Volume: 04, Issue: 13 02/22/2006 
Use this finder chart to locate Mercury in the evening sky this month. Be sure to find an observing location with an uobstructed western horizon. Image credit: Astronomy: Roen Kelly.
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Mercury met with the crescent Moon at 6:45 EST September 2, 2005. The waning Moon was only 1.57-percent illuminated when this shot was made from Dayton, Ohio. Image credit: John Chumack.
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The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. The probe was about 130,000 miles (210,000 kilometers) from Mercury. Image credit: NASA.
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Other Articles in This Issue:
NASA's Spitzer Finds Violent Galaxies Smothered in 'Crushed Glass'
Greenland Ice Loss Doubles in Past Decade, Raising Sea Level Faster
Gem of a Comet Particle
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Mercury Bright in the Evening Sky

The best time to see Mercury this year is right now. Mercury will brighten as an evening “star” in the western sky until it reaches its greatest eastern elongation, when the planet is as far east of the Sun as it can get. The evening of February 23, 2006, should provide the best view of Mercury as it shines at -0.5 magnitude, which makes it the second brightest object in the evening sky after the star Sirius.

Not only does Mercury appear bright in the sky at greatest elongation, but the closest plane to the Sun also rises to its highest point in the sky. At greatest elongation, Mercury reaches a maximum of 18 degrees above the horizon on February 23 and 24. (For comparison, your closed fist held at arm’s length spans 10 degrees.) After February 24, the planet dims nightly. On March 1, it glows at magnitude 0.3.

Mercury orbits the Sun at an average distance of only 36 million miles (58 million kilometers). Earth is nearly three times as far, so, from our perspective, Mercury always stays near the Sun. When Mercury is west of the Sun, we view it as a morning star in the east before sunrise. Some elongations are better than others because of Earth’s tilt and the stretched-out nature of Mercury’s orbit.

Through a telescope, observers can watch Mercury go through phases similar to the Moon’s. On February 8, the planet’s disk was more than 90 percent illuminated. At greatest elongation, Mercury’s disk will appear half-lit, and it will shrink to a 23-percent-illuminated crescent by March 1.

Most observers detect no surface markings on Mercury. It takes a seasoned observer and excellent atmospheric conditions to see anything at all on the planet even through the largest amateur telescopes. Experienced amateurs, however, have recorded dusky markings and occasional bright areas on the planet.

Here are some quick facts about Mercury:
• Mercury orbits the Sun every 87 days, 23 hours, and 18 minutes. For every one Earth “year,” Mercury experiences four.
• Mercury’s diameter is 3,032 miles, only 38 percent of Earth’s.
• The largest crater on Mercury, named Beethoven, is the largest known in the solar system. It is nearly 400 miles wide.
• From Mercury, the Sun is 6.3 times brighter than from Earth.
• Mercury has a total of 297 named features, of which 239 are craters.
• Next Mercury will pass in front of (or transit) the Sun on November 8, 2006. This event will be visible from the United States.

For more information about Mercury, please visit the following websites:

Space Explorers Mission: Solar System subscribers can also read more about Mercury and the other planets by visiting our Space Library at www.space-explorers.com.

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