A transit of Venus: early on the morning of June 8, the silhouette of Venus will slip across the Sun. The event, last seen in 1882, was once the key observation in determining the size of the solar system.

Publication: Natural History

Publication Date: 01-JUN-04

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COPYRIGHT 2004 Natural History Magazine, Inc.

Catching sight of celestial spectacles requires more than patiently sitting at a telescope set up in your backyard. Birdwatchers can pursue their passion almost wherever and whenever they choose. Flower and plant aficionados have only to stroll through their favorite park to enjoy the sights and scents of nature. But to experience the wonders of the heavens, the skywatcher must be in the right place at the right time. Moreover, even predictable astronomical events are often notoriously rare. A total eclipse of the Sun, for example, takes place on average only once every eighteen months. But to see it you may have to travel halfway around the globe to station yourself in the path of the Moon's shadow. Even then, you're still at the mercy of the elements. Some spectacular events, such as a Leonid meteor shower as splendid as the one in 2001, may happen only once in a lifetime--if at all.

Few celestial phenomena can compete in rarity and in historical interest with the passage of the planet Venus in front of the Sun: a "transit" of Venus. Only five times in recorded history has this event been witnessed: in 1639, 1761, 1769, 1874, and 1882. How lucky we are, then, to be around for Venus's next visit to the Sun, scheduled to begin at 05:13 Universal Time (1:13 A.M. Eastern daylight time), also called Greenwich Mean Time, on Tuesday, June 8, 2004, and to end at 11:26 Universal Time (7:26 A.M. Eastern daylight time) the same day.

The story of the transits of Venus begins in 1627, just three years before the death of the German astronomer Johannes Kepler. In that year Kepler finished his last major work, the Rudolphine Tables, a compilation of astronomical data on the positions of the Sun, Moon, and planets for every day of the year. On the basis of his tables, Kepler made a startling prediction, published separately: on November 7, 1631, the Sun, Earth, and the planet Mercury would be in perfect alignment, so that for a few hours Mercury's dark silhouette would be visible on the face of the Sun. That was not all: one month later, on December 6, the show would be repeated, with Venus in the place of Mercury.

Mindful of the rarity of each of these celestial alignments, let alone the near coincidence of two such events, Kepler issued an admonition to his fellow astronomers in 1629, urging them to watch the two events with the utmost care. Should each transit take place at the time he had predicted, it would verify the accuracy of his tables. Moreover, by observing Mercury and Venus starkly...

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