Courtesy of Sky & Telescope

Contact points: When the sun rises over North America tomorrow, the transit of Venus will already be more than half over. But skywatchers across the continent's eastern half will still be able to watch the planet's exit. Contact III denotes when the silhouette of Venus just touches the sun's outer edge; about 20 minutes later, at Contact IV, Venus will be completely off the sun. During its crossing Venus will look like a sharp, black circle. By contrast, sunspots look dark but usually have irregular shapes and less-distinct edges. Click photo for larger image. RELATED ARTICLE Information, precautions and Internet links for viewing the transit

When Richard Griffiths booked a flight for tomorrow from Pittsburgh to Denver, he made sure he chose one scheduled to depart at 6 a.m.

That way, explained the Carnegie Mellon University astrophysicist, he should be up above the clouds in time to get a clear view of a historic event -- the planet Venus crossing the face of the sun.

No one alive has ever seen such a thing. It's been 122 years since the last "Transit of Venus" and, though eclipse-like event is of far less significance today than it was to the astronomers of 1882, plenty of skywatchers are anxious to get their first glimpse of the planet as it crosses directly in front of the sun.

Venus already will appear as a black dot inside the solar disk by the time the sun rises over Pittsburgh at 5:50 a.m. and will have already completed most of its six-hour transit. By 7:05 a.m., the planet will reach the edge of the sun and by 7:25 it will have completed its exit.

Provided that the skies are clear and that proper safety precautions are taken, observers on the ground will be able to see the show. But given the propensity for cloudy skies over Pittsburgh, Griffiths figured an early morning flight might improve his chances of seeing it.

Clouds put a damper on the elaborate observation plans of Samuel P. Langley, then director of the Allegheny Observatory, during the last transit on Dec. 6, 1882. The transit was then a major astronomical event, one that had steadily grown in significance since the first one was detected in 1639.

Astronomers saw the Venus transit as an opportunity to pin down the distance between Earth and the sun ---- a standard, called the astronomical unit, that is used to measure distances in the solar system. Edmond Halley, of comet fame, had suggested that the distance could be calculated accurately by recording the exact moments that Venus entered and exited the solar disk as observed at various points of the globe.

Venus transits are infrequent, however. Venus and the Earth regularly line up with each other as they orbit the sun, but the plane of Venus' orbit is angled a bit relative to Earth's. So Venus usually appears to be above or below the sun's disk as viewed from Earth. The transits occur in pairs, about eight years apart; the next one will be June 6, 2012.

	Institute for Astronomy, University of Vienna
	The "black-drop" effect was recorded as the silhouette of Venus prepared to exit the disk of the sun during the 1874 transit. The transits are rare, but occur in pairs about eight years apart.

Major international expeditions were mounted in 1761, 1769 (including Capt. James Cook's observations in Tahiti) and 1874. It also became a cultural event; John Philip Sousa even composed the "Transit of Venus March."

In 1882, the Allegheny was one of the leading observatories and Langley one of the country's most prominent scientists (later, as director of the Smithsonian Institution, he infamously lost the race with the Wright Brothers to achieve powered, manned flight).

Not only did the Pittsburgh astronomers plan to help determine the astronomical unit, but the observatory was one of a handful equipped with spectroscopic instruments to study the planet itself.

The cloud cover over Pittsburgh and many cities that day proved troublesome, though Langley did cause some brief excitement by reporting a large bright spot on the planet ---- a spot that widened to 2,000 miles or more as Venus entered the solar disk.

But in an interview with the New York Times, part of a ProQuest digital archive, Langley said the phenomenon was "wholly inexplicable" and admitted he wasn't even sure the spot was totally on the planet's surface. "It might be due in some way to refraction, caused by an extraordinary protuberance of the atmosphere of Venus, though why such a protuberance should exist we are at a loss to say."

His assistant James Keeler, who would later succeed him as director of the observatory, told the Times that one of the most interesting findings came from the Princeton Observatory, where spectroscopic observations showed "unmistakable" evidence of water.

"The presence of water upon the planet Venus suggests the probability of inhabitants," Keeler explained. "No astronomer would care to say that Venus is not inhabited."

Subsequent research, of course, would show that despite similarities with Earth in size and composition, Venus is hostile to life. Encircled by sulfurous clouds, it is a dry, oceanless planet where surface temperatures are hot enough to melt lead and atmospheric pressure is 90 times that of Earth.

	U.S. Naval Observatory Library
	Astronomers gather at the U.S. Naval Observatory in Washington, D.C., in preparation for the 1874 transit of Venus. At the time, it was believed the event could help determine the distance between Earth and the sun, a standard that in turn is used to measure distances in the solar system.

Even the effort to calculate the value of the astronomical unit was less successful than hoped.

Though close to the actual value of 93 million miles, it was impossible to precisely time when the planet's edge touches the edge of the sun because of the "black-drop effect." This distortion stretches Venus' silhouette into the shape of a pear or water droplet, with a dark neck linking it to the sun's edge.

This was long thought to be caused by Venus' thick clouds, but recent studies show that it is due, in part, to distortions caused by the Earth's atmosphere and by a phenomenon called "limb darkening" which causes the brightness of the sun to dim near its edge.

At any rate, astronomers soon found better ways to determine the astronomical unit and, today, radar allows astronomers to precisely measure distances within the solar system.

Today, scientific excitement about the transit is limited. Scientists plan to use a NASA satellite, the Transition Region and Coronal Explorer, to further explore the black-drop effect. Timothy Brown, a scientist with the National Center for Atmospheric Research, will use a special solar telescope in the Canary Islands to study the composition and winds of Venus's upper atmosphere.

"Nowadays, it's primarily of interest to amateur astronomers," said David Turnshek, a cosmologist at the University of Pittsburgh. But, recalling his fascination as a high school student observing the transit of Mercury, he acknowledged that he likely will set up his telescope at home tomorrow to see the transit.

Kavan Ratnatunga, a CMU astronomer, isn't taking any chances. To see what no living person has ever seen, he has traveled to Luxor in Egypt, where clouds are scant and the entire transit can be seen start to finish.