It's a spectacle that won't repeat for another century _ the sight of Venus slowly inching across the face of the sun.
The silhouette of Venus will march across the face of the sun on Tuesday from the Western Hemisphere (Wednesday from the Eastern Hemisphere). Known as a transit of Venus, this won't happen again until 2117.
Venus will appear as a small black dot gliding across the disk of the sun. As in a solar eclipse, do not stare directly at the sun; wear special protective glasses.
The entire transit, lasting 6 hours and 40 minutes, will be visible from the western Pacific, eastern Asia and eastern Australia.
Skywatchers in the United States, Canada, Mexico, Central America, and the northern part of South America will see the beginning of the show before the sun sets. Europe, western and central Asia, eastern Africa and western Australia will catch the tail end after sunrise. Those who don't want to leave their homes can follow live webcasts by NASA and various observatories.
You can damage your eyes by staring at the sun. People need to remember that as they turn to the skies to watch and keep in mind a few tips.
Wear special viewing glasses such as solar eclipse glasses. You can buy them online or at your local museum. Alternatively, you can go to a hardware store and get a pair of welder's glasses, but make sure it's number 14 or darker. Or make a pinhole projector with cardboard. Do not watch the transit with regular sunglasses.
Peer through telescopes outfitted with special filters at viewing parties hosted by museums, observatories and astronomy clubs. Many will also have experts on hand who could talk about the history and significance of a Venus transit.
Tune in online. NASA, Slooh.com and the Exploratorium in San Francisco are among those that plan live webcasts.