|Volume: 01, Issue: 03|
Mercury to Pass In Front of Sun on May 7The planet Mercury will pass in front of the Sun on Wednesday, May 7 in a rare event called a transit. NASA's Solar and Heliospheric Observatory (SOHO) spacecraft will offer excellent, safe views of the transit online at http://soho.nascom.nasa.gov/hotshots/2003_05_07/.
"People will see a small, perfectly round, black dot slowly moving across the solar disk," said eclipse expert Fred Espenak of NASA's Goddard Space Flight Center in Greenbelt, Md.
Mercury transits are extremely infrequent, occurring only approximately a dozen times per century. The planet is about 1/160 the size of the Sun, so the transit cannot be seen just by looking into the sky. Since special precautions must be taken to avoid permanent eye damage, direct telescope viewing is not recommended.
Observers in Asia, Africa and Europe will have the best view of this transit, which will already be well in progress as the Sun rises over America (approximately 6 a.m. EDT). Although the entire transit lasts more than five hours, viewers in North America will only see the last 20 to 30 minutes. By the time the Sun rises across the United States, the transit will be finished.
SOHO will capture the transit from its orbit one million miles from Earth, in line with the Sun, where it makes continuous observations of solar activity. Its instruments will be able to see Mercury a few days before the transit and track Mercury's progress across the solar disk during the transit. The pictures will be available almost immediately on the SOHO Web site. The SOHO mission is a project of international cooperation between NASA and the European Space Agency.
As seen from Earth, Mercury and Venus are the only planets that appear to cross the face of the Sun, since both are closer to the Sun than Earth. Venus transits are also extremely rare, with just one pair eight years apart every 105 to 121 years.
"The last Venus transit was in 1882, so no one alive has seen one," said Espenak. "Happily, there will be a Venus transit June 8, 2004, so this year's Mercury transit can be taken as an appetizer for the main course."
The transits also have historical significance. "Venus transits were the Apollo project of the 18th and 19th centuries," said Espenak. "There were major international efforts, with scientific expeditions to remote corners of the world, in order to measure the apparent position of Venuson the solar disk. Using trigonometry and a careful analysis of observations, astronomers could determine the actual distances to Venus and the Sun.”
British explorer Captain James Cook recorded the transit of Venus in 1769. Espenak said Cook's observations were a major motivation for his expeditions around the world.
More information about transits of Mercury and Venus, including photographs, is available at: http://sunearth.gsfc.nasa.gov/eclipse/transit/transit.html .
For recommended safe telescope viewing instructions, visit: http://skyandtelescope.com/observing/objects/planets/article_921_1.asp.
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