File:Astronomy for high schools and colleges (1881) (14577337507).jpg

Identifier: astronomyforhigh00newc (find matches)
Title: Astronomy for high schools and colleges
Year: 1881 (1880s)
Authors: Newcomb, Simon, 1835-1909 Holden, Edward Singleton, 1846-1914, joint author
Subjects: Astronomy
Publisher: New York, H. Holt and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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arries another axis L D(the declination axis)^ which is fixed at right angles to it,but which can again be rotated about its axial line. To this last axis a telescope is attached, which mayeither be a reflector or a refractor. It is plain that such atelescope may be directed to any point of the heavens ;for we can rotate the declination axis until the telescopepoints to any given polar distance or declination. Then,keeping the telescope fixed in respect to the declinationaxis, we can rotate the whole instrument as one massabout the polar axis until the telescope points to any por-tion of the parallel of declination defined by the givenright ascension or hour-angle. Fig. 37 is an equatorial ofsix-inch aperture which can be moved from place to place. If we point such a telescope to a star when it is rising(doing this by rotating the telescope first about its decH-nation axis, and then about the polar axis), and fix thetelescope in this position, we can, by simply rotating the ASTRONOMY,
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Fig. 37.—equatorial telescope pointed towaud the pole. THE MICROMETER. 80 whole apparatus on the polar axis, cause the telescope totrace out on the celestial sphere the apparent diurnal pathwhich this star will appear to follow from rising to set-ting. In such telescopes a driving-clock is so arrangedthat it can turn the telescope round the polar axis at thesame rate at which the earth itself turns about its own axisof rotation, but in a contrary direction. Hence such atelescope once pointed at a star will continue to point at itas long as the driving-clock is in operation, thus enablingthe astronomer to observe it at his leisure.

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