Francis Baily

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Francis Baily (28 April 1774 – 30 August 1844) was an English astronomer, most famous for his observations of 'Baily's beads' during an eclipse of the Sun.



Baily was born at Newbury in Berkshire in 1774. After a tour in the unsettled parts of North America in 1796–1797, his journal of which was edited by Augustus de Morgan in 1856, Baily entered the London Stock Exchange in 1799. The successive publication of Tables for the Purchasing and Renewing of Leases (1802), of The Doctrine of Interest and Annuities (1808), and The Doctrine of Life-Annuities and Assurances (1810), earned him a high reputation as a writer on life-contingencies; he amassed a fortune through diligence and integrity and retired from business in 1825, to devote himself wholly to astronomy.

Astronomical work

By 1820, Baily had already taken a leading part in the foundation of the Royal Astronomical Society, and its Gold Medal was awarded him, in 1827, for his preparation of the Society's Catalogue of 2881 stars (Memoirs R. Astr. Soc. ii.). Later, in 1843, he would win the Gold Medal again. He was elected as president of the society for four consecutive two-year terms prior to his death.

The reform of the Nautical Almanac in 1829 was set on foot by his protests. He recommended to the British Association in 1837, and in great part executed, the reduction of Joseph de Lalande's and Nicolas de Lacaille's catalogues containing about 57,000 stars; he superintended the compilation of the British Association's Catalogue of 8377 stars (published 1845); and revised the catalogues of Tobias Mayer, Ptolemy, Ulugh Beg, Tycho Brahe, Edmund Halley and Hevelius (Memoirs R. Astr. Soc. iv,, xiii.).

His observations of "Baily's Beads", during an annular eclipse of the sun on 15 May 1836, at Inch Bonney in Roxburghshire, started the modern series of eclipse expeditions. The phenomenon, which depends upon the irregular shape of the moon's limb, was so vividly described by him as to attract an unprecedented amount of attention to the total eclipse of 8 July 1842, observed by Baily himself at Pavia.

In other work, he completed and discussed H. Foster's pendulum experiments, deducing from them an ellipticity for the earth of 1/289.48 (Memoirs R. Astr. Soc. vii.). This value was corrected for the length of the seconds-pendulum by introducing a neglected element of reduction, and was used, in 1843, in the reconstruction of the standards of length. His laborious operations for determining the mean density of the earth, carried out by Henry Cavendish's method (1838–1842), yielded the authoritative value of 5.66.

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