NEW YORK–It's something no one alive today will likely ever see again: The planet Venus crossing the sun—a small, black dot moving across the fiery face of our nearest star.
The transit of Venus across the sun is one of the rarest celestial sights visible from Earth, one that wowed scientists and amateur observers around the world Tuesday (June 5). The event, arguably the most anticipated skywatching display of the year, marked the last time Venus will cross the sun (as seen from Earth) for 105 years.
Only seven Venus transits have been witnessed since the invention of the telescope 400 years ago, and you'd have a long wait for the next one. It won't happen again until Dec. 11, 2117.
To celebrate the last transit of Venus in the 21st century, astronomers and skywatchers came together in many sites around the world. In the United States, NASA beamed images of the transit from an observatory atop Mauna Kea in Hawaii (just one of many webcasts from many countries) and welcomed the public to its various space centers, including the Ames Research Center in Moffett Field, Calif. [Amazing Venus Transit 2012 Pictures]
"It's truly inspiring to see so many faces here to share this moment with us," Natalie Batalha, the deputy science team leader of NASA's planet-hunting Kepler space telescope, told visitors at Ames. "It's going to give you, I hope, a profound feeling of the grandeur of our own solar system. You're going to see Venus in person, with a spotlight shining on her."
Tuesday's transit began just after 6 p.m. EDT (2200 GMT) and lasted about six hours and 40 minutes. It was visible across North America, Europe, Asia and eastern Africa. Because of the International Date Line, some parts of the world saw the transit on June 6.
A rare celestial sight
Venus transits occur when Venus reaches a point in its orbit that brings the planet directly between the Earth and the sun. Since the tilt of Venus' orbit isn't exactly the same as that of Earth, the events are rare, occurring just four times every 243 years.
The transits occur in pairs eight years apart. Since the June 5 transit followed a previous Venus sun crossing in 2004, this is the last one of the current cycle. Venus and Mercury are the only planets that can be seen crossing the sun from Earth since their orbits are between our planet and the sun. The next Mercury transit will be on May 9, 2016.
Despite the extreme rarity of Venus transits, they hold a wealth of information about Venus, the sun and our solar system. Since the first documented observation of a Venus transit in 1639, astronomers have used the events to measure the size of the solar system, the intricacies of Venus' atmosphere, the width of the sun and more. [Venus Crosses Sun's Hellfire in 2012 Transit (Video)]
In fact, NASA and other space agencies trained a fleet of satellites to watch the Venus transit in unprecedented detail. NASA's powerful Solar Dynamics Observatory captured spectacular photos and movies of the entire transit.
Astronomers and skywatchers were eagerly looking forward to seeing the so-called "Black Drop Effect," an optical illusion that occurs when just after Venus moves on to the sun's disk and just before it exits. During the illusion, the black disk of Venus appears to be connected to the edge of the sun due to a trick of optics, NASA officials said.