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Scientists prepare to observe rare alignment of Venus

The transit of Venus — which will be visible from 6:03 p.m. until the sun sets in the Boston area — is an exceedingly rare event.

AGRUPACION ASTRONOMICA DENEB

The transit of Venus — which will be visible from 6:03 p.m. until the sun sets in the Boston area — is an exceedingly rare event.

For weeks, astronomers have been putting out the alert: Now or never! See it while you can! Last chance in more than a lifetime! Before sunset Tuesday, amateurs and professionals around the world will watch closely as the planet Venus appears in silhouette — a tiny black spot that will slowly traverse the sun’s face.

The transit of Venus — which will be visible from 6:03 p.m. until the sun sets in the Boston area — is an exceedingly rare event. It occurs when our nearest planetary neighbor passes between Earth and the sun, blocking out a tiny circle of light as it makes the nearly seven-hour journey.

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In centuries past, the phenomenon drove explorers and astronomers to far corners of the world to make observations that could solve what one scientist called “the noblest problem in astronomy” — the distance from the sun to the Earth. But even today, it is more than just a curiosity. Teams of scientists are planning to use the event to help devise strategies that could be deployed in one of the most exciting quests driving astronomers today — the effort to detect and understand more about remote, Earth-like worlds orbiting other stars from afar. These exoplanets are seen as places where life, if it exists elsewhere in the universe, might be found.

“We have here a great opportunity. We have the type of planet we would like to look at — Venus. It’s very close to the Earth in terms of the size. It has an amosphere,” said Jean-Michel Désert, a postdoctoral scientist at the Harvard-Smithsonian Center for Astrophysics who will use the event to test techniques that could be used to determine the makeup of the atmospheres of Earth-sized exoplanets.

Scientists have been planning the experiments carefully because Venus transits come in pairs — eight years apart, and then not for more than a century. The next transit will occur in 2117.

“This experiment, it will set the path of what will be the future” of the instruments needed to detect and analyze in detail the atmosphere of terrestrial exoplanets, Désert said.

For years, astronomers have been using the slight dip in light that takes place when a planet passes in front of a star to search for planets orbiting remote stars. Because it’s not feasible to send a probe to such distant solar systems, researchers depend on measurements of the amount and length of dimming at different wavelengths of light to answer questions about how big the planet is, how far it is from its star, whether it is rocky or a gas giant, and the make-up of its atmosphere.

The transit of Venus provides a unique opportunity to see whether the technique works accurately, since scientists can compare the predictions they make from their models with the measurements that probes have sent back.

Désert and colleagues will utilize the Hubble Space Telescope. Instead of looking at the sun, which is too intense, the instrument will point toward the moon and use the lunar surface as a mirror. The moon’s face will reflect subtle changes in light caused when Venus crosses the sun. The researchers will analyze those changes and use their models to reconstruct the atmosphere of the planet, checking their findings against what is already known. This should enable them to refine their tools, instruments, and techniques for detecting the telltale signature of an Earth-like planet with an atmosphere that could support life.

Meanwhile, Jay Pasachoff, an astronomy professor at Williams College, will be observing the transit from an observatory that sits atop a dormant volcano on Maui, in Hawaii.

He is interested in getting as much data as possible — enough to keep astronomers busy and ready to ask new questions 105 years from now. The transit, he said, allows close measurements that are impossible to make when observing planets travelling around stars that are many light years away.

For example, Pasachoff said this crossing should allow scientists to better understand how sunspots, which can cause dimming on par with a small planet like Venus transiting the sun, might affect remote observations.

Owen Gingerich, an emeritus professor of astronomy and history of science at the Harvard-Smithsonian Center for Astrophysics, said he is hoping to witness firsthand an effect that boggled many of the historical attempts to measure the distance from the Earth to the sun: the “black-drop effect.” At the edges of the sun, the black dot of Venus appears to stretch and elongate. That made it difficult for long-ago observers to tell precisely how long it took the planet to cross the sun.

“That is so much the story of astronomers — feeling very faked out by this, not getting the results they had anticipated,” Gingerich said. “So I just would like to recreate that experience.”

The 82-year-old professor said that as a boy he would often think ahead to astronomical events, hoping that he would be alive to witness them: Halley’s comet, the transits of Venus in 2004 and 2012. He is planning to view it from Mount Wilson in California, where he has been promised a telescope.

“An eclipse, it’s wonderful but it’s all over in five minutes or so, and it’s highly addictive — you suddenly realize [you’ve] got to see more and you knock yourself out to get to the next eclipse, even if it means going halfway around the world,” Gingerich said. “This is not nearly as spectacular, but fantastically more rare.”

Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.
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