sun) 8:32 p.m. (Sun) to 9:20 p.m. (Sun) 96%April 30 (Mon) 8:33 p.m. (Mon) to 10:40 p.m. (Mon) 99%May 1 (Tues) 10:05 p.m. (Tues) to 11:50 p.m. (Tues) 100%May 2 (Wed)–May 3 (Thurs) 11:25 p.m. (Wed) to 1:00 a.m. (Thurs) 99%May 3 (Thurs)–May 4 (Fri) 12:37 a.m. (Fri) to 2:00 a.m. (Fri) 96%May 29 (Tues) 9:10 p.m. (Tues) to 10:50 p.m. (Tues) 97%May 30 (Wed) 10:26 p.m. (Wed) to 11:50 p.m. (Wed) 99%May 31 (Thurs)–June 1 (Fri) 11:33 p.m. (Thurs) to 12:55 a.m. (Fri) 100%June 1 (Fri)–June 2 (Sat) 12:39 a.m. (Sat) to 1:50 a.m. (Sat) 98%June 28 (Thurs) 10:35 p.m. (Thurs) to 11:25 p.m. (Thurs) 98%June 29 (Fri)–June 30 (Sat) 11:30 p.m. (Fri) to 12:20 a.m. (Sat) 100%June 30 (Sat)–July 1 (Sun) 12:20 a.m. (Sun) to 1:05 a.m. (Sun) 99%July 1 (Sun)–July 2 (Mon) 12:55 a.m. (Mon) to 1:45 a.m. (Mon) 96%The program typically predicts moonbows on four or five nights near each full Moon during the snowmeltrunoff period. While conservative, these predictions generally agree with the conclusions of sharp-eyed John Muir,who judged that:…magnificent lunar bows may be found for half a dozen nights in the months of April, May, June, and sometimes July.John Muir, “Yosemite in Spring,” New York Tribune, May 7, 1872Rain showers and waterfalls are not the only settings where this phenomenon occurs. Anyone can make a moonbowwith an ordinary garden hose. With a bright Moon high in the sky behind you, direct a fine mist toward a spot 42 degreesaway from the shadow of your head. It will be much easier to see the moonbow against a dark background such as a bushor wall. Once seen, the ethereal silver-white of the moonbow is not easily forgotten.Don Olson and Russell Doescher teach physics at Texas State University; Kellie Beicker is a student in theuniversity’s Mitte Honors Program. The authors have published four Yosemite-related articles in Sky & Telescope:“Dating Ansel Adams’s Moon and Half Dome” (December 1994); “Ansel Adams and an Autumn Moon” (October2005); “An Ansel Adams Encore” (January 2006); and “Moonbows over Yosemite” (May 2007), upon which thisarticle is based.YOSEMITE ASSOCIATION, SPRING 20075
Page 2: Cover: A moonbowat the LowerYosemit
Page 7 and 8: The Glacier Point lookout under con
Page 9 and 10: The Yosemite Museum againstthe Vall
Page 11 and 12: GRAND OPENING OF THE YOSEMITE VALLE
Page 13 and 14: “Cathedral Snow” by Stephen Lym
Page 15 and 16: PARK PROFILEDAN JENSENLAUREL REMATO
Page 17 and 18: VIRGINIA FERGUSONJuly 17, 1940 - Ja
Page 19 and 20: PARK NEWSMerced River Plan Public S
Page 21 and 22: Chiura Obata: Art of Yosemite 1927-
Page 23 and 24: ASSOCIATION DATESLegend:OA = Outdoo
Page 25 and 26: POSTERSYosemite Multi-Image Landsca
Page 27 and 28: SALEThe Joy of Yosemite—Selected
Page 29 and 30: Phillip Rocha, Therese & F. Rodrigu
Page 31 and 32: Robin Moores, Karl F. Munz, Robert

Spring 2007 - Yosemite Online

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