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248 41 Review of Chambers’s Pictorial Astronomy 26 November 1891 The Nation Pictorial Astronomy for General Readers. By George F. Chambers , F.R.A.S. Macmillan & Co. 1891. 16mo, pp. 267. There is no lack of popular books about astronomy by those who look upon the subject from the inside, as, Herschel, Secchi, Newcomb, Langley, Young, Lockyer, Ball. Mr. Chambers is none of these. He is not a scientific observer of the stars, nor has he an ordinary astronomer ’s acquaintance with celestial mechanics. He is a well-known compiler of astronomical books, useful in their way, but marked by incompleteness and a want of discrimination. The present little treatise will serve the purpose of a person who wants some light reading with pictures touching most of those important topics of astronomy that call for no mental exertion, about right in most of its statements, and not seriously unjust in many of its appreciations. To show how simple everything is here made, we annotate a short passage taken almost at random. The numerals in parentheses refer to our remarks below: In calculating the different positions of Mars (1), and comparing his own observations (2) with those of Tycho Brahe, Kepler was astonished at finding numerous apparent irregularities (3) in Mars’s orbit, and still more in its distance from the earth (4). He soon saw (5) that the orbit could not be circular, and eventually recognized that it must be (6) an ellipse, with the sun occupying one of the two foci. . . . The path of a planet once traced, the next thing (7) to determine was what regulated the irregularities observed in its course. Kepler, having remarked (8) that the velocity of a planet (9) seemed to be greatest when it was nearest to the sun, and least when it was most remote from the sun, proceeded to suggest that an imaginary line joining the centre of a planet and the centre of the sun would pass over equal areas in equal times. . . . He sought to discover if any relation subsisted between the diameters of the orbits and the times occupied by the planets in traversing them. After twenty-seven years 41. Chambers’s Pictorial Astronomy, 1891 249 (10) of laborious research (11), he found out that a relationship did subsist, and thus was able to assert his third law. (1) Kepler did not set out by calculating places of Mars from its elements , but on the contrary by endeavoring to deduce from the observations the eccentricity of the orbit. (2) At the time referred to, Kepler is not known to have observed Mars, and only a very few of his observations were used by him in the investigation of the motions of that planet. (3) What incited Kepler to his great work was not finding irregularities , but a belief that by a method of calculation different from that in use (based on apparent instead of mean oppositions) known seeming irregularities could be made to disappear. (4) The distance from the earth could not be a subject of observation, and consequently irregularities in this distance could not be detected. The only thing in the work with which we can connect this belongs to a later time, after a great part of the work had been done and a corrected theory of the earth’s motion had been made. (5) For “soon” read: after five years of diligent research. (6) This “must be” conveys no hint of the mode in which the opposite errors of two hypotheses directed Kepler’s suspicions to the ellipse as the form of the orbit. (7) Mr. Chambers writes as if Kepler first ascertained the form of the orbit and then introduced the principle of areas. But it was the other way. He had assumed this principle long before he dreamed of the orbit not being circular. Indeed, without some such assumption he would not have had sufficient data to determine the shape of the path, since the distance of Mars could not be determined except by an intricate procedure seldom applicable. Indeed, except for movements in latitude too slight to prove much, all that is observed is variable movements in longitude . (8) This remark was of course one of the earliest generalizations concerning planetary motion. (9) A superior planet is meant. (10) The discovery was made 1618, May 8. Twenty-seven years before, Kepler had not taken up the pursuit of astronomy. (11) Although he had puzzled long over the figures before he...

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