A magnificent rendezvous between the planet of love and the bright orb of the Sun. One of the most celebrated phenomena in astronomy. A sight unseen by anyone alive today. With superlatives like these describing a transit of Venus, it is little wonder that astronomers are eagerly awaiting the next one — June 8, 2004.

A Much-Scrutinized Planet

The earliest recorded sightings of Venus appear in a tablet dating back to the 16th century BC. This Venus Tablet, as it is known, was found among the ruins of Nineveh (near present-day Mosul, Iraq) and now resides in the British Museum.

Since then, 45 transits have taken place. Might Venus have been observed on one of these occasions and unwittingly set down as a sunspot? The planet's silhouette is large enough to appear as a small, naked-eye spot during a transit, and there are many records, especially by astronomers in ancient China, of "blemishes" on the Sun's face. It is a plausible idea, but to date no accounts of a round blemish, seen at the same time as a Venus transit, have been discovered.

For that matter, even the first transit of the telescopic era was missed. Johannes Kepler, a German astronomer and mathematician, predicted such an event for December 6, 1631. We now know that the transit's end was visible at sunrise from Italy, Austria, Germany, and Denmark. But it seems nobody in central Europe was looking, likely because of the turmoil caused by the ongoing Thirty Years' War. Kepler died a year before that transit, and Pierre Gassendi, the only astronomer known to have kept watch, did so from Paris, which was outside the region of visibility.

The next transit, eight years later, also nearly went unobserved, because Kepler had failed to forecast it. The possibility of a Venus transit was picked up only a month before it occurred, thanks to the calculations of a brilliant young English astronomer, Jeremiah Horrocks. Horrocks and his friend William Crabtree were the only ones to observe a transit of Venus in the 17th century.

Halley, Venus, and the Astronomical Unit

In contrast to the indifference of 17th-century astronomers, interest in transits soared in the 1700s after English astronomer Edmond Halley proposed using them to determine the distance from the Earth to the Sun.

As a young man, Halley journeyed to St. Helena, a bleak island in the South Atlantic, where Napoleon Bonaparte was exiled nearly 140 years later. There he mapped the southern stars and in November 1677 observed a transit — not of Venus but of Mercury. At once he realized that if two observers were widely separated in latitude, they would see a transiting planet move along different chords as it traversed the Sun. If each observer timed the transit from beginning to end, the shift in the planet's position — its parallax — could be calculated and used to determine the Earth-Sun distance, a separation called one astronomical unit (a.u.).

Although Mercury's transits are relatively frequent — on average, 13 of them occur each century — the planet's parallax is too small for the method to have great utility. But Halley realized that the rarer transits of Venus were a different matter, and he proposed that they could be used to measure the distance from the Earth to the Sun to an accuracy of 1 part in 500.

As with the comet whose return he successfully predicted, Halley knew that he would not live to see the upcoming transits of 1761 and 1769 — the first since the time of Horrocks and Crabtree. Halley died in 1742, but his method inspired other astronomers. The quest to use a transit of Venus to calculate the Earth-Sun distance became one of the great scientific obsessions of the 18th century.