Little Bits of History

Baily’s Beads

Posted in History by patriciahysell on May 15, 2014
Baily's Beads

Baily’s Beads

May 15, 1836: Francis Baily successfully describes Baily’s Beads. During an annular eclipse (when the Sun and Moon are exactly in line but the Moon appears smaller, it leaves a bright ring or annulus surrounding the Moon), beads of light appeared because the visible surface is uneven allowing for light to shine through in some places but not in others. When only one bead is left, a shining diamond ring pattern appears. The topography of the Moon is uneven due to mountains, craters, and valleys as well as other features. Today, astronomers are acutely aware of the unevenness of the “edge” of the Moon and know when beads will appear. They are seen only briefly for a few seconds at the center of the eclipse path.

Baily’s description of the phenomenon was so compelling that he also began the modern eclipse expedition craze. Because others wished to both see the beads and assess their cause, the total eclipse of July 8, 1842 garnered a great deal more than normal attention. Baily himself went to see the eclipse at Pavia, which is about 22 miles south of Milan, Italy. The Englishman was born in 1774 and managed a trip to the unsettled parts of North America in 1796-97. Upon his return, he joined the London Stock Exchange and wrote several books about finance. He retired from business in 1825 to devote all his time to astronomy.

Even before his retirement, he had been one of the leaders in founding the Royal Astronomical Society. He wrote several books about astronomy and encouraged others to make more accurate observations of the night sky. He helped create a catalog of stars which contained 57,000 entries. He also revised catalogs of previous astronomers including Tycho Brahe and Edmund Halley. He worked diligently to get a correct measurement via Foster’s pendulum experiments and corrected the standards of length in 1843. He also determined the mean density of the Earth with Henry Cavendish’s method and arrived at 5.66. Today, we believe the figure to be 5.52 grams per cubic centimeter.

There are other types of eclipses which do not show the beads as described by Baily. A total eclipse has the Moon completely obscuring the light from the Sun and allows a much fainter corona to be visible. This is seen only along a narrow track on the Earth. A hybrid eclipse combines a total and annular, depending on where one is stationed on Earth. They are comparatively rare. A partial eclipse occurs when the Sun and Moon are not perfectly aligned and can be seen over a much larger portion of the planet.

Astronomy compels the soul to look upwards and leads us from this world to another. – Plato

Superstition is to religion what astrology is to astronomy the mad daughter of a wise mother. These daughters have too long dominated the earth. – Voltaire

So far as hypotheses are concerned, let no one expect anything certain from astronomy, which cannot furnish it, lest he accept as the truth ideas conceived for another purpose, and depart from this study a greater fool than when he entered it. – Nicolaus Copernicus

The history of astronomy is a history of receding horizons. – Edwin Powell Hubble

Also on this day: A Cattle Trail Grows Up – In 1905, Las Vegas is established.
Friends Hospital – In 1817, the first private psychiatric hospital in the US opened.
Puckle Gun – In 1718, the first machine gun was patented.
Plane Crazy – In 1928, Mickey Mouse starred in a silent, black-and-white cartoon.


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