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The following works, among others, have been freely consulted in preparing this volume:
The Heavens Guillemin.
Introduction to Astronomy Hind.
Solar System Hind.
Popular Astronomy Airy.
Popular Astronomy Arago.
Astronomy ... Norton.
Age of Fable Bulfinch.
Poetry of Science Hunt
Outlines of Astronomy HerscheL
Popular Astronomy Mitchell.
Astronomy and Physics Whewell.
Annual of Scientific Discovery Kneeland
The Chemical News.
SUGGESTIONS TO TEACHERS.
This work is designed to be recited in the topical method. On naming the title of a paragraph, the pupil should be able to draw on the blackboard the diagram, if any is given, and state the substance of what is contained in the book. It will be noticed that the order of topics, in treating of the planets and also of the constellations, is uniform. If a portion of the class write their topics in full upon the blackboard, it will be found a valuable exercise in spelling, punctuation, and composition. Every point which can be illustrated in the heavens should be shown to the class. No description or apparatus can equal the reality in the sky. After a constellation has been traced, the pupil should be practised in star-map drawing. Much profitable instruction can be obtained in this way. For the purpose of more easily finding the heavenly bodies at any time, Whit All's Movable Planisphere is of great service. It may be procured of the publishers of this work. "Orreries are of little account" A tellurian is invaluable in explaining Precession of the Equinoxes, Eclipses, Phases of the Moon, etc. Messrs. A. S. Barnes & Co., New York City, furnish a good instrument at a low price. The article on " Celestial Measurements," near the close of the work, should be constantly referred to during the term. In the figures, the right-hand side represents the west and the left-hand the east. When it is important to obtain this idea correctly, the book should be held up toward the southern sky.
Never let a pupil recite a lesson, nor answer a question, except it be a mere definition, in the language of the book. The text id designed to interest and instruct the pupil; the recitation should afford him an opportunity of expressing what he has learned, in his own style and words.
FRESH FACTS AND THEORIES.
THERE has been for some time a growing conviction among astronomers that the currently-accepted solar parallax (8".94) and the corresponding mean solar distance (91,430,000 miles), as given on page 307 of this work, are incorrect. The methods recently adopted for approximating the solar parallax give a smaller angle, and hence increase the estimate of the sun's distance. It was hoped that the transit of Venus, Dec. 8,1874, would furnish data for determining the question definitely. As yet, the reported results are not satisfactory. Authorities agree that the mean solar distance is more than 91J million miles, but vary in the amount. Thus, Newcomb makes the solar parallax S".848, and the mean solar distance 92,393,000 miles; while Airy considers the former to be not far from 8''.76, and the latter 93,300,000. Mr. Tod, of the Washington Observatory, in a recent article, says, " All the experimental determinations of the velocity of light hitherto made, when combined with the appropriate astronomical constants, give the mean equatorial horizontal parallax at 8".808 and the mean solar distance 92,800,000 miles." Some astronomical writers have recently gone so far as to state the sun's distance broadly at 95,000,000 miles. American authorities now generally base their calculations on a distance, in round numbers, of 92 V million miles. As the solar distance is the foot-rule for celestial measurements, the discrepancy here noted occurs in the other elements— diameters and distances of the planets, etc. Meanwhile, it has not seemed desirable to change the data of this book until there is a substantial agreement among astronomers themselves. There is a satisfaction in noticing that the facts and theories of Astronomy are untouched by this want of uniformity, it being only a question of a few miles more or less in a distance whose vastness in any case is beyond the grasp of the mind.
In this connection it may be added that the velocity of light (p. I82'), which has been estimated in part on the basis of the solar distance (Physics, p. 150), is now placed at about 186,000 miles per second.
(P. 40.) The colures being meridians, are by many assigned to the Equinoctial system, and the term "vertical circle" is therefore limited to the Horizon system.
(P. 45, 173.) The number of minor planets discovered at this time (Feb. 1, 1881) is 219.
(P. 64.) For the views of various authorities on the constitution of the sun, solar spots, etc., see Newcomb's Astronomy, third edition, p, 271. The heat of the sun could be maintained by an annual contraction of 250 feet in its diameter—a decrease so insignificant as to be imperceptible with the best instruments; or by the annual impact of meteors equal in amount to } the mass of Mercury. (See article by Prof. Young in Popular Science Monthly, Nov. 1880.) Newcomb says that in 5,000,000 years, at the present rate, the sun will have shrunk to half its present size.
P. 82.) During the total eclipse of July 29, 1878, Prof. Watson of the Ann Arbor Observatory, and Dr. Lewis Swift, of Rochester, each saw two Intra-Mercurial planets.
(P. go.) The greatest brilliancy of Venus will occur next in Feb., 1886.
(P. 143.) The brilliancy of the moon is now estimated to be about eoo^o100 'nat of the sun. Whether or not the moon radiates any heat to the earth has long been a mooted question. The best authorities, at present, estimate the average heat of the moon-beam at about ssofdoo that which we receive from the sun, or sufficient to raise the temperature of a sensitive black-bulb thermometer -g^-g of a degree. Of course it is absurd to suppose that this slight amount of heat can have any appreciable effect upon the weather.
(P. 170.) Prof. Hall, as the result of his observations on the satellites of Mars discovered by him (p. 171), fixes the density of that planet at .776.
(P. 181.) Jupiter and Saturn are older planets than the earth and Mars, but being so large have cooled more slowly and are yet only partially solidified and still shine with much of their primeval fire. Mars typifies the middle age, Saturn and Jupiter the youth, and Uranus and Neptune the infancy of planetary existence.
(P. 204.) The theory explained by Fig. 63 is now generally discarded, and the tendency is to adopt the view of the physical association of comets and meteors. The orbit of the August meteors is identical with Comet III 1862 (Swift's), and that of the Nov. 14th shower corresponds with Comet I 1866 (Tempel's). The showers of Nov. 24 and 27 are thought to be produced by meteors traveling in the path of the two dissevered parts of Biela's comet. Several hundred meteoric rings have been identified, and the grand problem of meteoric astronomy today is to detect their allied comets. Prof. Newton, the great advocate of this theory, broadly asserts that every meteoric stone was once a part of a comet, and every meteoric shower consists of broken fragments of some known or unknown comet. "The meteoroids (as these bodies are called before being ignited) of the November ring have a period of 33} years, but the great showers occur once in 33 years, or, more correctly, 33 years and one day. The precession of the equinoxes and the motion of the nodes of the ring, throw them one day later at each period. It will be noticed that it is not necessary that the period of the showers and that of the meteors should exactly agree."—(Swift.)
(P. 224.) It is now asserted by Proctor and others that there is no proof that the Pleiades is the great center of the solar system.
(P. 282.) A considerable modification of the Nebular Hypothesis is probable, leaving its general idea, however, intact. One view supposes that the rupture of the parent body (sun or planet) did not take place until it was partially consolidated and had attained nearly its present dimensions. Thus the moon was produced by a rupture of a primeval planet composed of the present earth and moon. It is now generally conceded that the several planets were not "thrown off," but merely detached and left behind.
(P. 287.) Dr. Draper of New York has detected in photographs ol the solar spectrum, evidence of the presence of oxygen in that luminary.
Attention is called to the new star chart in this edition, which has been redrawn according to the latest authorities.
TABLE OF CONTENTS.