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 this 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.

The photograph in question, "Autumn Moon: the High Sierra From Glacier Point," has appeared in a half-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. PDT. 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 also determined where and when Vincent van Gogh set up his easel to paint some of his most famous portraits of the heavens.

"This is a slightly different kind of anniversary — and in some ways more meaningful than those we arbitrarily pick," Harvard professor Owen Gingerich said. "There's something exact involved here that isn't repeated often."

Olson, who received his doctorate at University of California, 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 a 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.

In 1994, they pinpointed the exact moment he shot his iconic photograph of the moon rising over Half Dome in Yosemite Valley, and alerted the curious about a rare return engagement that December. About 40 photographers braved Yosemite Valley's cold to walk in the steps of Adams.

The two professors teamed up this year with three honors students to tackle the truth about "Autumn Moon."

The shot depicts an ethereal mix of land and sky, looking away from Yosemite Valley southeast toward the jagged peaks of the Clark Range.

The scene is breathtaking, the darkroom work flawless. But there's no exact date in the record.

"Dates never meant much to Ansel," recalled Mary Street Alinder, for five years the photographer's assistant before his death and later the author of an Adams biography. "It meant something to the historian, but not the artist. It simply wasn't on his radar."

The university team, based in San Marcos, Texas, began its astronomical investigation by using topographic maps and sky photographs to triangulate the location of Adams' camera to a spot near the Geology Hut, a stone building halfway between Glacier Point's popular cliff-side railing and a parking lot. Armed with that information, they could plot the moon's location in Adams' photo to narrow the list of possible evenings.

From there, the lunar face helped pin it down further. "Autumn Moon" is of such rich detail that the moon's lava-strewn seas and rimmed craters were easily discernible as a point of reference. Given the moon's penchant for rocking and nodding ever so slightly as it cycles through the skies, the astronomers could toss out a third of the suspected dates.

The final clues, Olson said, were earthbound shadows.

A sharp triangular shadow cast on the distant ridge by the setting sun sent Olson and his team scurrying to their computer. Accounting for atmospheric refraction and the curvature of the Earth, they concluded that "Autumn Moon" was shot in mid-September 1948. A trip to Glacier Point by the team in June firmed up those findings.

"It was the only date that fit everything," Olson said.

Number crunching also revealed the upcoming anniversary. For only the third time since the shutter snapped on "Autumn Moon," a waxing gibbous moon and setting sun will conspire between 6:50 and 6:52 p.m. today to reprise the same golden scene captured by Adams.

"It's like you're re-creating a moment in time," Olson said. "I can envision myself up there on Glacier Point with Ansel Adams."