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.
Such acts of cosmic detective work have become a scholarly mission for Olson and his collaborator, physicist Russell Doescher of Texas State. "Forensic astronomy," it's called, and the astrophysicists have conducted more than two dozen studies of suspect dates in history, literature and the arts.
They theorized that Confederate Gen. Stonewall Jackson fell to friendly fire because his own troops failed to recognize their general's silhouette against a full moon. They ferreted out why Marine Corps landing craft unexpectedly ran aground short of the beach at Tarawa in the South Pacific during World War II (a rare lunar cycle caused an extremely low tide).
The Texas astronomers, whose exploits have earned them a bit of fame, also determined where and when Vincent van Gogh set up his easel to paint some of his most famous portraits of the heavens.
"They have a very good track record," said Owen Gingerich, a Harvard University professor and emeritus astronomer at the Smithsonian Astrophysical Observatory. To the common man or scientist, Gingerich added, "it's fascinating" to precisely pin down a great moment in time -- and see it reprised.
"This is a slightly different kind of anniversary -- and in some ways more meaningful than those we arbitrarily pick," Gingerich said. "There's something exact involved here that isn't repeated often."
Drew Johnson, curator of photography at the Oakland Museum of California, said Adams' foibles with the calendar weren't unusual among photographers of his day.
Dorothea Lange, the great Depression photographer, "wasn't so great about recordkeeping," Johnson said, noting how a Lange photo for years thought shot in Alabama was later determined to have been taken two states away in North Carolina.
The date correction for "Autumn Moon" "is a footnote," Johnson said. "In a sense, you could say it misses the point, which is art. But I totally respect people who take on a project like this. Footnotes can cumulatively add up to something big."
Olson, a Renaissance man at heart who received his doctorate at UC Berkeley, said the fusion of astronomy with art, literature and history has "made my study of science richer."
A decade ago, he and Doescher dabbled for the first time into Ansel Adams and the photographer's habit of recording incomplete or contradictory data about the location, time and place he shot those famous black-and-white negatives.