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By Dr. George Gatewood, Director of the Allegheny Observatory
As you drive along a country road you notice that the phone poles seem to move backward with respect to distant mountains. This is a perspective effect called parallax. As the Earth carries us around the Sun, the positions of the nearest stars exhibit a similar effect swinging back and forth along a small ellipse, which is a reflection of the Earth's orbit. A major test of the Copernican theory that the Earth and other planets orbited the Sun, was its prediction of the parallactic motion of the stars (e.g., Hirshfeld, A.W. 2001, "Parallax, the Race to Measure the Cosmos, Freeman & Company, NY, NY). Normally measured in arc seconds, a star's parallax is the inverse of its distance in parsecs. A parsec is equal the distance that light travels in 3.26 years. The distances from the Sun to the nearest stellar neighbor is so great that all stellar parallaxes are less than 1.0 arc second. This angle is so small that it took 300 years of trial and error before Bessel successfully measured the first stellar parallax in 1838.
Despite early experiments with photography, the precision of the measurements of stellar parallax did not improve much until the experiments of Frank Schlesinger with the Yerkes refractor between 1903 and 1905. These were so successful that he was quickly appointed as the director of the Allegheny Observatory during the design phase of the Thaw 30-inch (0.76-m) refractor. At his insistence the instrument became the first refractor specifically designed for the photographic determination of stellar parallax. Only one other refractor and one reflector would ever be designed for this purpose, the Yale 26-inch refractor in Australia and the United States Naval Observatory 61-inch reflector in Arizona. The two refractors feature objective lenses that are positionally defined with respect to their optical axis, in independent cells, by weight driven levers that offset the effects of gravity. Both were designed to be free of magnitude and coma effects across the field of the reference stars and to work at a wavelength not requiring a bandpass filter. The reflector uses a long-focal length primary mirror and a flat secondary. These features enhance the optical stability and thus their suitability to the task. The result has been that these telescopes have been frequently sighted as the most precise in the field.
Allegheny Observatory Photographic Parallax Program
As set up by Frank Schlesinger, a major goal of the Allegheny Observatory Photographic Parallax program was to determine all of the parallaxes of a well-defined sample, the bright stars. Thus the parallax of most of the stars brighter than about the 5th magnitude, which can be observed from Pittsburgh, will be found in the Publications of the Allegheny Observatory of the University of Pittsburgh. As this task neared completion, usually with short series of plates in the 1910's and 20's, the interest of the staff shifted to the study of stellar clusters, multiple stars, and intrinsically fainter nearby stars. A major contributor to the program and its evolution was Nicholas E. Wagman, director during most of the program and whose initials "NEW" can be found in both the observing books and on the jackets of measured plates.
Compilation Catalogs of Star Parallaxes
Those seeking the most accurate parallax of a star are directed to the compilation catalogs of the Yale Observatory (The General Catalogue of Trigonometric Stellar Parallaxes, 4th edition, William F. van Altena, John Truen-liang Lee, and E. Dorrit Hoffleit, 1995, Yale University Observatory New Haven, CT 06520) or to the Hipparcos Catalogue of the European "Hipparcos" astrometric satellite (The Hipparcos and Tycho Catalogues, SP-1200, 1997, ESA Publications Division, c/o ESTEC, Noordwijk, Netherlands). Data from the Publications of the Allegheny Observatory is incorporated into the Yale Catalogue. Its importance, in that compilation of the work of all ground-based parallax studies, may be noted from the fact that the 1952 version of the Yale Catalog gives the data collected in the Publications the highest weight of any parallax program in existence at that time. By far the largest and most accurate stellar parallax work is that of the Hipparcos Catalogue which contains more than 110,000 stars with a mean internal standard error of approximately 0.001 arc seconds. While a few current parallax programs using modern detectors have matched this precision, their combined programs total only a small fraction of the number of stars contained in the Hipparcos Catalogue.
Publications of the Allegheny Observatory
The material found in the Publications of the Allegheny Observatory is meant to be used by the investigator seeking to better understand the origin of the measurements given in modern catalogs. It is through the detailed study of this data that one gains a better assessment of both its statistical nature and the characteristics of the instrumentation and the reduction algorithm on which is it is based. The Publications reflect the results of one of the largest ground-based studies of the distances of the stars in history. It is through the statistical investigation of this data, in comparison to those of other observatories, that our understanding of the intrinsic characteristic of the stars was obtained. The data is important in the study of the compilation catalogs to which it contributed, to the weighted mean parallax of modern studies of these stars, and to the understanding of the strengths and limitations of the program, which contributed so profoundly to the determination of the scale of the Universe.
When creating the electronic version of the Publications, staff at the Digital Research Library noted the parallax and radial velocity data contained in the 10-volume set. The parallax database records the same information for each star: star name, Allegheny Observatory number, BD number, right ascension hours, minutes, and seconds, declination degrees, minutes and seconds (like the Yale Catalogue, these are for the equinox of 1900), Henry Draper number, relative parallax, and parallax error.