PHILADELPHIA — Philip Mead was online late one night in May, looking for possible artifacts from the American Revolution, when a painting up for auction caught his eye and got his heart racing.

The chief historian at the American Revolution Museum had spied an unsigned watercolor from 1782. It was a panorama of an army encampment, and to his expert eye seemed to feature the only known wartime depiction of the tent George Washington used as his command center during the Revolutionary War.

The tent is the marquee exhibit at the museum, which opened in April. And, thanks to Mead’s sharp eye, the museum now owns the painting that will anchor an exhibition next year.

Mead said the discovery seemed almost “too good to be true.”

“I’ve had this level of excitement only a handful of times in my 30 years of looking for this stuff,” Mead said.

When Mead saw the painting, he immediately emailed the image to Scott Stephenson, the museum’s vice president of collections, exhibitions and programming.

“My heart leapt into my throat when I realized what this painting was,” Stephenson said.

They had to quickly line up donors to bid on the piece, which was going up for auction just days after they spotted it. They were concerned maybe they weren’t the only people to spot the rare work, and they weren’t 100 percent sure the painting was exactly what they had hoped.

“Our motto is you must kiss every frog in case it is a prince,” Stephenson said.

In this case, it was a prince.

With only one other bidder, they landed the painting easily, for $12,000. Once in hand, the museum’s curatorial team was able to conclude the painting shows the Continental Army’s fall encampment at Verplanck’s Point, New York, and was created by Pierre L’Enfant, the French-born engineer best known for laying out the nation’s capital.

Before he created the blueprint for Washington, D.C., L’Enfant served in the Continental Army. He was wounded at the Siege of Savannah, taken prisoner at the surrender of Charleston and upon his release went back to serve George Washington for the remainder of the war.

The painting depicts hundreds of military tents arrayed across a rolling Hudson Valley landscape. Perched on a hilltop rising about the scene on the painting’s left side is Washington’s field headquarters, including the telltale tent.

Most artwork from the war was afterward, so the images didn’t necessarily depict actual events, the historians said.

“To have such a detailed depiction of the scene painted by an eyewitness — and engineer, nonetheless — from an age before photography is like having a Google Street View look at a Revolutionary War encampment,” Mead said.

Although the painting isn’t signed, it mirrors a L’Enfant panoramic painting from August 1782 of troops at West Point, New York, which was donated to the Library of Congress by the Maryland family who cared for L’Enfant at the end of his life. The style of brushwork, the timeframe of both paintings and handwriting comparisons helped to attribute the newly discovered watercolor to L’Enfant.

The original panoramic painting was at some point cut into six sheets of paper and mounted into a folder. A conservationist is working to clean the painting and remount the sheets adjacent to each other so the painting can be displayed as it was intended.

It will be the centerpiece of an exhibit called “Among His Troops: Washington’s War Tent in a Newly Discovered Watercolor,” on display from Jan. 13 to Feb. 19. The West Point painting will be on loan from the Library of Congress to round out the showing.