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Rare alignment for Venus

By Alan M. MacRobert
Globe Correspondent / June 2, 2012
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On Tuesday afternoon, Venus will cross the face of the sun for only the seventh time since the invention of the telescope. You can join millions of people who will be watching through safe solar filters, or by projecting the sun’s image onto paper with binoculars or a telescope, or via one of countless live broadcasts planned on the Web.

This rare alignment of the sun, Venus, and Earth is stirring up worldwide interest not just as a spectacle in its own right, or because of its rarity (it last happened in 2004 but won’t again until 2117), but also because of cultural memories that we have inherited from generations past.

In the 18th and 19th centuries, transits of Venus promised the best chance to pin down one of the greatest astronomical unknowns of the time: the size of the solar system. In the era of sailing ships, the world’s great powers vied with each other, sending heroic expeditions to the farthest ends of the earth to triangulate on Venus and the sun, and the public followed the news of these efforts keenly.

Astronomical distances were hard to measure. Ever since ­Kepler worked out his laws of planetary motion (completed in 1619), astronomers had a precise scale model of the relative distances of the sun and planets, and later of the planets’ sizes themselves and the orbits of their moons. But until an absolute measurement — in miles — could be made to just one of these bodies, the whole scale model lacked a scale.

Edmond Halley, of Halley’s Comet fame, announced in 1716 that the best hope of succeeding in this grand project would be to time the exact moments when a transit of Venus began and ended as seen from widely separated loca­tions. This would allow a triangulation on Venus and the sun similar to the way surveyors worked on Earth. The basic unit of solar system distances — the average distance of the sun from Earth — was given the grand name the astronomical unit. Finding its length was as great a quest of the 18th and 19th centuries as finding the size, age, and expansion rate of the universe (the “Hubble constant”) was for most of the 20th.

Sequence of events

In the Boston area, the Venus transit begins at 6:03:42 p.m. Tuesday, when the leading edge of Venus’s silhouette first touches the Sun’s top edge. It will take several more seconds for the tiny dent in the Sun’s rim to become evident in a filtered telescope.

For the next 17 minutes, more of Venus’s black disk will cross the Sun’s edge, until the last bit of Venus enters the Sun completely.

For the next two hours Venus will creep across the sun’s face, until sunset (at 8:15 for Boston) puts an end to the show for us.

On hazy days the setting sun can be so dimmed and reddened that it is safe to gaze at with the naked eye, but on other days it is still too bright. Use caution.

Watchers farther west can ­observe the rest of the transit. You can keep watching via webcams until Venus departs the sun’s face around 12:45 a.m., some 6 hours and 20 minutes ­after the event began.

How to watch

The silhouette of Venus will appear 1/30 the sun’s width. That is big enough to see as a distinct black dot if you have a safe solar filter to look through. A solar-­filtered telescope will give a much better view.

It is pretty late to obtain a proper filter; you need to use a filter specifically made for sun viewing to avoid permanent damage to your eyesight. Or watch by one of the telescopic webcams around the globe.

See the growing list at SkyandTelescope.com/transit­webcams.

For more about all ­aspects of this event, visit the site SkyandTelescope.com/tov.

Alan M. MacRobert is a senior ­editor of Sky Telescope magazine (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.

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