5:51 pm. That's the moment on V-J Day in 1945 when a sailor planted his lips on a woman in New York City's Times Square in one of the 20th century's most famous photos, according to a study by astronomers from Texas State University and Iowa State University. And by finding the precise time, they might make it possible to disprove at least one claim about who was pictured in Alfred Eisenstaedt's Life Magazine photo from the New York City celebration of allied victory over Japan in World War II.

The astronomers, in an article published in the August issue of Sky & Telescope, set out to show how much you can learn about a photograph by poking around in the shadows in its frame. The investigation actually began in the comment thread of a New York Times article in 2010 about the photo.

Steven D. Kawaler, one of the authors of Sky & Telescope article, got involved debating when the photo was taken. The clue that made this deeper inquiry possible, said Donald Olson, another author, was the shadow cast on a building in the top right corner of the photo.

"Every tall building in Manhattan acts like the gnomon of a sundial," the authors explain in the article.

From the image, they could assess that the top of the shadow was 94 feet above the street on what was then the Loew's Building at West 45th Street and Broadway.

By working out what structure caused the shadow, the scientists could compute the sun's position, and thus the time the photo was taken. With a topographical analysis of Manhattan's layout at the time, they concluded that the shadow was cast by a sign atop the Hotel Astor and across the intersection from the Loew's Building. By constructing a scale model of Manhattan and considering other details from accounts of the day, the sleuths arrived at 5:51 p.m. as the moment when the sun would have fallen at the correct angle to create the shadow on the Loew's Building.

Dozens of people have claimed to be either the sailor or the nurse. But Mr. Eisenstaedt did not ask for the identity of the pair when he photographed them. One pair, George Mendonsa and Greta Zimmer, became the subject of a book, "The Kissing Sailor." The book claimed to offer "irrefutable proof to identify the couple." The Sky & Telescope analysis, pointing to their claims of meeting and kissing around 2 p.m., say that this analysis refutes the book's account.

If not Mr. Mendonsa and Ms. Zimmer, who are the sailor and nurse? That's a question that the astrophysicists can't answer. Nor can looking at the shadows in the photo work out whether the kiss was consensual. But Dr. Olson was hopeful his techniques for investigating photos, paintings and other visual works of art had potential applications in other contexts.

"What we need to do it especially is a distinctive foreground, and then we can work with it," he said by phone. "If you ask me to analyze a painting, I would look for distinctive buildings. When they paint anything with a distinctive feature, you can find it."

In the past, Dr. Olson and his co-author have used similar techniques from astronomy to explore questions to study why the Titanic was sunk by an iceberg and how a volcanic eruption contributed to the painting of Edvard Munch's "The Scream."