The solar system was in motion. The crowd was not.

About 6 p.m. Tuesday, hundreds of tourists, schoolchildren and fresh-from-the-office workers milled about on the steps of the National Air and Space Museum.

They looked up. They looked up again. An ugly deck of clouds blocked their view.

They were there for a historic glimpse of Venus inching across the face of our sun — one of the rarest events in the heavens. For space buffs and science lovers, it was now or never: The next transit of Venus, as it’s called, is not until 2117.

“A little bummed out,” said Sean Greene, 17, of Silver Spring. “I was really excited.” Young and optimistic, Greene added, “But we’ll have until sundown.”

The museum was ready, setting up five sun-safe telescopes outside. About 200 people stood in line. But the clouds persisted.

Michael Halpern, 34, and his friend Kalen May-Tobin, 33, loaded a NASA transit webcast on their iPads.

Others drifted inside to the museum’s Moving Beyond Earth gallery, where images from a telescope in Hawaii were beamed to a big screen. Museum educator Agustin Baldioli said that 770 people had come in. It was standing-room only. A cheer went up when a bright orb popped onto the screen. A black dot — a beauty mark on the face of the sun — sat inside the disk.

D.C. resident Jeff Villa, 29, tossed his arm around his fiancee, Lundy Khoy, 31. He held out his camera and snapped a couple’s photo, backdropped by an enormous bright sun and a tiny dark Venus.

“This is a once-in-a-lifetime event,” said Villa, an amateur astronomer, “and she’s once-in-a-lifetime, too.”

Then someone yelled, “It’s got a ways to go!”

The transit of Venus is not an event for the easily distracted. It takes seven hours for our sister planet to finish her lazy crossing.

At the U.S. Naval Observatory in Northwest Washington, a small crowd lingered until the sun sank beneath the trees.

They came to watch through a famous instrument: the five-inch Alvan Clark Transit of Venus telescope, which saw the previous three transits: in 1874 in Vladivostok, Siberia; in 1882 in San Antonio; and in 2004 at the Naval Observatory.

Geoff Chester, public-affairs officer for the observatory, said he believed that it is the only telescope to witness four transits of Venus.

Through a gap in the clouds, Chester saw the black dot. “It lasted about five seconds,” he said. “I am happy.”

By 7:15 p.m., the crowd on the museum steps had thinned. The clouds were low and unyielding.

But then, the patient were rewarded.

A crack opened. Beams shone through.

“Oh, yes!” shouted Ramon Miro of Rockville, a member of the National Capital Astronomers club. He had hauled his six-inch telescope downtown.

“Yes!” shouted Miro. “Quickly! Top center! Top center!”

The knot surrounding Miro’s shiny red telescope squeezed in.

Sofya Leonova, 26, of the District pressed her eye to the eyepiece. “The dot was bigger than I expected,” she said. “It’s glorious to see something so wonderful.”

Then the clouds swallowed the sun again.

Back in the museum, Venus slid toward the sun’s interior on the big screen, in no hurry at all.

An older gentleman hobbled past a father bottle-feeding an infant. He leaned on his cane and said, “Maybe that baby will see the next one.”

Only 105 years to go.

Staff writer Jason Samenow contributed to this report.