Last Friday marked the 70th anniversary of VJ Day or Victory over Japan Day. On August 14, 1945 New Yorkers rushed out into the streets to celebrate as word of the end of World War II spread.

That jubilation was captured by photographer Alfred Eisenstaedt in an image of a young sailor planting a kiss on a white-clad girl in the middle of Times Square. In the chaos of the moment, Eisenstaedt failed to get the names of the pair and their identities remained a mystery for many years.

Last year the US Naval Institute published The Kissing Sailor, the most complete investigation into the identities of the sailor and the girl in white to date. The book concludes that the sailor is George Mendonsa and the dental assistant (not nurse as widely assumed) is Greta Zimmer Friedman. But another detail remained unsolved: when was that photograph taken?

A direct clue can be found in the background of the photo. A stylized clock is embedded in the “O” of the Bond Clothes sign. The minute hand shows around 50 minutes past the hour but the usually short hour hand of this clock is obscured, leaving historians to continue speculation about how news of the war’s end unfolded that day and ultimately when this iconic image was taken.

Popular assumption has been that the photo was taken after the White House’s 7:00 p.m. announcement of the war’s end. However the sun set on New York at 7:56 pm making such a well lit photo impossible. The Kissing Sailor theorizes the photo was taken around 2 p.m. based on showtimes for the movie Mendonsa was attending when he joined crowds bursting into the streets. But this still did not seem right to physicists and astronomers at Texas State University and Iowa State University.

The team called upon previous experience using astronomical clues to pinpoint a date and time. Analysis of artwork of Monet, van Gogh, Munch and the photographs of Ansel Adams. Every mountain, building, tree or flagpole acts like the gnomon (pronounced: no-man) or upright portion of a sundial. If you know the height of the object casting the shadow and which way is north, the position of the sun can be calculated, which can be used to identify the date and time the image was recorded.

Vintage photographs and maps were scoured and blueprints of buildings in the Time Square were referenced for clues. A breakthrough came when they found a horizontal shadow cast onto the 8th floor of the Lowes building in the background of the photo.

The shadow was cast by the sign atop the Hotel Astor across the street. The shadow was measured and after a little math the upper limb of the sun was determined to be at azimuth 270° (exactly due west) at an altitude of 22.7° above the horizon. On August 14, 1945, the sun was at this position at 5:51 p.m. Eastern War Time. (daylight saving time was observed year round from 1942 to 1945 as a wartime energy saving measure)

The team checked their math by constructing a scale model of 1945 Times Square using an adjustable mirror as a light source. This finding fits the Bond Clothes minute hand. It also fits shadows on buildings in the background of two additional Eisenstaedt photos on the same roll of film submitted to Life Magazine that evening. The 5:51 p.m. timing also aligns with arrival of U.S. Navy photojournalist Victor Jorgensen, who photographed the iconic couple from a slightly different angle.