Ansel Adams was a genius with a camera, but he wasn’t so great about taking notes. The famous 20th century landscape photographer did not keep careful records of the dates he took his photos, leading to some debate over the origin period of certain images, including Denali and Wonder Lake (below), taken in Denali National Park in Alaska sometime in the late 1940s.

Denali and Wonder LakeCollection Center for Creative Photography, The University of Arizona, © The Ansel Adams Publishing Rights Trust

To settle a debate about when the photograph (known as Mount McKinley and Wonder Lake until the mountain's name was officially changed in 2015) was taken, Texas State University astronomer Donald Olson looked to the sky, using astronomical hints to determine the exact date, time, and location it was shot. Olson—who has solved other cultural mysteries related to topics such as Edvard Munch's paintings and Chaucer's writing using the night sky—writes about the process in his new book, Further Adventures of the Celestial Sleuth.

Adams did take some technical notes during his photography shoots, writing down the exposure time, film type, filters, and other settings used to capture the image, but he wasn’t as meticulous about the more mundane parts of the shoot, like the date. However, during his research, Olson found that another photo, Moon and Denali, was taken the night before the image in question. Because that one featured the moon, he could use it to calculate the date of both images—once he figured out where Moon and Denali was taken.

Moon and DenaliCollection Center for Creative Photography, The University of Arizona, © The Ansel Adams Publishing Rights Trust

To do so, Olson used topographical features such as cirques, hollowed landforms carved by glaciers, that were visible in Moon and Denali to identify several areas of the park where Adams may have been working. He and his student, Ava Pope, wrote a computer program to calculate the view from each possible location along the park road Adams drove along during his trip, eventually determining the coordinates of the location where the photographer shot Moon and Denali.

He could then estimate, using the position of the waxing gibbous moon in the photo, the exact time —8:28 p.m. on July 14, 1948—that Moon and Denali was taken. Denali and Wonder Lake would have been taken the next morning, and Olson was able to calculate from the shadows along the mountain where the sun would have been in the sky, and thus, when the photo was taken.

The answer? Exactly 3:42 a.m. Central Alaska Standard Time on July 15, 1948.