Just Passing By

By Michael Lemonick

In 1627, the great German astronomer Johannes Kepler first predicted a transit of Venus, but he died before he could witness the 1631 event. In 1769, the explorer Captain James Cook-just a lieutenant at the time-made his first voyage to the South Pacific in order to view that year's transit from Tahiti. And more than 50 expeditions were launched from the U.S., Britain, Russia and other nations to every corner of the earth to see the 1874 transit.

There was an important reason to make such efforts: by recording the moments a transit began and ended from different vantage points on Earth, astronomers could use trigonometry to precisely calculate the distance from Earth to the sun. That was easier in theory than in practice, though, and nowadays astronomers use other methods to measure the distance to the sun. The world will be watching next month, but mostly out of curiosity and wonder at seeing a planet move across the face of the sun-firsthand proof that the seemingly two dimensional sky is anything but. Another transit is coming June 6, 2012. If you miss that one, you're out of luck. There won't be another transit until 2117.

Best seats in the house The transit takes only about six hours. The U.S. has a partial view; the West Coast misses it because it happens there at night.

U.S. As the sun rises, New England sees Venus two-thirds of the way through its passage. As you head south and west, less and less of the transit is visible.

WORLD Europe and most of Africa and Asia are ideal for viewing

TRIP PAST THE SUN Venus passes between Earth and the sun more than once a year on average, but because its orbit is tilted with respect to Earth's, a lineup precise enough to permit a transit is much rarer.

VENUS Its size and composition make it almost a twin of Earth, but its atmosphere is so thick that pressure at the surface is 90 times that on Earth. The atmosphere is mostly carbon dioxide flavored with sulfur and sulfuric acid, which has caused a runaway greenhouse effect. The result: temperatures of more than 800DEGF (430DEGC).

VENUS'S TRANSIT

June 8

1 A.M. (E.T.) 2 A.M. 3 A.M. 4 A.M. 5 A.M. 6 A.M. 7 A.M. 8 A.M.

How to view the transit safely

DO NOT WATCH THIS EVENT WITHOUT ADEQUATE EYE PROTECTION The sun may look dim through sunglasses, unexposed photographic film and CDs, but it can cause permanent blindness. The only safe way to look at the transit is through No. 14 welder's glasses or a Mylar filter certified for sun observation (some manufacturers have made such filters into goggles). If you aren't absolutely sure that's what you've got, you can avoid frying your retinas by making a pinhole projector, right, and watching indirectly.

Source: Transitofvenus.org

TRANSITS THROUGH THE AGES

--1600s An English astronomer named Jeremiah Horrocks, left, studied Kepler's planetary tables and realized a transit would happen within a month. In 1639, he and a friend are the first humans known to have seen one.

--1716 Edmund Halley, who would give his name to the celebrated comet, proposed a technique for gauging the distance from Earth to the sun by viewing transits from different vantage points. He died in 1742, before his idea could be tried.

--1760s Following Halley's suggestion, explorers- including, in 1761 and '69, Jeremiah Dixon and Charles Mason, who later surveyed the Mason-Dixon line, and, in 1769, James Cook-headed for remote regions, above, to see the transits that occurred in those years. Unfortunately, observers had to know their precise longitude and latitude, which were hard to measure at the time. As a result, astronomers couldn't calculate the distance from Earth to the sun with much accuracy.

--1870s and 1880s The transits of Venus in these decades were media events, featured in popular articles all over the world. The U.S. Naval Observatory alone sent out several expeditions, left. By then, it was easy to figure out longitude and latitude, and good telescopes were available. But it was still impossible to get precise timing measurements because just before Venus leaves the sun's disk, an optical illusion makes the planet seem to ooze toward the edge.