More than half the world will have a chance to watch a rare cosmic event today as Mercury crosses the face of the Sun.

The crossing, called a transit, can be viewed from backyards with safe viewing equipment [Viewer's Guide]. Looking directly at the sun without a proper filter will damage your eyes.

Mercury's transit will begin within a minute of 19:12 GMT (Greenwich Mean Time) at every site from which it is visible. That's 2:12 p.m. ET and 11:12 a.m. PT. The entire transit will last 4 hours and 58 minutes.

Observers throughout the Americas and across eastern Asia, Australia and New Zealand will be able to see all or part of the crossing, weather permitting. [Map]

"From the United States, those situated to the east of a line running from roughly Bonners Ferry, Idaho to El Paso, Texas will be able to see the beginning stages of the transit, as Mercury moves onto the lower left part of the Sun," explained Joe Rao, SPACE.com's Skywatching Columnist. "Unfortunately, for most of the United States, local sunset will intervene before Mercury can move off the Sun's disk."

Mercury will appear as a small black dot against the Sun [image].  

For those who cannot see the event due to bad weather or lack of safe viewing equipment, there are several webcasts planned. See a list of webcasts, along with a complete viewer's guide and safe sun-watching advice, here.

From our vantagepoint, Mercury transits the Sun every seven years on average. The last such event, however, was in 2003.