Skywatchers young and old partied like it was 1882 at the University of Colorado's Fiske Planetarium on Tuesday afternoon as they gathered to view the transit of Venus, an astronomical event that won't happen again for more than 100 years.

The Venus transit refers to the passage of the planet between the Earth and the sun.

Through telescopes, Venus appeared Tuesday as a tiny black dot that slowly traversed the sun's surface. The event happens in cycles that repeat every 243 years, separated alternately by gaps of 121.5 and 105.5 years.

The last two transits took place in 1882 and 2004, and the phenomenon won't be seen again until Dec. 11, 2117.

By 5 p.m. Tuesday, the rare event had drawn hundreds of people to CU's Fiske Planetarium, the grassy area in front of the building and the neighboring Sommers-Bausch Observatory. A thin veil of clouds was present early in the afternoon, but the transit was visible with telescopes, binoculars and the same special glasses used for viewing last month's solar eclipse.

Julie Andrew, of Boulder, checked out the transit with her daughter, Parker Hardee, 8, and her friend Madelyn Hoobler, 7, from the observation deck atop the Sommers-Bausch Observatory. They had the options of looking through several specially outfitted 4-inch and 6-inch telescopes as well as CU's 16-inch and 18-inch models.

"Rocking," was how Madelyn described the transit.

"I thought it was pretty cool," Parker said. "Venus is definitely very small compared to the sun."

The two girls were co-founders of a space club at Boulder's Eisenhower Elementary School, Andrew said, and were both excited for Tuesday's viewing.

Loveland resident John Matis, 67, said he was planning to observe the transit of Venus from a viewing area outside Loveland after he finished up some business in Boulder on Tuesday. When he heard smoke from wildfires was obscuring the view there, he said, he made a U-turn on the Diagonal Highway and came straight to the CU campus.

"I'm absolutely, totally overjoyed," Matis said as he checked out the transit from the observatory deck. "I'm not disappointed. This is a once-in-a-lifetime astronomical event."

Matis said he called, texted and emailed friends and family members across the country urging them to check out the ultra-rare transit.

Brandon Bell, a CU senior studying astrophysics, was among the volunteers at the observatory, helping operate CU's largest telescope. The 24-inch telescope was used to direct a smaller telescope that was providing a view of the transit broadcast live at Fiske and on the Sommers-Bausch Observatory website.

"I'm very impressed with the turnout today. I'm thrilled with it," Bell said as people patiently lined up to check out the large telescope and look at the images of the transit it was projecting on the observatory's dome. "I think it's a fascinating subject for anybody, and we're fortunate to have the facilities here to take full advantage of it and really enjoy it."