Unless you hope to be alive in 2117, tonight is the last chance of your lifetime to witness the transit of Venus across the face of the sun.

The phenomenon will be visible in most of the United States beginning in the last few hours before sunset on June 5.

Transits of Venus, which happen when the  planet crosses between Earth and the sun, occur in pairs, and tonight's is the sister transit of the one that occurred in 2004, according to a Huffington Post article.

The next one will occur in December 2117.

Scientist Johannes Kepler was the first person to predict a transit of Venus.

"A maligned scientist during his day, Kepler's mother was charged with witchcraft, he was all but ignored by many of his contemporaries, notably Galileo, and he died a penniless wanderer," reported Ellicott City Patch Editor Brandie Jefferson. "To add insult to injury, he didn't even live to see Venus cross the face of the sun, as he had predicted, in 1631."

In the eastern U.S., the transit will begin at about 6:03 p.m.

It will take about 18 minutes for Venus, visible as a black spot against the sun, to move completely inside the sun's upper left edge, according to a Fox News report.

From the upper-left spot, Venus will move diagonally from the upper left to the lower right of the sun's surface, according to the Fox report.

The transit will take place over about a six-hour period.

Viewers are reminded to not look directly at the sun, whether viewing the transit with the naked eye or through a telescope of camera lens.

NASA offers the following tips for sage viewing of the astronomical rarity:

• Experts suggest that one widely available filter for safe solar viewing is number 14 welder's glass.
• If you're using a telescope, be sure it has a solar filter.
• Pinhole projectors, though a good way to observe the sun safely, suffer from shortcomings when Venus approaches the edges of the Sun. Small features like the halo around Venus will not likely be discernible.

The National Weather Service is forecasting scattered showers and cloudy skies night, which could interfere with viewing opportunities.

Check NASA's interactive viewing event map for a list of additional transit viewing events.

Find anything you need to know about the transit at NASA's 2012 Transit of Venus website.

Other resources include the Transit of Mars website, space.com and discovery.com.

If you plan to take pictures of the transit, post them here by clicking the "upload photos and videos" tab or send them to marge.neal@patch.com.