The black dot of Venus punctuates the setting sun in a picture of the 2012 transit of Venus taken near Salt Lake City, Utah, on Tuesday.
Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than Earth, undergo this unusual alignment.
Due to the planet’s tilted orbit, Venus transits are so rare that only six have been observed since the invention of the telescope more than 400 years ago. (See a telescope time line.)
The 2012 transit of Venus saw the planet glide across the sun’s face for the last time for 105 years. Some countries saw the transit on Tuesday, while others saw it Wednesday morning.
The planet Venus sits near the edge of the solar disk on Wednesday in a high-definition picture of the sun taken by NASA’s Solar Dynamics Observatory.
The orbiting telescope snapped the shot in extreme ultraviolet wavelengths, capturing solar activity such as bright coronal loops—coils of electrified gas—in addition to the planetary silhouette.
Children wait in line to use binoculars set up at Science Center Singapore for viewing the 2012 Venus transit.
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.
Towatch any sun event safely, observers should always use special “eclipse glasses” or telescopes and binoculars equipped with solar filters.
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.
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.
Astronomers planned to use the 2012 Venus transit to collect data on the planet’s atmosphere. Later they’ll compare their findings to measurements from the European Space Agency’s Venus Express orbiter.
The orbiter has returned information on intriguing weather patterns in Venus’s dense atmosphere, but at close range the craft can see only one region at a time. (See “Venus Craft Reveals Lightning, Supports Watery Past.”)
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.
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.
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.
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.
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.