File:The elements of astronomy; a textbook (1919) (14595735467).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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they belong ; at least, so far as known, for we do notknow with certainty the position of the equatorial planes ofUranus and Neptune. Moreover, all the satellites but themoon, Hyperion, and those recently discovered, move in orbitsthat are practically circles. 313. Classification of Planets. — Humboldt has classifiedthe planets in two groups,—the terrestrial planets, as hecalls them, and the major planets The terrestrial group 218 CLASSIFICATION OF PLANETS. (§313 contains the four planets nearest the sun,—Mercury, Venus,the Earth, and Mars. They are all bodies of similar magni-tude, ranging from 3000 to 8000 miles in diameter; not verydifferent in density and probably roughly alike in physicalconstitution, though probably also differing very much in theextent, density, and character of their atmospheres. The four major planets, Jupiter, Saturn, Uranus, and Nep-tune are much larger bodies, ranging from 32,000 to 90,000miles in diameter; are much less dense; and, so far as we can
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Fig. 78. — Relative Sizes of the Planets. make out, present only cloud-covered surfaces to our inspec-tion. There are strong reasons for supposing that they are ata high temperature, and that Jupiter especially is a sort of semi-sun ; but this is not certain. As to the asteroids, the probability is that they represent afifth planet of the terrestrial group, which, as has been alreadyintimated, failed somehow in its evolution, or else has beenbroken to pieces. § 313) herschels illustration. 219 Fig. 78 gives an idea of the relative sizes of the planets. Thesun on the scale of the figure would be about a foot in diameter. 314. Tables of Planetary Data. — In the Appendix we presenttables of the different numerical data of the solar system, derivedfrom the best authorities and calculated for a solar parallax of 8.80:the suns mean distance being, therefore, taken as 92,897,000 miles.These tabulated numbers, however, differ widely in accuracy. Theperiods of the planets and their distan

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