This Tuesday or Wednesday, depending on where you live, sky-watchers around the world will be able to see a cosmic spectacle known as a transit of Venus. The events are so rare that only six Venus transits have been observed since the invention of the telescope more than 400 years ago. (See a telescope time line.)
Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than Earth, can undergo this unusual alignment.
The last Venus transit was in 2004—above, the planet glides across the rising sun in a picture taken during the event from the North Carolina coastline. After 2012, we won't see another transit of Venus until 2117.
"People watching this event through some form of safe solar viewer will see the small, dark silhouette of Venus crossing the sun's face over the course of about six hours," said Jay Pasachoff, an astronomer at Williams College in Massachusetts. (Read a Q&A with Pasachoff about Venus transits.)
"The effect won't be visually impressive, but that black dot against the sun is a remarkable thing to see."
Photograph by David Cortner, Galaxy Picture Library/Alamy
Venus hangs low in the evening twilight near a razor-thin crescent moon in an undated picture taken from Troms County in northern Norway.
As a prelude to this week's transit, nighttime observers may have noticed that Venus seems to be sinking toward the western evening horizon, as the planet appears to approach the sun.
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.
A crescent Venus shines in an ultraviolet snapshot taken by the NASA-ESA Hubble Space Telescope in 1995. As Venus circles the sun, it appears to go through phases that mimic those of our moon when seen through a telescope. (Related: "'Hot Jupiter' Planet's Phases Seen—A First.")
Venus has been getting closer to Earth ahead of the 2012 transit, and the planet's disk has been appearing to not only grow bigger but also to morph from a full disk to a thinning crescent.
In the final few days before the transit, the ends of the crescent will appear to wrap around the planet's disk, forming a ring. This phenomenon is caused by dust in Venus's upper atmosphere scattering sunlight.
Image courtesy ESA/NASA
Safety in Stargazers
Japanese schoolchildren huddle around a projected image of the sun at the Yokohama Science Center during the 2004 transit of Venus.
Staring directly at the sun without eye protection can concentrate incoming ultraviolet and infrared radiation, damaging retinal cells and possibly causing blindness. To watch safely, observers should always use special "eclipse glasses" or telescopes equipped with solar filters.
To do so, cut a hole about a quarter-inch (0.6-centimeter) wide in a piece of cardboard paper, and use the hole to project an image of the sun onto a flat surface, such as a wall or sidewalk.
Photograph by Itsuo Inouye, AP
Venus—and a pelican—glide across the solar disk in 2004, when viewers on Flagler Beach Pier, Florida, saw the sun rise with a Venus transit already underway.
During the 2012 transit, "Venus's diameter will appear only about a 30th the diameter of the sun, so it will be ... like a pea in front of a watermelon," barely visible to the naked eye, Pasachoff said.
Photograph by Jim Tiller, Daytona Beach News-Journal/AP
NASA's sun-watching satellite Transition Region and Coronal Explorer (TRACE) caught an extraordinarily clear closeup view of Venus entering the sun's disk during the 2004 transit.
Scientifically, the most important parts of a Venus transit are the moments when the planet appears to enter and leave the sun's disk, Pasachoff said.
Timing exactly when the planet crossed the solar limb from different locations helped 18th-century astronomers more precisely measure the distance between Earth and the sun. Today scientists hope to use the Venus transit to study the planet's upper atmosphere.
An ultraviolet view from TRACE shows Venus as a dark dot on the sun during the 2004 transit.
For the 2012 transit of Venus, scientists will use the Hubble Space Telescope to help improve techniques for finding and characterizing planets around other stars, aka exoplanets.
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.
A satirical 1793 print plays on the public interest in Venus transits, which were of great historical significance.
During the transits of the 18th century, astronomers went on treks across the globe to time how long it took for Venus to cross the solar disk. (Related: "Journals of Captain Cook Go Online.")
"In 1716, Edmond Halley figured out how, if you could time a transit's duration from places both far north and far south on Earth, you could make a long skinny triangle to Venus and figure out how far away it is, by seeing where on the sun Venus's silhouette is projected," Pasachoff said.
This simple trigonometry allowed scientists to calculate a more exact distance between Earth and the sun, which in turn gave astronomers a way to measure distances to the other planets in the solar system.
Photograph from Science Museum/SSPL/Getty Images
Orbiting spacecrafts can map the planets of our solar system in detail, as seen, for instance, in this 1991 topographic map of Venus by NASA's Magellan probe.
But transits remain among the best methods for hunting and characterising planetary systems around other stars.
NASA's Kepler, for example, has racked up 61 confirmed planets and more than 2,300 planetary candidates by watching for the tiny dips in starlight as exoplanets transit their hosts.
Image courtesy SVS/Magellan/NASA
Rays from the sun pierce Venus's thick, cloud-covered atmosphere and light up the barren landscape in an artist's illustration.
Transits of planets around other stars are being used to search for worlds that may harbor life. A tiny portion of the light from an exoplanet's host star will pass through that planet's atmosphere, which could reveal properties such as the telltale presence of water or methane. (Related: "First Exoplanet With CO2 Heats Up Hunt for Other Earths.")
However, "since the stars are so far away that no details can be seen, those exoplanet transits are visible only in the total light of the star," Pasachoff said. Scientists therefore have to make a number of estimates when analyzing transit data to tease out a planet's atmospheric properties.
Watching how the sun's light changes during the 2012 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.
"Understanding the details of a transit in our own system can be the key to unlocking the transits of exoplanets in faraway solar systems," Pasachoff said.
Image courtesy CIL/GSFC/NASA
Day at the Beach
Telescopes at the ready, amateur astronomers stake out a picturesque outcrop to watch the 2004 transit of Venus from Huntington Reservation in Cleveland, Ohio.
The 2012 transit promises to be one of the most observed and most recorded astronomical events in history, in part because it'll be visible in its entirety from the western Pacific, eastern Asia, eastern Australia, and high northern latitudes. (See a visibility map for the transit of Venus.)
For North America, the first half of the transit will be visible late Tuesday afternoon, while for much of Europe the sun will rise on Wednesday with the transit already in progress.