Venus Transits the Sun (June 2004)
The last time the planet Venus crossed the face of the Sun, Chester Arthur was president, the American flag boasted just 38 stars, and both the telephone and the electric light bulb were brand new. And astronomers were looking for a precise measurement of the distance from Earth to the Sun. They hoped the Venus transit would give them just that.
 A photographic plate from the U.S. Naval Observatory records the transit of 1882.
Now, Venus is poised to cross the face of the Sun again. In the early morning hours of June 8, it will cross the face of the Sun, looking like a small black dot against the bright solar disk. Skywatchers in the eastern United States will get a look at the transit's end -- weather permitting, of course.
Transits of Venus are possible because the planet's orbit around the Sun is inside that of Earth. But Venus's orbit is tilted with respect to the Sun's path across the sky. So most of the time, Venus passes above or below the Sun as seen from Earth. But occasionally, the geometry is just right, and Venus passes directly in front of the Sun.
Venus transits occur in pairs, with the second coming eight years after the first. But the pairs are separated by either 105.5 or 121.5 years. So the next transit comes in 2012, with the one after that in 2117.
Viewing the Transit
 The transit begins when Venus first appears to "touch" the Sun's disk at 12:13 a.m. CDT, which is well before the Sun rises in the United States. As a result, the transit will be in progress as the Sun rises east of a line from about Houston to Helena, Montana. The transit will be over by the time the Sun rises to the west of that line. Check our list of some sunrise and end-of-transit times to find out if the transit will be visible from your location.
Viewed with the unaided eye, Venus will look like a tiny black dot near the edge of the bright solar disk. It's important to note, however, that you should never look at the Sun without proper eye protection, because unfiltered sunlight is bright enough to damage your eyes. Instead, look at it through a piece of welder's glass, No. 14 or darker.
Some web sites will present live images of the transit. Some links are provided below.
Transit History
The first recorded transit in history was observed in 1639 by Britons Jeremiah Horrocks and William Crabtree. Horrocks found that astronomers of the day varied widely in their opinions of if or when a transit would take place, so he calculated the time on his own, and was accurate to within a few minutes.
Later, Sir Edmund Halley (for whom Halley's Comet is named) predicted that astronomers could use the next Venus transits, in 1761 and 1769, to determine the distance from Earth to the Sun with a high degree of accuracy. As a result, astronomers mounted several dozen expeditions to view these events. Some of the more interesting include the expeditions of Captain
Cook, Guillaume Le Gentil, Jean Chappe, and Jeremiah Dixon and Charles Mason, who later surved the famous Mason-Dixon line in the United States.
Despite the best efforts of these teams, the uncertainty in the Earth-Sun distance was still several million miles. So new expeditions were launched in 1874 and 1882, including many by the United States Naval Observatory. Each expedition was equipped with the same set of instruments, including cameras for photographing the transit on glass plates. Some of these artifacts and plates are on display at the National Museum of American History. Despite some political infighting within the Naval Observatory, it eventually produced an Earth-Sun distance that was less than 100,000 miles off the true value.
-- Damond Benningfield
Resources
Live Views
European Southern Observatory
Exploratorium, San Francisco
Worth Hill Observatory
Other Viewing Techniques
Viewing the transit on a paper plate
Using a mirror to project an image of the transit
Viewing Details
NASA Goddard Space Flight Center Eclipse Home Page
General Transit Sites
Transit of Venus
Many links to other pages on viewing, history, and other transit aspects
The Venus Transit 2004
Information from the European Southern Observatory
Venus Transit 2004: Sun-Earth Day
Transit History
Chasing Venus: Observing the Transits of Venus 1631 - 2004
Exhibit at National Museum of American History
Jeremiah Horrocks
Le Gentil and the Transits of Venus, 1761 and 1769
Le Gentil and other expeditions of 1769
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