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Solar 'scopes probe Mercury for sodium

Little planet crosses big star

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Yesterday, Mercury passed between Earth and the Sun, giving astronomers a rare opportunity to study the planet's very tenuous atmosphere.

By studying the sunlight as it is filtered through the atmosphere, and comparing the data to the Sun's known spectral signature, astronomers can determine the atmospheric composition.

The atmosphere was only discovered in 1985 by Andrew Potter and Thomas Morgan. It is thought to be mostly composed of sodium.

Potter, an astronomer at the National Solar Observatory in Arizona, studied the planet during the transit using the Kitt Peak's McMath-Pierce Solar Telescope. He is hoping to determine exactly how much sodium the atmosphere contains.

Kevin Reardon of Osservatorio di Arcetri in Arcetri, Italy, used the same facility to observe the Sun's features, using Mercury as a knife edge to enhance spatial resolution.

If Earth and Mercury orbited the Sun on the same plane, Mercury would pass between us and our star several times a year. However, the two planets' orbital planes are slightly inclined, so Mercury only transits the Sun 13 times a century. The phenomenon won't be seen again until 2016.

The transit was visible from much of the Americas, Australia, and eastern Asia. Those on the west coast of the US got the best view, as the transit was completed before the Sun went down.

Mercury began its passage across the face of our star shortly after 7pm, GMT. That's a much more sunlit 11am, if you happened to be in California. Observers equipped with proper H-Alpha filters, or pinhole cameras, were able to watch as the tiny black circle of Mercury spent the next few hours tracing a path across the Sun.

The dark circle, taking up just 1/200th of the sun's surface, finally disappeared from the view of West Coast observers just after 4pm, or midnight, GMT. ®

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