A long exposure shot, taken from where Adams snapped Autumn Moon, helps to work out the declination of the Moon that night. Credit: © Russell Doescher of Texas State University

Texas astronomers have pinned down the exact date and time that a famous Ansel Adams photograph was taken.

Using astronomical clues such as the position of the Moon and the length of shadows in the snap, a group from Texas State University has worked out that Adams shot Autumn Moon, the High Sierra from Glacier Point at 19:03 (2:03 GMT) on 15 September 1948. In a travelling exhibition, currently in California, the photograph is dated to 1944.

Although Adams took detailed notes on the exposures for his works, he was often frustratingly vague about when and where the shots were taken, says team leader Donald Olson. But Adams's love of sky phenomena, particularly the Moon, sometimes provides enough clues for a little astronomical sleuthing.

"The Moon with its rapidly changing phases and position is really a big cosmic clock," says Dennis di Cicco, an editor at Sky & Telescope magazine. "When you've got a picture with a Moon in it, you can do these calculations."

Moon hunters

The Moon with its rapidly changing phases and position is really a big cosmic clock. Dennis di Cicco , Sky & Telescope magazine

In 1991, di Cicco pinpointed the time and place of Adams's iconic Moonrise, Hernandez, New Mexico. Inspired by that work, in 1994 Olson's team tackled Moon and Half Dome, taken in Yosemite National Park in California, and then moved on to Autumn Moon.

The Moon is clearly on its way to becoming full in the picture. And its position above the mountain peaks helps to narrow down when the shot could have been taken.

Less obviously, a close look reveals that a dark region near the Moon's north pole, called Mare Frigoris, is captured in the snap. Although the Moon always keeps one side roughly facing the Earth, it does wobble a bit, so the position of this dark patch helps to restrict the possible dates.

Colour scheme

The astronomers then unearthed a rare colour version of Autumn Moon, which appeared in Fortune magazine in 1954. From the position of the Moon they could tell that the colour image must have been taken first, and the black-and-white version just over two minutes later.

By looking at shadows in the two, the team pinpointed the altitude and azimuth of the sun, to further determine the timing of the photo.

The researchers verified their theories with a trip to Yosemite in June, during which they confirmed where Adams took the shot from, as well as measuring the distances to the mountains and the Moon?s location from there.

Back in the picture

Adams took the photos from near a stone building called the Geology Hut atop Glacier Point, the team reports in the October issue of Sky & Telescope. The colour image was taken at 19:01, and the black-and-white version at 19:03.

The celebrated photographer captured a unique glimpse of the High Sierra in those brief moments, says Olson. "There are just a few minutes when you can capture both detail in the Moon and the last shadows cast by the setting sun," he says. "Getting the balance of all that is very hard to do, and Adams knew how to do it."

On 15 September, celestial events will coincide to recreate the dramatic lighting seen in Autumn Moon. The Texas State group plans to be there to witness the scene that Adams captured 57 years ago.