Early this week sky-watchers around the world will be able to witness a transit of Venus—a celestial event that won't be seen again for more than a century.

Visible either Tuesday or Wednesday, depending on where you live, the transit will offer astronomers a chance to refine our understanding of Venus as well as to tweak models for searching for planets around other stars. (Pictures: See what the Venus transit will look like.)

Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than our planet, can undergo this unusual alignment.

With its relatively tight orbit, Mercury circles the sun fast enough that we see the innermost planet transit every 13 to 14 years. But transits of Venus are exceedingly rare, due to that world's tilted orbit: After the 2012 Venus transit, we won't see another until 2117.

During the upcoming transit, Venus will look like a black dot gliding across the face of the sun over the course of about six hours.

"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," 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."

The entire transit of Venus will be visible from Hawaii, Alaska, New Zealand, Japan, the Philippines, most of Australia, and parts of eastern Asia. Countries in the Western Hemisphere will see the transit on Tuesday, while those in the Eastern Hemisphere will see it on Wednesday.

Watch a live broadcast of the 2012 transit of Venus.



(See a visibility map for the transit of Venus.)

Viewers in North America will see Venus start to cross the sun in the late afternoon on Tuesday, but the sun will set with the planet still in transit.

Observers in Europe, Africa, and western Australia, meanwhile, will see the sun rise Wednesday morning with Venus already on its face.

Venus on the Edge

The 2012 transit of Venus will be visible even to the naked eye—although astronomers caution that people should never look directly at the sun without proper protection.

To watch safely, observers should always use special "eclipse glasses" or telescopes equipped with solar filters.

Perhaps the safest way to watch the transit of Venus is to make a pinhole camera, said Pasachoff, who is also a National Geographic Society Committee for Research and Exploration grantee. (National Geographic News is a division of the Society.)

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.

During the transit, the leading edge of Venus's silhouette will first touch the upper left side of the sun's disk.

In less than half an hour, Venus's opposite edge will touch the same point of the solar limb. At this stage, the planet's circular shape will appear to be distorted into a teardrop for a few minutes—something astronomers call the black drop effect.

"It's believed that this is an effect of blurring by Earth's atmosphere, combined with the apparent slight darkening of the sun's visible surface near its edges," said Ben Burress, a staff astronomer at the Chabot Space and Science Center in Oakland, California.

Depending on local sky quality, the altitude of the observer, and the size of the telescope, observers may also glimpse Venus's atmosphere during the transit, Burress said.

"A bright rim around the edge of Venus against the background of space may be visible just as Venus is entering or leaving the solar disk," he said. "This is caused by sunlight refracting"—or bending—"in the dense upper atmosphere of Venus."

Measuring the Solar System

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 happen in pairs spaced eight years apart, and they alternate whether Venus crosses the top or the bottom of the solar disk, Williams' Pasachoff said. This year, for instance, the planet will transit the top of the sun.

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.

Observations from different locations on Earth allowed scientists to not only triangulate the true size of the sun but also to more accurately determine the distance between the sun and Earth.

"Prior to that, the estimates of those scales were mostly educated guesswork," Chabot's Burress said.

Based on the 18th-century transit, 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).

"Since we already knew the relative spacing between the orbits of all the planets, once we determined the Earth-sun distance, in one fell swoop we were able to calculate the distances to all the other planets."

Venus Transit a Key to Planetary Puzzles

Today, 21st-century astronomers hope to use the 2012 transit of Venus to collect data on the planet's atmosphere and 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.

The transit, meanwhile, will allow astronomers to get a broader picture of Venuvian weather in the planet's upper atmosphere and see how different regions interact.

In addition, scientists using the NASA-ESA Hubble Space Telescope will use the transit to help improve techniques for finding and characterizing planets around other stars, aka exoplanets.

With its sensitive instrumentation designed to peer deep into the cosmos, Hubble can't look directly at the sun. Instead astronomers will have the orbiting observatory aimed at the moon to watch for the slight drop in reflected sunlight during the transit.

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 prolific planet hunter, Kepler has already racked up 61 confirmed planets and more than 2,300 planetary candidates. (Related: "NASA Finds Smallest Earthlike Planet Outside Solar System.")

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," Williams's Pasachoff said.

Scientists therefore have to make a number of estimates when analyzing Kepler data to tease out a planet's size and atmospheric properties.

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.