Identifier: astronomyforuseo00gill (find matches)
Title: Astronomy for the use of schools and academies
Year: 1882 (1880s)
Authors: Gillet, J. A. (Joseph Anthony), 1837-1908 Rolfe, W. J. (William James), 1827-1910
Subjects: Astronomy
Publisher: New York : Potter, Ainsworth, & Co.
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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eye ; but, if the whole twoseconds were caused by atmospheric refraction, this wouldimply a horizontal refraction of one second, which is only onetwo-thousandth of the earths horizontal refraction. It is pos-sible that an atmosphere competent to produce this refractionwould not make itself visible in any other way. But an atmosphere two thousand times rarer than our aircan scarcely be regarded as an atmosphere at all. The con-tents of an air-pump receiver can seldom be rarefied to agreater extent than to about a thousandth of the density of airat the earths surface; and the lunar atmosphere, if it existsat all, is thus proved to be twice as attenuated as what wecommonly call a vacuum. H4 ASTRONOMY. The Surface of the Moon. 112. Dusky Patches on the Disk of the Moon. — Withthe naked eye, large dusky patches are seen on the moon,in which popular fancy has detected a resemblance tohuman face. With a telescope of low power, these darkpatches appear as smooth as water, and they were once
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Fig. 130. supposed to be seas. This theory was the origin of thename mare (Latin for sea), which is still applied to thelarger of these plains ; but, if there were water on the sur-face of the moon, it could not fail to manifest its presenceby its vapor, which would form an appreciable atmosphere.Moreover, with a high telescopic power, these plains present ASTRONOMY. 115 a more or less uneven surface ; and, as the elevations anddepressions are found to be permanent, they cannot, ofcourse, belong to the surface of water. The chief of these plains are shown in Fig. 130. They areMare Crisitim, Mare Fcectinditatis, Mare Nectaris, Mare Tran-quillitatis, Mare Serenitatis, Mare Imbrium, Ma7-e Frigoris,and Oceanus Procellarum. All these plains can easily be rec-ognized on the surface ofthe full moon with the un-aided eve. 113. The Terminator of the Moon. — Theterminator of the moonis the line which sepa-rates the bright and darkportions of its disk.When viewed with atelescope of even mod-er

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