Newswise — The planet Venus will cross the sun next week in a rare event many people all over the world will be able to witness.
Unfortunately, people in Arizona won't be able to see it at all. Which is why University of Arizona associate astronomer Glenn Schneider has flown to Greece.
Schneider, who is known to chase anywhere in the world to view a solar eclipse, is partly responsible for solving the "black drop effect" problem that comes with the Venus transit.
For about six hours on Tuesday, June 8, Venus will appear to eclipse the sun, a phenomena that hasn't been seen from Earth since 1882. From Earth, the event is seen only at intervals of 8, 121.5, 8, and 105.5 years. We don't often see Venus when it passes in front of the sun because its orbit is inclined by 3.4 degrees to the plane of Earth's orbit. Therefore, the sun, Venus and Earth rarely line up. The next Venus transit visible from Earth will be June 6, 2012. There won't be another until 2117.
The June 8, 2004 transit will begin at 5:20 a.m. GMT (10:20 p.m. June 7 MST). The entire continent of Europe, along with most of Asia and much of Africa, will be in the prime viewing area to see the entire transit from start to finish. Western Africa and the easternmost parts of North and South America will be able to see the transit at sunrise. Australia will see it at sunset. Western North American, including Arizona, won't see it at all.
As the transit begins, the leading edge of Venus first touches the bottom left side of the sun's disk. Astronomers call this Contact I. About 20 minutes later, Venus' opposite edge touches the same point on the edge, or limb, of the sun as Contact I. This creates a "neck" where the appearance of the planet appears a little distorted " the so-called the "black drop effect."
As the transit ends, Venus' leading edge touches the bottom right side of the sun's limb, again creating a dark neck along the edge, during Contact III. Venus' opposite edge touches the sun's limb during Contact IV, as Venus completes its journey across the sun.
When the "black drop effect" occurs, Schneider said, Venus no longer resembles a disk. "It kind of looks like a tear drop," he said.
Astronomers once attributed the black drop effect to Venus' thick atmosphere. But then they discovered a black drop effect during Mercury's transits, which can be seen from Earth 13 or 14 times a century. Because Mercury has almost no atmosphere, astronomers realized the idea that the transiting planet's atmosphere creates the black drop effect is wrong.
Schneider, along with Jay Pasachoff of Williams College and Leon Golub of the Harvard-Smithsonian Center for Astrophysics, saw Mercury's faint black drop effect in 1999 data from NASA's Transition Region And Coronal Explorer (TRACE) spacecraft observations of a Mercury transit.
Schneider and his colleagues determined that there were two factors causing the black drop effect. The first was due to the point spread function, which occurs when the optics in smaller telescopes used on Earth cause the object being viewed to appear blurry. The second factor they attributed to limb darkening, when the brightness of the sun decreases towards the edge of it disk.
Astronomers in the 18th and 19th centuries tried to use the 1882 Venus transit to determine the size of the solar system to scale. They figured if they knew exactly how long it took Venus to cross the sun, they could figure out how to draw the solar system to scale by using Kepler's third law.
However, the black drop effect hindered their attempts at making exact measurements, even though they were only about one minute off the time it took Venus to cross the face of the sun.
Astronomers have since discovered the distances between the sun and all the planets of the solar system. However, Schneider said, observing the transit of Venus still plays an important role in astronomy today. Those who search for extrasolar planets can use the same observational methods to find terrestrial planets in other star systems where hot, gaseous, Jupiter-like planets have been found, he said.
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