File:Astronomy for high schools and colleges (1881) (14761457504).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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oursof right ascension. To map out the path with greater precision, we have toobserve the position of the moon from night to night %vitha meridian circle. We thus lay down her course amongthe stars in the same manner that we have formerly shownit possible to lay down the suns path, or the ecliptic. Itis thus found that the path of the moon may be consideredas a great circle, making an angle of 5° with the ecliptic,and crossing the ecliptic at this small angle at two oppo-site points of the heavens. These points are called themoons nodes. The point at which she passes from thesouth to the north of the ecliptic is called the ascendirignode; that in which she passes from the north to thesouth is the descending node. To illustrate the motion ofthe moon near the node, the dotted line a a may be takenas showing the path of the moon, while the circles showher position at successive intervals of one hour as she is ap-proaching her ascending node. Position number 9 is exactly 160 ASTBOJsroMr,
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at the node. If wecontinue following liercourse in this way fora week, we should findthat she had movedabout 90°, and attainedher greatest north lati-tude at 5° from theecliptic. At the endof another week, weshould find that shehad returned to theecliptic and crossed itat her descending node.At the end of the thirdweek very nearly, weshould find that she hadmade three fourths thecircuit of the heavens,and was now in hergreatest south latitude,being 5° south of theecliptic. At the endof six or seven daysmore, we should againfind her crossing theecliptic at her ascend-ing node as before. Wemay thus conceive offour cardinal points ofthe moons orbit, 90°apart, marked by thetwo nodes and the twopoints of greatest northand south latitude. Motion of the Nodes.—A remarkable prop- MOON8 NODES, 161 erty of these points is that they are not fixed, but are con-stantly moving. The general motion is a little irregular,but, leaving out small irregularities, it is constantly towardthe west. Thu

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