Why We See Baily's Beads in a Total Eclipse
The phenomena of Baily's Beads are part of the breathtaking beauty and mystery of a total solar eclipse. Here's why they happen.
What's Really Happening?
During a solar eclipse the Moon slowly covers the face of the Sun. The process takes about an hour or two. Just before the Sun disappears completely a few beads of brilliant light are seen at the edge of the Moon. These last for only a few seconds. The last bead before the Sun disappears gleams like a diamond and is called the diamond ring effect.
These effects happen because the very last bit of the Sun’s photosphere, or surface, is shining through the valleys and craters of the Moon. These phenomena are followed by the pink light of the Sun’s chromosphere, a layer of gas spikes just above the photosphere. An observer might even see solar prominences, flares of plasma that erupt from the Sun’s surface. After this, the corona, or crown, can finally be seen as the Moon blocks the Sun's disk. When the eclipse begins to end, the process is reversed.
The Best Way to View
An eclipse should only be looked at through special filters, for there are parts of it that are too bright to look at without damaging the eyes, especially when the Sun is only partially covered by the Moon. Only when the diamond ring effect is over, is the eclipse safe to look at again.
When the diamond ring and Baily's beads return, which signals the end of the eclipse, the filters must be put on again. A good, cheap filter is cardboard glasses with aluminized Mylar lenses. Aluminized mylar, which is mylar coated with a thin film of aluminum, filters out all of the Sun's harmful ultraviolet and infrared rays and nearly all of the visual light that can permanently damage the retina. The buyer should check to see that the Mylar doesn’t have any holes or tears in it first. Number 14 welder’s glass is also good to look through. A pinhole camera, where the image of the eclipse comes through a telescope, passes through a piece of cardboard with a pinhole in it and onto a piece of paper, is also very safe to use. All of these filters can also be used to view sunspots.
Not Rare But Spectacular
Solar eclipses aren’t rare, but they're spectacular. Partial eclipses happen about three times a year while a total eclipse happens about every eighteen months. What is rare is a total solar eclipse happening in the same spot on Earth within a short period of time. The shadow, or umbra of the Moon is so tiny that it only passes over a very small portion of Earth. At best, the track is hardly ever wider than 150 miles and the time that the Sun is completely covered, or totality, never lasts longer than 7 minutes and 31 seconds. Eclipses are possible because of the fascinating fact that though the Sun is four hundred times the size of the Moon, it’s four hundred times farther way from the Earth, so the Moon can cover it perfectly. Baily's beads aren't seen during partial eclipses because the Sun doesn't have a chance to just peek through the valleys of the Moon.
The beads were first described by British astronomer Francis Baily. He was born in Berkshire in 1774 and helped found the Royal Astronomical Society. Baily was such an important figure at the Society that he was made President four times. Interestingly, astronomy began as a hobby for him. He originally worked at the London Stock Exchange and made so much money that he was able to retire early and devote his life to astronomy.
On May 15, 1836 Baily saw the beads of light that would bear his name during an annular eclipse of the Sun in Roxburghshire. An annular eclipse is when the shadow, or umbra, of the Moon doesn’t reach all the way to the Earth during an eclipse, and there’s a ring, or annulus, of light round the Moon. This happens when the Moon is a bit too far in its orbit to cover the Sun completely. Baily’s beads can be seen with this sort of an eclipse, sometimes as a complete, beautiful necklace, but not the diamond ring effect.
Baily was so awed by the event that his writing about it inspired other people to travel to see solar eclipses. Baily himself traveled to Pavia, an ancient college town in Lombardy, Italy, and on July 8, 1842 saw the beads again during a total eclipse of the Sun. He died two years later, on August 30, 1844.
Herschel, John. Memoir of the late Francis Baily, http://adsabs.harvard.edu//full/seri/MNRAS/0006//0000089.000.html
Kaufmann III, William J. Universe: Fourth Edition, 1994
Schaaf, Fred. The 50 Best Sights in Astronomy and How to See Them, 2007
Menzel, Donald. Pasachoff, Jay. A Field Guide to the Stars and Planets,1983
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