Transits of Venus, when that planet passes in front of the Sun as viewed from Earth, are themselves infrequent: they occur only 13-14 times in a thousand years. There have been only five transits of Venus ever observed: in 1639, 1761, 1769, 1874, and 1882—the last was nearly 122 years ago. Not only is this event so rare that no one alive has seen one, but possibly less than a thousand people have ever observed Venus’ transit and the black drop.
The black drop has considerable historical significance. Edmund Halley first proposed measuring Venus’ path across the solar disk by timing Venus’ entrances and exits, known technically as the 2nd and 3rd contacts. Observations made across the globe could determine the solar parallax by triangulation. This quantity is simply related to the Sun-Earth distance, and thus provides a benchmark for the solar system. Although this was attempted at the next two opportunities, 17th century astronomers’ results were far less precise than hoped. Uncertainty in contact timings introduced by the black drop, an error of up to one minute in some cases, was enough to introduce considerable inaccuracy in the Sun-Earth distance.
Astronomers tried again during the 18th century transits, this time as part of the first international scientific expeditions sponsored by their governments. Indeed, this is why Lieutenant James Cook was in Tahiti and soon to discover terra australis, what today we know as New Zealand and Australia. Even for Cook and his astronomer Charles Green, the black drop rendered the measurements imprecise. 19th century astronomers had better equipment, although their timings were scarcely better. Photographs documented the black drop as a real effect, proving that it was not just an optical illusion. But by the late 1800’s, there were superior methods for determining the solar parallax, and it was realized that transit timings were no longer scientifically critical. In the 20th century, interplanetary distances were measured by radar, and the solar parallax was determined to great accuracy. Transit timings no longer have any role in determining this cosmic yardstick.
Unlike past transits, nowadays the spectacle is made for amateurs and history buffs. No doubt millions will watch webcasts of the first of two transits of Venus this century. On 8 June 2004, Venus will cross the face of the Sun, taking about 6 hours. While observers in the eastern hemisphere will be able to see the whole event, we on Delmarva will not miss out entirely. As the sun rises over the Atlantic Ocean, the transit will be well underway: we will be able to view the last quarter of the transit. 3rd contact and the black drop will occur around 7:06 EDT with the Sun about 15º above the horizon. A mere 20 minutes later, Venus will have left the Sun, and this much anticipated conjunction will be finished. The next, and our last, opportunity will be eight years hence, 5-6 June 2012.
To help prepare for this June’s extraordinary sight, I’d recommend: William Sheehan and John Westfall’s The Transits of Venus, Eli Maor’s June 8, 2004: Venus in Transit, and Guy Ottewell’s Astronomical Calendar 2004. Sky & Telescope magazine has published numerous feature articles this year: see June pp. 73, 108 and 112, May pp. 32 and 137, and February p. 46. Plenty of historical and astronomical information is available on the web: visit Fred Espanak’s authoritative 2004 Transit of Venus page or www.transitofvenus.org for graphics and other resources. Be sure to use safe observing techniques to view the transit and the black drop. Never look at the Sun directly or through a telescope without an aperture filter. See astronomy books, magazines and web sites for how to observe the Sun safely. Low clouds and fog are common in summer near the coast, so check a local forecast and choose alternate viewing locations to maximize your chances. I’ll be on the beach with the surf fishermen hoping for an unobstructed view of a most memorable sunrise.
Moondark is written by Doug Miller, published on the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 May 2004. Text and images copyright © 2004 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.
