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The so-called "transit of Venus" is expected to begin at 3:06 p.m., when the planet will appear as a small black dot slowly moving across the sun. The planet will remain visible until the sun sets at 8:02 p.m.

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, but it wasn't visible from Southern California. The last time such a transit was visible from the Southland was 1882, and the next time it is scheduled to happen is Dec. 11, 2117, according to the Griffith Observatory.

Much like an eclipse, the transit of Venus is dangerous to view with the naked eye because it involves looking directly into the sun. Viewers must use properly filtered glasses or other viewing devices to witness the event safely.

The Reuben H. Fleet Science Center in Balboa Park was selling the special glasses for $2, but officials say the glasses were sold out as of Tuesday afternoon.