Weather permitting, the transit was visible even to the naked eye—although astronomers caution that people should never look directly at the sun without proper protection.
To watch any sun event safely, observers should always use special "eclipse glasses" or telescopes and binoculars equipped with solar filters.
Photograph by Tim Chong, Reuters
Mission to Venus?
A jetliner seems ready to fly over Venus in a picture of the 2012 transit taken through a telescope with a solar filter from Colorado. Partially cloudy skies give the sun a mottled appearance.
Astronomers first used telescopes to observe a transit of Venus in 1639.
But it wasn't until 1769 that dozens of scientists scattered across the globe to make detailed measurements of the event, including the famous voyage of British lieutenant James Cook, who had astronomers collecting transit data from the island of Tahiti during his South Pacific expedition. (Related: "Journals of Captain Cook Go Online.")
Observations from different locations on Earth allowed scientists at the time to not only triangulate the true size of the sun but also to more accurately determine the distance between the sun and Earth.
Seen close up, a thin ring around the edge of Venus shows sunlight being refracted, or bent, in the planet's upper atmosphere, as revealed by the Japan Aerospace Exploration Agency's Hinode spacecraft.
The transit, meanwhile, should have allowed astronomers to get a broader picture of Venuvian weather in the planet's upper atmosphere and see how different regions interact.
Image courtesy JAXA
Using cardboard eclipse glasses, Hindu holy men watch the 2012 Venus transit from the banks of the Ganges River in India.
Transits of Venus are so rare because the planet's orbit is tilted just over three degrees from the plane of the solar system. This means that most of the time Venus passes above or below the sun's disk, as seen from Earth.
On average, we see four transits of Venus within 243 years. The events occur in pairs, with each of the two transits spaced eight years apart.
Photograph by Rajesh Kumar Singh, AP
Clouds create a gentle blur in a picture of the rising sun, plus Venus, taken from Sweden on Wednesday.
Based on the 1769 transit of Venus, astronomers calculated that the sun is 95 million miles (153 million kilometers) away—only slightly off from the true Earth-sun distance of 93 million miles (150 million kilometers).
Perhaps the safest way to watch the transit of Venus is to make a pinhole camera. This usually involves cutting a hole about a quarter-inch (0.6-centimeter) wide in a piece of thick paper and using the hole to project an image of the sun onto a flat surface, such as a wall or sidewalk.
The effect can also be achieved with binoculars, as pictured above. Here, a pair of binoculars gives a dual projection of the Venus transit on a white envelope outside Griffith Observatory in Los Angeles on Tuesday.
Photograph by Robyn Beck, AFP/Getty Images
Venus's silhouette stands out against the deep red and vibrant yellow of the sun's chromosphere—its middle atmospheric layer—in a high-definition picture from NASA's Solar Dynamics Observatory taken Tuesday.
The bright yellow patch on the sun is a region of heightened magnetic activity.
Image courtesy SDO/NASA
The 2012 transit of Venus serves as a backdrop to the Kansas City skyline on Tuesday.
Scientists using the NASA-ESA Hubble Space Telescope used the transit to watch for the slight drop in reflected sunlight on the moon. The hope is that Hubble's activity will be a good parallel to observations currently being carried out by NASA's Kepler spacecraft, which looks for dips in starlight caused by planets transiting their host stars, as seen from Earth.
Watching how the sun's light changes during the Venus transit can show astronomers whether their calculations capture the known properties of a nearby planet, helping them to refine their models for studying more distant worlds, said astronomer Jay Pasachoff of Williams College.
"Understanding the details of a transit in our own system," Pasachoff said, "can be the key to unlocking the transits of exoplanets in faraway solar systems."