Early evening sky watchers may have noticed that Venus is now appearing lower in the sky each evening. This happens because Venus is orbiting the sun and, within only a week or so, it will cross over to the opposite side of the sun and will soon become visible as an early morning "star."

What we don't usually see is the actual crossover. But on June 5, we will. Astronomers know the phenomenon as a "transit of Venus," and it's one of the rarest celestial events you'll ever see. If you miss this one, you'll have to wait 105 years for your next chance.

During a transit, a properly protected eye can see a tiny black dot slowly crossing the face of the sun. While it will be just possible to spot Venus' silhouette without optical aid as it crosses the Sun -- the planet will appear only 1/32 of the sun's diameter -- you'll find that properly filtered binoculars or a telescope will produce a more exciting view.Much like viewing an eclipse, observing a transit with safety is extremely important. Never view the sun with the naked eye, sunglasses, double thickness of darkened film, smoked glass or other homemade filters. To learn more about safely viewing and photographing the sun, check out Fred Espenak's website: mreclipse.com/MrEclipse.html#Sun.

, when sharp-eyed observers may spot the edge of Venus entering in front of the solar disk. By 6:27 p.m., the planet's entire disk will become visible. The sun will have set for all of the continental U.S. by the time the transit ends. Those living on Pacific islands, Asia and east Africa will enjoy the rest of the show.

The transit times are in Universal Time (UT), which is that of Greenwich, England, and you'll need to convert to your own local time. Eastern Daylight Time is four hours behind UT, so a simple subtraction is all you'll need to do.

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