MOUNTAIN VIEW (CBS 5) – Another must-see event for casual sky-watchers and budding astronomers alike was in the skies above California on Tuesday.

The so-called “transit of Venus” — when the planet Venus passes between the Earth and our sun — began shortly after 3 p.m. when the planet will appeared as a small black dot slowly moving across the sun.

Historically, the transit of Venus was used by astronomers to measure the size of the solar system by measuring the amount of time it takes for Venus to pass by the sun.

The last time Venus made such a pass between the Earth and sun was in 2004. Tuesday night’s trip will take almost 7 hours. The event will not be visible again until 2117.

“It’s such a rare chance,” said William Borucki, a space scientist at NASA Ames Research Center. “Never again in their lives will they see this, the actual of event of Venus crossing the surface of the sun.”

Scientists warn that people should not look directly at the sun due to risk of eye damage. There are several local events where watchers can safely view the event, including:

The NASA Ames Research Center at Moffett Field
Chabot Space And Science Center In Oakland
The Lafayette Library And Learning Center
Mare Island Shoreline Heritage Preserve
Additional Information From The California Academy Of Sciences

This is the seventh transit visible since German astronomer Johannes Kepler first predicted the phenomenon in the 17th century. Because of the shape and speed of Venus’ orbit around the sun and its relationship to Earth’s annual trip, transits occur in pairs separated by more than a century.

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