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Mercury Crossing Paths with Sun
The planet Mercury will pass in front of the Sun on May 7 in an unusual event called a 'transit'. Called the 'Mercury Transit', it will be visible from North America as the Sun rises (a few minutes after 6 am EDT for Washington, DC).
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Projected view of Mercury Transit by the Solar & Heliospheric Observatory (SOHO) spacecraft.
Credit: NASA / ESA
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The transit can't be seen with the naked eye because Mercury is so small - only about 1/160 of the Sun's diameter. But anyone with an Internet connection can watch it live from the SOHO spacecraft.
"People will see a small, perfectly round, black dot very slowly moving across the solar disk," said eclipse expert Fred Espenak of NASA's Goddard Space Flight Center, Greenbelt, Md.
These events are rare, occurring approximately a dozen times a century. Although the entire transit lasts over five hours, viewers in North America will only see the last 20 to 30 minutes of it. To watch the event, log in to:
http://soho.nascom.nasa.gov/hotshots/2003_05_07/
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The November 15, 1999 Mercury Transit as seen by the Transition Region and Coronal Explorer (TRACE) spacecraft.
Credit: NASA / LMSAL
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The SOHO spacecraft orbits the Sun at one million miles from Earth to constantly track solar activity. One of its instruments, the Large Angle and Spectrometric Coronagraph (LASCO), will be able to see Mercury a few days before it crosses the Sun. Still other instruments will be able to track Mercury's progress across the solar disk during the transit.
These pictures will be made available almost immediately at the website listed above. The mission is an international cooperative project between NASA and the European Space Agency.
The planets Mercury and Venus are the only ones that appear to cross the face of the Sun as seen from Earth, since both are closer to the Sun than Earth. Venus transits are extremely scarce, 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 were important historically. "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. By analyzing Venus' transit, astronomers could determine the actual distances to Venus and the Sun.
Captain James Cook, the legendary British navigator and explorer, recorded the transit of Venus from Tahiti in 1769. That observation was a major motivation for his expedition to the South Pacific and the circumnavigation of the globe.
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