YOSEMITE NATIONAL PARK — Ansel Adams, the venerated photographer, was notably scrupulous about recording the details of his craft -- camera apertures, shutter speeds, film type -- as he documented the Western outback in monochrome.
But he also was notoriously poor at writing down dates.
Now a team of Texas astronomers has found that one of Adams' photos of the Yosemite backcountry, a solitary shot from Glacier Point of the moon rising over saw-toothed peaks beside a pillow of clouds, was misdated by four years.
The Texas State University astronomers, who have built a reputation for pinpointing historical dates and events, also determined that the celestial clock is ticking toward a rare encore performance early on Thursday evening, re-creating the same dance of moon and mountains Adams captured on the same date more than half a century ago.
That cycle repeats itself only once every 19 years, so folks in Yosemite are expecting a crowd of amateur photographers, astronomers and Adams aficionados atop Glacier Point, eager for a brief chance to relive a scene documented by one of the 20th century's greatest photographers.
Matthew Adams, the photographer's grandson and president of the Ansel Adams Gallery in Yosemite, considers it a fitting tribute to the lifelong environmentalist, who died on Earth Day 1984.
"It's wonderful," he said. "It's interesting [that] astronomy can do this. And it's great [that] there's this ongoing interest in Ansel. We're planning to go out and see it for curiosity's sake."
The photograph in question, "Autumn Moon: the High Sierra From Glacier Point," is not among Adams' best-known, but has appeared in half a dozen books and magazines over the years. It long was believed to have been shot in 1944.
But the Texas State astronomers sleuthed through celestial history, plotted lunar phases, crafted a special computer program and calculated angles of shadows cast by the setting sun to determine the exact time, date and spot where the photography legend snapped the shutter on his bulky view camera.
It actually was Sept. 15, 1948, at 7:03 p.m. Pacific Daylight Time. Give or take a few seconds.
"Ansel Adams' genius was in getting there at the right time and the right day," said Donald Olson, the Texas State astrophysicist who led the study, detailed in the October issue of Sky & Telescope magazine. But, Olson adds, the photographer was actually there four years later than everyone believed.
