Skywatchers on all seven continents will get a chance to witness an astronomical rarity on June 5 or 6, the transit of the planet Venus. It is likely the last time anyone alive today will have a chance to view one.
That's because after this year's transit of Venus, the next one is in 2117.
Transits of Venus have happened only six times since the phenomenon was first observed with a telescope in 1639, and only once since the invention of the television.
People around the world turned out in 2004 to watch as Venus passed across the face of the sun - seen from Earth for the first time since 1882.
Venus appeared in silhouette, casting a small black dot on the sun.
It's an uncommon event, but not necessarily a spectacular one.
"What actually happens to the casual person who doesn't know it's happening is nothing," explains David DeVorkin, the senior curator of the history of astronomy at the Smithsonian National Air and Space Museum in Washington. "Okay, there won't be any change in the brightness of the sky and, you know, birds won't come home to roost or anything like that. But if you know it's happening, it's really quite interesting."
The event is interesting, in part, because it is so rare. Transits occur in pairs, and the pairs are separated by more than a century. The upcoming transit bookends the transit of 2004.
"If you're looking safely at the sun, you will see a spot appear on the edge of the sun that is about 1/30th the size of the disc of the sun itself, and over a few hours' time, it's simply going to drift across the sun and keep going," DeVorkin says.
The key, as DeVorkin notes, is to view safely. You should not stare directly at the Sun, but you can use glasses and telescopes with protective filters.
Scientists in previous centuries used the transits to answer questions about the solar system.
"Astronomers realized way back in the 17th century that if you could very accurately time this appearance of Venus going across the sun, and time it as accurately as you can from two widely different parts of the Earth, you could use the different timings to triangulate to find out how far we are from the sun, and that was a fundamental question," says DeVorkin.
Scientists, astronomers and adventurers, including famed British explorer Captain James Cook, launched expeditions to observe and gather data about the transit pair in the 1760s.