Tuesday’s transit of Venus—a celestial passage of the planet across the Sun—attracted thousands of people from across the Bay Area to Chabot Space and Science Center, all eager to see an orbit that will go back into hiding until December, 2117. The line of people stretched so far outside of the observatory that workers inside were calling out the headcount by the hour on walkie talkies, saying that the day could set the attendance record for America’s largest public telescope facility.

At the center, people watched the transit live through telescopes on the observation deck. The most popular of the telescopes was “Leah,” the center’s oldest telescope, which dates from 1883.

“Venus is passing directly between the Earth and the sun,” explained Ben Burress, a staff astronomer at the center, who studied physics and astronomy at Sonoma State University. “On Earth we’ll see Venus crossing over the sun’s face—a little dark silhouette circular dot. The planet Venus, at about 3:04, first touches the edge of the sun and then for about the next six hours crosses the sun’s face. At 9:46 tonight it will all be over and Venus will leave the sun.”

The last transit of Venus, also lasting about six hours, happened in 2004.

The wait to look through the telescopes allowed spectators to reflect on the rare occasion—which won’t happen for another 105 years—and the history of the science center. “It was an old wooden telescope building,” said Carla Paganelli, an adjunct professor at Dominican University who remembered the space center’s original building on Mountain Boulevard. “I was so fascinated as a kid when they turned the telescope to point to a star, the whole thing, on this creaky wooden track. It was magical.”

“I’m not going to be around for the next one,” she said of the transit of Venus. “I want to live a long life, but I don’t think I’m going to quite make it for 105 years unless I go into cryogenics or something like that.”

Throughout the day, people were warned to take precautions when viewing the transit, which were similar to those given for viewing an eclipse—don’t stare directly into the sun without protective eyewear.

The day was an affair for kids, too. “We go to the children program here [at the science center] called Tykes and they learn about planets and all kinds of things space related,” said Carol Behr, a visitor at the center. She brought her daughter, Sarah, a preschooler at Temple Beth Abraham in Oakland’s lower hills, to witness the transit. “We wanted to see it for real. It’s really great what they’re doing here,” she said.

In the Ask Jeeves Planetarium, the seats were filled with young and old space enthusiasts watching a live webcast from Hawaii on the planetarium’s full-dome digital projection system. In Oakland, the sun would set before Venus finished crossing its disk, but in Hawaii, the transit remained visible, so once the sun began to set in Oakland, people moved inside to watch the webcast. “NASA and the Exploratorium have each set up on different volcano tops—Mauna Loa and Mauna Kea—so they can televise through their telescopes the Venus transit,” Burress said.

Several researchers observing the transit were working to improve their ability to find extrasolar planets orbiting other stars. “We find planets going around other stars when they transit their star by watching the slight dip in light of the star when the planet crosses in front of it,” Burress said.

“The sun will get slightly dimmer when Venus blocks out a small piece of it,” he continued. “We detect other planets around other stars the same way—and Venus is very close compared to far-away stars.”

Venus is sometimes called Earth’s twin: it’s about the same size and composition, but the fundamental difference is that Venus has a very thick atmosphere. “The pressure of air at the surface of Venus is about the same if you were half-a-mile under the Earth’s oceans,” Burress said. “It’s also very hot—900 degrees Fahrenheit—day and night, north pole and equator.” The heat on Venus comes from its runaway greenhouse effect—the atmosphere is largely carbon dioxide, which captures solar heat.

Not everyone in the planetarium knew the transit would be a day-to-night affair. “This isn’t a quick event,” Burress told people as they stared at the 70-foot diameter screen. “It takes six hours, so you wont miss it if you blink. You can come in here and relax a bit, watch the webcast, and—when you’re ready to go back out for the sun—look through the telescopes again.”

By 6:30 pm hundreds were still making there way to the science center, many choosing to park down the step, curvy hill and walk in case the traffic jam didn’t clear by sunset. One visitor found herself a little jealous of the attention Venus was getting. “I’m here because I want to dance with the stars,” said Anat Shamash, a dance instructor who teaches Brazilin samba. “It’s just a little black dot—but I must admit it’s cute.”