File:The elements of astronomy; a textbook (1919) (14781911332).jpg

Identifier: elementsofastro00youn (find matches)
Title: The elements of astronomy; a textbook
Year: 1919 (1910s)
Authors: Young, Charles A. (Charles Augustus), 1834-1908 Young, Anne Sewell, b. 1871., ed
Subjects: Astronomy Constellations
Publisher: Boston, New York (etc.) Ginn and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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cted for precession, aberration, etc. The operationis, however, a very easy one with modern tables and formulae, involv-ing perhaps from five to ten minutes work. 425. Star-Charts and Stellar Photography. — For some pur-poses, accurate st&i-charts are even more useful -than cata-logues. The old-fashioned and laborious way of making suchcharts was by plotting the results of zone observations, butat present it is being done by means of photography, vastlybetter and more rapidly. A co-operative campaign began in1889, the object of which is to secure a photographic chart ofall the stars down to the 14th magnitude. The work is nowwell advanced, the greater part of the necessary 23,000negatives having been already made. One of the most remarkable things about the photographicmethod isthat with a good instrument there appears to be nolimit to the faintness of the stars that can be photographed:by increasing the time of exposure, smaller and smaller stars 308 STELLAR PHOTOGRAPHY. (§425
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FlG. 105. — Photographic Telescope of the Paris Observatory. § 425) PROPER MOTIONS. 309 are continually reached. With the ordinary plates and expo-sure-times not exceeding twenty minutes, it is now possibleto get distinct impressions of stars that the eye cannot pos-sibly see with the telescope employed. Fig. 105 represents the photographic telescope (14 inches apertureand 11 feet focus) of the Paris Observatory. The others engaged inthis star-chart campaign are all of identical optical power. Severalother enormous photographic telescopes, ranging in diameter from24 to 32 inches, and from 40 to 60 feet long, have recently beenmounted in European observatories. The Bruce telescope (namedfor its donor, the late Miss Bruce of New York) has a four-lensobjective of 24 inches diameter and 14 feet focal length. It is atpresent mounted at Arequipa, Peru. STAR MOTIONS. 426. The stars are ordinarily called fixed in distinctionfrom the planets or wanderers, because they keep their posi-tions

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