Solar Eclipse; Sunday, May 20

Sunday will come to a close with a spectacular solar eclipse across much of the United States with the Southwest enjoying the best view and weather.  This view (above) was from Japan's Hinode spacecraft showing the solar eclipse of January 4, 2011.

The moon will reduce the sun to what resembles a thin ring late Sunday along a path from Medford, Oregon to Redding, California, to Reno, Nevada, to Cedar City, Utah, to Albuquerque, New Mexico, to Lubbock, Texas.

While officially deemed a partial eclipse elsewhere across the Western United States, the astronomical show will still be amazing for Los Angeles, Phoenix, Salt Lake City, and Boise, Idaho, with more than 80% of the sun being blocked.

The eclipse will first travel across a huge swath of the uninhabited Pacific Ocean before becoming visible on the West Coast of the United States.

A solar eclipse occurs when the Moon passes between the Sun and the Earth. The Moon fully or partially blocks the Sun. This can only happen during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. In a total eclipse, the disk of the Sun is fully obscured by the Moon.

An Annular Eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bring ring, or annulus, surrounding the outline of the Moon. This differs from a Total Eclipse of the Sun when the silhouette of the Moon completely obscures the light of the Sun, allowing the much fainter solor corona to be visible.

The Sun's distance from the Earth is about 400 times that of the Moon, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and Moon as seen from Earth appear to be approximately the same size: About 0.5 degree of arc in angular measure.

Solar Eclipse Viewing Dangers

Looking directly at the photosphere of the Sun (the bright disk of the Sun), even for a few seconds, can cause permanent eye damage, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in permanent impairment of vision, up to and including blindness. Under normal conditions, the Sun is so bright that it is difficult to stare at it directly. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare.

Looking at the Sun during an eclipse is as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun's disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse like the upcoming May 20, 2012 event).

Viewing the Sun's disk through any kind of optical aid (binoculars, a telescope, or even an optical camera view finder) is extremely hazardous and can cause irreversible eye damage in a fraction of a second. Special eye protection or indirect viewing methods should be employed (above). Sunglasses are not sufficient protection.

1919 Observations and Einstein's Theory of Relativity 

The observation of the total solar eclipse of May 29, 1919 helped confirm Einstein's theory of general relativity. By comparing the apparent distance between stars, with and without the Sun between them, Arthur Eddington stated that the theoretical predictions about gravitational lenses were confirmed. The observation with the Sun between the stars was only possible during totality, since the stars are then visible. Though Eddington's observations were near experimental limits of accuracy at the time, work in the later half of the 20th century confirmed his results.