It's the hottest astronomical performance on nature's big screen, it won't happen again for more than a century, and untold millions across more than half of the world will have the chance to watch.
But don't look at it.
Unless, of course, proper safety precautions are taken.
A rare transit of Venus, when the planet appears to move across the face of the sun as seen from Earth, will be visible to Pittsburgh-area residents early this evening, weather permitting. Observers with safe solar filters, such as a No. 14 welder's filter or sun shades made with Mylar -- but not regular sunglasses or smoked glass, which cannot filter out harmful solar radiation -- can see the planet begin to touch the upper left portion of the sun around 6:04 p.m.
Just after 6:21 p.m., the entire disk of Venus will be visible in front of the sun. The planet will look like a silhouette equal to about 3 percent of the solar diameter. A perspective to keep in mind when safely viewing the spectacle: Venus virtually is the same size as Earth.
The event will be visible until around local sunset, about 8:47 p.m. in Pittsburgh. Trees, buildings, hills or other objects along an observer's western horizon will bring a premature end to seeing the transit.
In all, Venus will take about 6 hours, 30 minutes, to move across the sun. The transit can be seen in its entirety in eastern Asia, about half of Australia, and most of the Pacific Ocean.
Venus transits occur only in June or December and only four times in 243 years, according to Sky & Telescope magazine. Because of the varying motions of Earth and Venus as they relate to each other and the sun, the times between the transits are 121 1/2 years, eight years, 105 1/2 years, then eight years.
The last transit of Venus was June 8, 2004; the next occurs Dec. 11, 2117 (but not visible for most of North America) and Dec. 8, 2125.
The first known observation of a Venus transit was Dec. 4, 1639. Later transits were related to history -- in 1761, Charles Mason and Jeremiah Dixon worked together for the first time to observe the event. They later surveyed the Mason-Dixon Line, effectively Pennsylvania's southern border. In 1769, British Capt. James Cook observed the transit from Tahiti on his first voyage to the South Seas. And in 1882-83, famous American composer and conductor John Philip Sousa wrote his "Venus Transit March."
Properly filtered binoculars and telescopes will enhance the view of the transit without endangering human eyes. For those without such eye protection, public observing of the transit will be available at:
• Wagman Observatory, operated by the Amateur Astronomers Association of Pittsburgh, in Deer Lakes Park near Russellton; 724-224-2510 or www.3ap.org. The association's Mingo Creek Park Observatory in Washington County also will be open to the public; 724-348-6150.