People gather at Dark Star Park to see the shadows from the large spheres line up with the impressions on the ground in Rosslyn on Aug. 1. (Linda Davidson/The Washington Post)

The group huddled at tiny Dark Star Park in Rosslyn on Saturday morning anxiously watched the passing clouds. They wanted sunshine. They needed it for just a minute. At 9:32 a.m., to be precise.

They cheered as the rays cooperated, breaking through the gray skies. For a fleeting moment, the art display in the park put on a special show, as shadows from towering concrete spheres and metal poles aligned precisely with metal impressions on the ground.

“I just wanted it to be sunny, and see it up close,” said Catholic University student Ariadne Cerritelli, who is planning an independent study on land art in the fall.

“It’s kind of Rosslyn’s answer to Stonehenge,” said Melinda Carlson, who watched with a group of friends, all of whom were experiencing the event for the first time.

The poles and spheres decorating the 0.4-acre park, next to the Park Place office building, are interesting enough to look at any day. But each Aug. 1 at 9:32 a.m., the perfect shadow alignment celebrates Rosslyn’s past.

The park’s artist and designer, Nancy Holt, chose Aug. 1 to commemorate the day in 1860 when William Henry Ross purchased the land that became Rosslyn, according to Angela A. Adams, the director of Arlington Public Art. (Adams said 9:32 a.m. was chosen simply because Holt liked the morning light.)

The towering black metal poles reach the heights of nearby trees. Two are more than 16 feet tall, with another two reaching 20 feet. The concrete spheres dwarf the cars in traffic just feet away. One is more than 6 feet tall, and its neighbor reaches 8 feet in diameter.

Holt spent years planning and installing the project, which opened in 1984. She even sought the help of astrophysicists.

“Suddenly it just locks into place and lines up perfectly,” Adams said. “You look at it thinking there is no way it will align, and then it snaps into resolution, creating an uber-dark impression on the ground, lasting seconds.”

Jerome Gonzalez of Northeast Washington was new to the tradition. He said an event in Rosslyn just a week ago led him to the little-known event. Gonzalez and a friend biked several miles to watch the few moments of perfect shadows.

“I happened to stumble upon a sign in the park about the event,” Gonzalez said. “It was lucky that I could make it this year.”

Holt, an environmental artist who used sunlight and shadows in other work, died in February 2014 at 75.

This was Holt’s most urban project; many of her pieces are set in desolate, rural settings, Adams said. It is one aspect that draws artists such as Cerritelli to admire the work.

“To see a piece of land art, one that is significantly big, there is none really around here,” Cerritelli said. “It’s nice to be able to see one in person rather than having to look at pictures.”

Holt named Dark Star Park for the spheres that represent fallen stars, but it also has become part of Grateful Dead fan culture — a 1969 song by the band shares its name with the park. Some incorrectly believe that is the main reason for the park’s name, furthered by the coincidental fact that the band’s lead guitarist, Jerry Garcia, was born on Aug. 1.

On Saturday, father and daughter Tony and Margaret Trombly sported colorful band T-shirts for their annual visit.

“It’s her birthday. She shares it with Jerry,” Tony Trombly said. “This is how I get away with observing Jerry’s birthday on hers.”

Margaret Trombly, who turned 19, and her dad repeated a routine they have had for the past three years: grabbing a cup of coffee, taking a stroll to the park and crossing their fingers for crisp shadows.

“It’s nice that my birthday happens to match up with the event this day,” Margaret Trombly said.

On major anniversary dates, such as the 30th anniversary last year, the town puts on a celebration to commemorate the event. On the years in between when there is less promotion, some locals, art fanatics and fans of the Grateful Dead still show up like clockwork.

“It is very much in the spirit of what Nancy would have wanted,” Adams said. “She appreciated that there would be quieter times and more celebratory events. That some people would remember unprompted to come out and draw in a crowd.”

Rosslyn has recently reaffirmed its commitment to Holt’s August tradition. Last month, Arlington County adopted an urban plan, Realize Rosslyn, including measures to protect and maintain the park for years to come.

“It’s a cool community event that happens without planning,” Masood Mortazavi, who rode from the District with Gonzalez, said about this year’s informal gathering. “We’ll be here next year — Aug. 1, 9:32.”