By Agencies

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Planet Venus is set to pass directly between the sun and Earth, a transit that will occur next in 2117.Transits of Venus happen in pairs eight years apart, with more than a century between cycles.

During the pass, Venus appears as a small, dark round spot moving across the face of the sun, like a bug on a dinner plate.

Tuesday's transit, which bookends a 2004-2012 pair, begins at 22:09 GMT and lasts for six hours and 40 minutes. Times can vary by seven minutes depending on the location of the observer.

Skywatchers on seven continents, including Antarctica, will be able to see all or part of the Venus transit, which should only be observed with telescopes outfitted with solar filters to protect the eyes.

The event will be covered extensively online, with live video and pictures from an armada of space- and ground-based observatories.

Even astronauts aboard the International Space Station are joining in the event.

"I've been planning this for a while," space station flight engineer Don Pettit said in a NASA interview. "I knew the transit of Venus would occur during my rotation, so I brought a solar filter with me."

Scientists will also be monitoring the transit to collect experimental data, including for a study intended to help in the search for habitable planets beyond Earth.

'Hellish environment'

Telescopes, such as NASA's Kepler space telescope, are being used to find so-called extra solar planets that pass in front of their parent stars, much like Venus will pass by the sun. During the transit, astronomers will be able to measure Venus' thick atmosphere and use the data to develop techniques for measuring atmospheres around other planets.

Tuesday's transit, which bookends a 2004-2012 pair, begins at 22:09 GMT and lasts for six hours and 40 minutes. Times can vary by seven minutes depending on the location of the observer.

Skywatchers on seven continents, including Antarctica, will be able to see all or part of the Venus transit, which should only be observed with telescopes outfitted with solar filters to protect the eyes.

The event will be covered extensively online, with live video and pictures from an armada of space- and ground-based observatories.

Even astronauts aboard the International Space Station are joining in the event.

"I've been planning this for a while," space station flight engineer Don Pettit said in a NASA interview. "I knew the transit of Venus would occur during my rotation, so I brought a solar filter with me."

Scientists will also be monitoring the transit to collect experimental data, including for a study intended to help in the search for habitable planets beyond Earth.

'Hellish environment'

Telescopes, such as NASA's Kepler space telescope, are being used to find so-called extra solar planets that pass in front of their parent stars, much like Venus will pass by the sun. During the transit, astronomers will be able to measure Venus' thick atmosphere and use the data to develop techniques for measuring atmospheres around other planets.

Armchair astronomers have a rich choice of websites to monitor. NASA's compilation will also be viewable.

Slooh Space Camera will broadcast 10 real-time feeds of the transit from solar telescopes located in Australia, Japan, New Zealand, Hawaii, Norway, Arizona, and New Mexico. The webcast begins at 2200 GMT.

And, of course, there is an app for that. Owners of mobile devices using Apple and Android operating systems can download a free application to learn about the transit, interact with skywatchers and track the transit as it occurs around the world. The app is available online and several other websites.