Thursday, 22 June 2017

Countdown to the American Eclipse: The story behind “BAILEY’S BEADS”

H402/0547ENGLISH ASTRONOMER FRANCIS BAILY (28 April 1774 – 30 August 1844).


The first American expedition was orga­nized and sent out from Harvard College for the eclipse of October 27, 1780. As this took place during the war of the American Revolution, an appeal was made to the govern­ment of the Commonwealth that a vessel might be prepared to convey proper observers to Penobscot-Bay; and that ap­plication might be made to the officer who commanded the British garrison there, for leave to take a situation con­venient for this purpose. The letter of Appeal read:

“Though involved in all the calamities and distresses of a severe war, the government discovered all the attention and readiness to promote the cause of science, which could have been expected in the most peaceable and prosperous times; and passed a resolve, directing the Board of War to fit out the Lincoln galley to convey me to Penobscot, or any other port at the eastward, with such assistants as I should judge necessary.”

Probably on account of an error in the tables, the eclipse was not total where the Harvard party was located. Be­tween the first and second contacts Professor Williams measured the angular length of the moon subtended by the decreasing crescent of the sun. He gives the following de­scription of what appeared shortly before the total phase was expected: " The sun's limb became so small as to ap­pear like a circular thread or rather like a very fine horn. Both the ends lost their acuteness and seemed to break off in the form of small drops or stars some of which were round and others of an oblong figure. They would separate to a small distance, some would appear to run together again and then diminish until the whole disappeared.”


Apparently this is a clear description of the so-called " Baily's Beads " observed by Francis Baily at the eclipse of 1836. An excellent description of this phenomenon is given by Agnes Clerke in her History of Astronomy during the Nineteenth Century, page 74. Baily gave the correct explanation of the phenomenon he saw as being due to ir­radiation. This same effect is seen when one holds up his hand to the sunlight. In making the fingers come close to­gether, they appear to touch each other before one feels they are actually in contact. An analogous manifestation is called the " black drop " which caused surprise at the transits of Venus in the years 1761 and 1769, and was the source of great trouble to astronomers at the transits of 1874 and 1882, so widely observed for the purpose of determining the solar parallax.

The appearance of " Baily's Beads" is a phenomenon well worth watching and should be atten­tively looked for just before totality begins and just after it ends. Very excellent observations may be made with a good pair of field glasses or with a small telescope, a large telescope being unnecessary.

Baily was not an astronomer by profession. He was a stock-broker, and fortunately he had been successful in the making of money, with the result that he was able to devote the maturer years of his life to astronomy which he took up as his hobby. His work is but one of the many instances of the great debt of science to the amateur astronomer.

One important result of his observations in 1836 was to show professional astronomers that at the time of the total eclipse of the sun there were other phenomena to observe than the mere times of contact of the limbs of the sun and moon.

The eclipse of 1836 witnessed not only ,the phenomenon of " Baily's Beads " but also an attempt by Forbes to test the physical constitution of the sun's atmosphere by means of the spectroscope. A new era for astronomy had accord­ingly dawned. An eclipse occurred in Southern Europe on July 8, 1842, and into the narrow track were collected the foremost astronomers from England, France, Germany and Russia. What was observed in 1836 was as nothing com­pared with the wonders of the eclipse of 1842!

One of the strangest portions of the history of astronomy before the middle of the nineteenth century is the. evident lack of interest in, or perhaps one should say, the dearth of accurate observations of the phenomena visible at the time of a total eclipse of the sun. The startling suddenness of the apparition, coming in the early days of civilization without warning, must have brought terror to the hearts of the populace and caused them to fear war or pestilence or the death of a favorite prince.

It is but natural that the prehistoric superstition of the dragon swallowing the sun should have spread during the middle ages from the far East to all of the civilized world. One fleeting glance, however, should have revealed, even to the most timorous minded, the pearly-Gray light of the corona and brought to view the glow of the rosy-hued prominences. To those of the present generation, the nine hundred and ninety-nine out of a thousand who have never had the good fortune to witness a total solar eclipse, it might not be out of place to point out that a telescope is entirely unnecessary for viewing the beauties of the corona, this being a spectacle that derives its glory from the wide-spread splendour and slight gradations of contrast. A telescope, small or large will of course magnify any particular portion — but to see and enjoy the beauty of the corona as a whole nothing is actually needed but the normal naked eye.

Solar_eclips_1999_6Published references to the corona in the early literature are exceedingly rare. Plutarch and Philostratus give allu­sions which unmistakably refer to the corona, but appar­ently the first to take any scientific cognizance of the crown of glory was Kepler who seems to have witnessed the solar eclipse of 1605 iri Naples. A hundred years later at the eclipse of 1706, Cassini, who was a practised observer, de­scribes the " crown " of pale light, and he decides that it must be caused by the illumination of zodiacal light; and eleven years thereafter, Halley saw the corona and also prominences, but he was unable to decide whether the corona belonged to the sun or to the moon.

Image Credit: © Luc Viatour Belgium, Brussels

If so little attention was paid to the corona, it is not sur­prising that even less should be taken of the " red flames," though if one refers to the plate facing page 130 he will see what a brilliant spectacle they afforded in the eclipse of 19x8. The first reference to them seems to be at the eclipse of 1706 when they were apparently observed by Stannyan who wrote a description of them to Flamsteed. The first vivid portrayal was by Vassinius of Sweden who observed them in 1733. The Spanish admiral Ulloa observed them while at sea during the eclipse of June 24, 1778, and he furnished a valuable account, with the added explanation that the rosy; hues were caused by the sun's light shining through some hole or crevice in the limb of the moon!

The astronomers who witnessed the eclipse of 1842 were entirely unprepared for the phenomena that met their gaze. Baily repaired to Pavia, and made his observations from one of the rooms of the University. One of the professors, out of the goodness of his heart, offered to assist him in any way possible, but Baily informed him that all he wanted was to be " left alone, being persuaded that nothing is so in­jurious to the making of accurate observations, as the intru­sion of unnecessary company." Not being content with this gentle hint, the key was taken from the outside of the door and it was securely locked on the inside. Baily's report of the observations made at the eclipse is found in the Memoirs, R. A. S., 15, 4, 1846, as follows: " The beads were distinctly visible. ... I was astounded by a tremendous burst of ap­plause from the streets below, and at the same moment was electrified at the sight of one of the most brilliant and splen­did phenomena that can be imagined. For at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory. ... I had indeed antici­pated a luminous circle round the moon during the time of total obscurity, but I did not expect, from any of the ac­counts of previous eclipses that I had read, to witness so magnificent an exhibition as that which took place. . . . The breadth of the corona, measured from the circumference of the moon, appeared to me to be nearly equal to half the moon's diameter. It had the appearance of brilliant rays. Its colour was quite white, not pearl colour, nor yellow, nor red. …

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