The opportunity to witness one of the rarest predictable celestial event will present itself shortly after 6 p.m. this evening: a transit of Venus.

A "transit" of Venus occurs when Venus passes between the Earth and the Sun in such a way that we can see Venus's silhouette backlit by the Sun's brilliant light.

The last transit of Venus happened in 2004, but if you are thinking you will just catch it next time, you may want to think again. The next transit will not occur until 2117. 

Often referred to as the "Evening Star" or "Morning Star," Venus is the brightest natural object in our sky after the Sun and the Moon. As the second planet from the Sun, it is closer to the Sun than the Earth is. 

Were Venus either large enough or close enough to block out the Sun's light as it passed, we would call this event an eclipse, as we do when the Moon passes between the Earth and the Sun.

Venus, however, is a little bit smaller than the Earth and about 27 million miles away. When its tiny silhouette is viewed against the Sun, which lies another 66 million miles beyond, it can offer viewers a dramatic sense of the solar system's vast scale.

Assuming that skies are sufficiently clear, the transit will be visible for us starting at about 6:04 p.m. on Tuesday and will remain so until the sun sets.

Those in the central and western U.S. will be able to enjoy it longer, while viewers in Alaska, Japan, and large sections of Australia, China, and Russia will be able to see it in its entirety.

By the time the Sun rises on the East Coast on Wednesday, Venus will have completed the transit.

How to watch

Never look directly at the sun with your naked eyes. You can damage your eyes.

Likewise, viewing the sun with either binoculars or a telescope can direct the sun's magnified rays directly into your eyeball and cause serious injury―think about what happens to ants under a magnifying glass.

Sunglasses do not provide sufficient protection. If you know someone who works in plumbing or construction, ask them if they have any #14 welder's glass. You can look directly at the sun through this material without risking injury.

If you are feeling particularily industrious, newsworks.org posted some videos of Franklin Institute Chief Astronomer Derrick Pitts demonstrating how to make your very own device to safely view the transit, using a manilla folder, transparent tape, some tin foil and a nail. 

And while it's not quite the same as viewing the phenomenon in person, there are several places to watch the transit of Venus online:

• The Slooh Space Camera will offer an 8-hour webcast of the transit that includes real-time video feeds from 10 telescopes around the world.
• Astronomers Without Borders will carry a video stream of the transit from the Mount Wilson Observatory in California.
• NASA will offer a live video feed from Mauna Kea observatory in Hawaii with expert commentary.
• The San Francisco Exploratorium will host an online video stream from the Mauna Loa telescope in Hawaii.

Lastly, there's Don Pettit, an astronaut currently aboard the International Space Station.

Pettit's not doing a video feed, but he will become the first person to ever photograph a transit of Venus from outer space. 

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