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Nancy Neal
Headquarters, Washington         May 2, 2003
(Phone: 202/358-2369)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)

RELEASE: 03-152

NASA OBSERVATORY TRACKING RARE SOLAR EVENT

     The planet Mercury will pass in front of the sun on 
Wednesday, May 7 in an unusual event called a transit. 
NASA's Solar and Heliospheric Observatory (SOHO) spacecraft 
offers excellent, safe views of the rare occurrence to 
anyone with an Internet connection.

"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, Greenbelt, 
Md.

Mercury transits are rare, occurring only approximately a 
dozen times per century. Observers in Asia, Africa and 
Europe will have the best view of this transit, which will 
already be in progress as the sun rises over America 
(approximately 6 a.m. EDT).

The transit can't be seen with the unaided eye, since 
Mercury's apparent size is only about 1/160 of the sun's 
diameter. Direct telescope viewing is not recommended, since 
special precautions must be taken to avoid permanent eye 
damage.

Although the entire transit lasts over five hours, viewers 
in North America will only see the last 20 to 30 minutes of 
it. The event will be finished by the time the sun rises 
west of a line from the Great Lakes to the Carolinas, but 
the complete transit is safely viewable on the SOHO Web site 
at:

http://soho.nascom.nasa.gov/hotshots/2003_05_07/

SOHO orbits a special point in space one million miles 
(about 1.6 million kilometers) from Earth, in line with the 
sun, to make continuous observations of solar activity. One 
of its instruments, the Large Angle and Spectrometric 
Coronagraph, will be able to see Mercury a few days before 
it transits the sun. Other instruments will 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.

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 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 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, in 
order to measure the apparent position of Venus on the solar 
disk. Using trigonometry and a careful analysis of 
observations, 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. The observation was a 
major motivation for his expedition to the South Pacific and 
the circumnavigation of the globe," Espenak explained.

For more information about transits of Mercury and Venus, 
including photographs, refer to:

http://sunearth.gsfc.nasa.gov/eclipse/transit/transit.html

For information on the Internet about Captain Cook's 
expedition, visit:

http://www.geocities.com/TheTropics/7557/cook.html

For recommended safe telescope viewing instructions, refer 
to:

http://skyandtelescope.com/observing/objects/planets/article
_921_1.asp

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