The planet Mercury will pass between the Earth and the sun on November 15, making a nearly hour-long trek across the solar disk -- an uncommon occurrence that has delighted astronomers only 13 other times this century.
For viewers in most of North America, Mercury will appear as a silhouette grazing the suns northern edge, or limb. Although transits of Mercury are rare, such a graze is even rarer.
"Im told this is the first time since the invention of the telescope," said John Westfall of the Association of Lunar and Planetary Observers.
Seen from Earth -- the third planet from the sun -- transits of both Mercury and Venus are possible. However, the latter is much less frequent, having occurred only six times since Galileo peered skyward with his first telescopes in the early 1600s.
During the transit, Mercury will be quite small, a black spot less than 1/195 the diameter of the sun, making it impossible to view without the aid of telescopes -- equipped, naturally, with adequate solar filters. Viewing a transit without adequate precautions is every bit as dangerous as staring at the sun at any other time.
The event will be best seen from areas around the Pacific Rim, including nearly all of the Americas, Australia and New Zealand. Viewers on the Pacific coast of Russia and South Korea, as well as portions of Japan, the Philippines and Indonesia, will be able to catch the end of the transit just at sunrise.
In North America, the event will begin at approximately 4:11 p.m. ET and last until 5:10 p.m. ET. At the time of the Nov. 15 transit, the Earth will be 63 million miles (101 million kilometers) from Mercury and 92 million miles (147 million kilometers) from the sun.
From an historical standpoint, astronomers have long been fascinated with transits because of the information that can be derived from them, including measurements of the suns distance from Earth as well as its polar diameter.
"Transits provided people -- who didnt have spacecraft -- with accurate tools to measure the distance" to the sun, said Patrick So, an astronomer at the Griffith Observatory in Los Angeles.
Similar events -- whether occurring when the moon transits the sun, otherwise known as an eclipse, or when the Galilean moons cross the disk of Jupiter -- also hold a more basic fascination.
"People are very intrigued by them because, for example, in the case of the moons and the shadows they cast on the atmosphere of Jupiter, it implies a connectivity that people instinctively are interested in," said Gil Clark, an astronomer at the Jet Propulsion Laboratory in Pasadena, California.
"Im not sure what that is, but it is another way of viewing the object," Clark said.
Mercury is the innermost of the four terrestrial planets, which include Venus, Earth and Mars. Mercury zips around the sun once every 88 days, lapping the Earth every 116 days. Since its orbital inclination is tilted at 7 degrees relative to the Earths, only once every 23 times does its passing between the sun and the Earth make for a transit. The most recent was in 1993; the next will be May 7, 2003.
The last Venus transit was in 1882; the next will be June 8, 2004.
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