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