On Wednesday, sky watchers throughout the United States will have a chance to see the planet Mercury in broad daylight. Not because it will shine brightly, but because, on that afternoon, the planet will pass in front of the sun and will appear in silhouette against the sun's brilliant surface.
Mercury orbits the sun every 88 days, so it appears to transit the face of the sun about 13 times every century. The last time this occurred was in 2003; the next time won't be until 2016. So if the sky is clear Wednesday, be sure to check it out.
Viewing the transit directly is not difficult, but it requires eye protection to prevent serious eye damage or permanent blindness – just as with a solar eclipse. Homemade eye protection is not safe for viewing the sun. Devices such as sunglasses, double thicknesses of darkened film, smoked glass, uncoated mylar, tinted glass and others may appear to cut the sun's glare, but may allow the blinding (and invisible) rays known as ultraviolet and infrared to enter the eye.
Only certified solar filters cut visible and invisible radiation sufficiently to be safe. These are available through several sources around the country. Go to www.mreclipse.com/ Totality/ TotalityApC.html #Solar_Filters
for more information.
The first indication that the transit has begun will be the appearance of a tiny notch on the southeastern edge of the sun. It won't be long, however, before you can see the entire planet's disk against the sun's face. It will appear quite tiny, and you may need a filtered telescope or binoculars to see it at all.
During the afternoon, the planet's silhouette will appear to drift slowly westward across the face of the sun. Most of the U.S. will see the sun set before the transit is complete; only on the West Coast will sky watchers be able to see the entire event, including the planet's exit from the sun's western limb.
To learn more about planetary transits in general and the Mercury transit in particular, go to http://sunearth.gsfc.nasa.gov/ eclipse/ transit/ transit.html.
You can also watch the transit live as seen from Kitt Peak observatory in Arizona. Researchers from San Francisco's Exploratorium will Web cast the event beginning at 11 a.m. at www.exploratorium.edu/transit.
© Copley News Service
