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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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The Olcott Meridian Circle was ordered by the Dudley Observatory's first director, Benjamin Apthorp Gould, from the scientific equipment manufacturers Pistor and Martins of Berlin, Germany, in 1855. It was built in the next year and installed at the Observatory's original site in North Albany in 1860. It was named for Thomas Olcott, an Albany banker who was a founder, trustee and major financial supporter of the Dudley Observatory. A meridian circle telescope is used to measure very precisely the positions of stars. It is a refracting telescope that is fixed to point along a single north-south direction, while capable of rotation in the vertical direction from the horizon to the zenith (the point directly overhead). This varies the astronomical coordinate called declination. By positioning a star exactly halfway between the top and bottom of the telescope's field of view, the declination can be read on large and precisely mounted engraved circles, attached to the telescope . Meanwhile the turning of the earth moves the chosen star across the telescope's field of view. By precisely noting when that star crosses a fine wire stretched across the telescope's aperture, an astronomer can very precisely measure the other astronomical coordinate, right ascension. The result was the 19th century's most accurate method of measuring star positions. The object-glass of the Olcott Meridian circle is a clear eight inches in diameter, and its focal length is ten feet. The two circles used to measure declination are each thirty six inches in diameter, initially inlaid with silver, and capable of being read, with the aid of microscopes, to within tenths of seconds of arc (one tenth of a second is one 36,000th of a degree). Using an electrical device controlled by a push button called a chronograph, the time when the star crosses the wire stretched across the telescope's aperture, and therefore the star's right ascension, can be determined with similar accuracy. Only in the late 1870s was the Olcott Meridian Circle first used to carry out significant astronomical research. At that time, Lewis Boss, the Dudley Observatory's fourth director, used it to measure the position of 8245 stars as part of an international sky survey effort coordinated by the Astronomische Gesellschaft in Germany. In 1893, it was moved to the observatory's second site, on the south side of Albany, and was extensively refurbished, again thanks to the generosity of the Olcott family. This led to the use of the Olcott Meridian Circle to carry out a more ambitious project, the determination of the positions of 33,342 stars arrayed around the entire celestial sphere. In order to achieve full sky coverage, the Olcott Meridian Circle was taken off its mountings in Albany in 1908, and shipped to San Luis, Argentina. There, over the next two years, 15,333 southern stars were observed, after which the telescope was returned to Albany. This supplemented the observation of 20,811 stars from Albany, an effort that concluded in 1918, No other high precision telescope on earth has ever made a survey of comparably extensive coverage (a modern more extensive higher accuracy survey was accomplished from space in the 1980s by the fully automated and computerized Hipparchos satellite). The results of these observations were combined with those of 238 other observatories world wide to produce the General Catalogue of 33,342 Stars. After the sale of the Dudley Observatory's second building in 1965, the Olcott Meridian Circle was disassembled and stored in the New York State Museum in Albany.
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Dudley Observatory |
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