Nov. 8, 2006 — The tiny spot moving across the face of the sun today isn't some newly discovered celestial canker sore or a weird roving sunspot. It's sister planet Mercury in a rare, but totally predictable transit in front of the sun.
Of all the planets in the solar system, only Mercury and Venus, which orbit closer to the sun than Earth, can be seen passing across the face of the sun. The next transit won't occur until 2016.
Beginning at 2:12 p.m. EST and lasting for nearly five hours, Mercury will appear as a small dot moving left to right across the bottom-third portion of the sun.
Don't try to find it with your naked eyes, however. Looking directly at the sun can cause severe damage. Besides, Mercury is too small to be seen without magnification.
Dozens of observatories, planetariums and websites plan live coverage of the event (http://www.exploratorium.edu/transit/) and many astronomers will be using special instruments on telescopes for private and public viewings. Scientists, meanwhile, plan to take advantage of the transit for studies of Mercury and the sun.
"There is still so much that we don't know about Mercury," said Deborah Domingue, a Johns Hopkins University researcher and the deputy project scientist of a NASA probe named MESSENGER that is scheduled to enter into orbit around Mercury in 2011.
The last visit to the inner-most planet in the solar system took place in the mid-1970s when Mariner 10 made a quick flyby. Only half the planet was imaged during that pass, leaving many mysteries.
For example, scientists are curious if ice exists in deep craters at the planet's poles and if odd wrinkle-like features on Mercury's surface are signs that the planet is collapsing in on itself as its iron core cools and contracts.
Scientists also plan to use Mercury's transit to hone techniques for finding and studying planets beyond our solar system, which occasionally can be spotted as they pass in front of their mother stars. Another team at the Sacramento Peak Observatory in Sunspot, N.M., plans to use the transit to measure the true sizes of small features in the solar atmosphere.
Meanwhile, astronomers will use a light-splitting spectrograph to determine the sodium content in Mercury's atmosphere as it passes in front of the sun. Several solar-orbiting probes will be making simultaneous observations as well, some of which will be broadcast on the Internet. (http://sunearthday.nasa.gov/2007/events/mercurytransit.php)
The whole transit will be visible in Hawaii and in the extreme western United States. From New Mexico eastward to the Atlantic, the sun will set with Mercury's disk still silhouetted against it.
