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ART. III.-POPULAR ASTRONOMY.

Popular Astronomy. By SIMON NEWCOMB, LL.D., Professor in the United States Naval Observatory. With One Hundred and Twelve Engravings, and Five Maps of the Stars. Octavo, pp. 566. New York: Harper & Brothers. 1878. WE are prepossessed with this volume at first sight. We like the theme: "The works of the Lord are great, sought out of all them that have pleasure therein." Astronomy is the science of grand ideas. It deals with vast magnitudes, immeasurable distances, and inconceivable durations. The author names it Popular Astronomy; that is, knowledge for the people; and for all the people, not merely the select few. Certainly, and emphatically, among the American people, and measurably throughout the broad earth, the spirit of the age demands light on all possible subjects, and the universal diffusion of light. Nor can we complain that science is slow to respond to the call. She comes out of her cell. "Wisdom crieth without; she uttereth her voice in the streets; she crieth in the chief place of concourse, in the openings of the gates."

And astronomy is not only among the most fascinating of the sciences, but is even better adapted than some others for general cultivation. Mathematics, for instance, deals in abstract numbers and quantities; it requires intense application, and few minds grow enthusiastic over its problems. Chemistry requires costly apparatus and materials. Botany is loaded down. with names and definitions, and demands meadow and marsh and forest, and leisure to explore them. But the glorious heaven is over all heads, and open to all eyes. The deep things of astronomy, indeed, involve study such as few are able to devote to them. The time is probably somewhat distant when every village lyceum will own a telescope of twenty-six inch aperture, and the arithmetic class of every country school calculate eclipses. Nevertheless, the sun in its glory, and the moon in its beauty, and the stars in their faroff realms of silence, shine for all; the wandering comets, and the auroral flames, the marvels of the day and the night, and the revolving seasons, are visible to all that look upon them; and the mind must be slow to perceive, and the heart slow to feel, which are not moved by the wonders of earth and sky.

When we hear the Psalmist exclaim: "O Lord, how wondrous are thy works; in wisdom hast thou made them all," we think of the author not as the wise king reigning in Jerusalem, but rather as the simple shepherd-boy, watching his flocks by night in the fields of Bethlehem the Fruitful. In this our day any man of average mental power, and as ignorant on the subject as any such man can be, and yet willing to listen for an hour to a lecturer like Dr. Newcomb, or devote an hour to the thoughtful perusal of a book like his, will go from the lecture, or rise from the book, a larger man, with more of thought and more of mental breadth than he before possessed.

Professor Newcomb divides his work into four parts. The first, entitled, "The System of the World Historically Developed," traces the growth of astronomy as a science. China puts forth claims to the greatest antiquity, asserting that two thousand years before the Christian era the science was cultivated there to such an extent that the Government maintained astronomers, whose duty was to note the motions of the heavenly bodies, and give timely notice of any unusual phenomenon, that the religious ceremonies proper on such occasions might be performed; and that two of these scientific officials, whose names were Hi and Ho, gave themselves up to riotous living, and neglected their duty; and that an eclipse of the sun came which had not been announced, and religious rites were not performed, and the whole empire was consequently exposed to the wrath of the gods, and that the royal scientists were put to death for their crime. This tradition may possibly be authentic. If true, it is the earliest of its kind on record. The Hindus, also, claim to have cultivated astronomy in very ancient times, but their records are vague and unreliable. The Chaldeans and the Egyptians were close observers of the heavenly bodies, and it is surprising to see how much was learned without the aid of telescopes or other instruments now indispensable.

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It is, in fact, difficult to tell where or when the science of astronomy originated. As we follow the line back through ages we find Copernicus learning of Ptolemy, and Ptolemy of Hipparchus, and Hipparchus of Pythagoras, and Pythagoras of Egyptian and Chaldean priests, until all traces are lost in the shadow of wild tradition and fable. Ptolemy, an Egyp

tian astronomer of the second century after Christ, taught a theory which was universally accepted for fourteen centuries. after the death of its author. His system describes the Earth as the center of the universe, and the Sun, Moon, and other heavenly bodies as making a daily revolution around it. In 1543 Copernicus, a Prussian astronomer, published a work entitled, "The Revolution of the Celestial Orbs," in which he attacked, somewhat timidly, the Ptolemaic theory, and advocated another, which places the Sun in the center of the system, and reduces our world to the position of a planet circling about it.

The new view met with great opposition. Every body has heard how Galileo was arrested by the Inquisition, compelled to recant, and to promise, under oath, to teach no more the dangerous doctrines of Copernicus. A fact not so well known is that the opposition was not confined to the reactionary guides of the Romish Church, but was found, also, among scientists and the adherents of the Reformation. Years after the death of Copernicus his theory was debated and opposed; one mighty reasoner among the rest stoutly arguing that the earth cannot revolve daily on its axis, else all manner of confusion would follow; for instance, the sky-lark, rising on its wings to sing its morning carol, would never find its nest again, the revolving surface having meanwhile borne it miles away. The discovery of Newton, that all the worlds of the family to which the Earth belongs are bound together by one law, that of universal gravitation, may be said to complete the theory of the solar system, considered as a piece of enginery. This is the mysterious force which marshals the worlds "in order infinite," and attunes their motions to a harmony which is the true music of the spheres.

Part second treats of Practical Astronomy. Under this head the author discusses the telescope and its uses, also the motion of light, and the spectroscope. When Galileo, in the year 1610, discovered the moons of Jupiter, the instrument which he used magnified objects only thirty times, and was a mere spy-glass. Now the astronomer scans the heavens with telescopes which magnify a thousand times or more. There is, indeed, no theoretical limit to the size of the telescope, and its magnifying power. Herschel is said to have employed

a power of 6,000 times in one of his great telescopes; but practically it is found that the disturbing influences of the atmosphere are magnified even in a greater ratio when we go beyond a certain limit. A night in which celestial objects appear clear, distinct, and steady under a telescopic power of 400, is regarded as a favorable time for observation, while nights in which a power of 1,000 and over can be employed to advantage are of rare occurrence.

A list of the "Principal Great Telescopes of the World" is given, from which it appears that there are forty-three worthy of mention. Nine of these are reflectors, that is, telescopes which magnify by means of concave mirrors. The largest of these was built by the Earl of Rosse, in Ireland, in 1844, and has an aperture of six feet. The only American instrument of this kind named in the list is of twenty-eight inches aperture, and belongs to Professor Henry Draper, of Dobb's Ferry, New York. Of the thirty-four refractors, magnifying by means of lenses, the two largest in the world are, one in the United States Naval Observatory at Washington, and the other in the Imperial Observatory at Vienna. These have an aperture of twentysix inches. Of the others, seventeen are in Europe, fourteen in this country, and one in South America. Two of the American telescopes are among the apparatus of the universities at Middletown and Ann Arbor.

One of the chief uses of the telescope, and practically the most important of all, is in measurements, celestial and terrestrial.

Should the reader ask what Practical Astronomy is, the best answer might be given him by a statement of one of its operations. Place an astronomer on board a ship; blindfold him; carry him by any route to any ocean on the globe, whether under the tropics or in one of the frigid zones; land him on the wildest rock that can be found; remove his bandage, and give him a chronometer regulated to Greenwich or Washington time, a transit instrument with the proper appliances, and the necessary books and tables; and in a single clear night he can tell his position within a hundred yards by observations of the stars. This, from a utilitarian point of view, is one of the most important operations of practical astronomy. -Page 103.

The determination of the distance of the Earth from the Sun is the great problem of astronomical measurement. This

distance, whatever it may be, enters into almost all calculations in regard to the Solar System, and becomes the unit of measurement, when we attempt to estimate the distance of the fixed stars. Till this is ascertained, therefore, we cannot deterinine the magnitude of the Sun, or the planets, or estimate, with any pretense of probability, the depths of the stellar universe. Aristarchus tried a solution, two thousand years ago, and his clumsy methods indicated a distance of only five millions of miles. Ptolemy tried another imperfect method, and reached about the same result. This huge mistake remained unchallenged for fourteen centuries after Ptolemy. Huyghens, two hundred years ago, tried the problem in still another way, and estimated the distance to be ninety-nine millions.

One modern method is based upon observations of the parallax of the two planets which from time to time approach nearest the Earth. The first trial of this method was made by French astronomers in 1672, observations of Mars being taken in France, and, also, in South America, by an expedition sent thither for that purpose. The result showed a distance of about eighty-five millions of miles. This estimate was far from being accurate; yet for a hundred years it was regarded as probably near the truth. Meanwhile astronomical instruments and methods were steadily improving, and the attention of scientists was directed to an approaching transit of Venus, as furnishing the means of a solution of the difficult problem. When the transit of 1761 occurred, observations were made in various parts of the world by parties sent out for that purpose by the principal European Governments; but the results so differed that the problem seemed as far from final solution as ever. Another transit of Venus occurred in 1769, and still more extensive arrangements were made for its observation, the Americans, this time, taking part in the work, under the lead of Rittenhouse.

The results of the observations of 1769 agreed better than those of 1761; and yet to utilize these results in fixing the distance of the Sun required the solution of so many intermediate problems that it was not until 1824 that the calculations were completed in a satisfactory manner by Encke, of the Royal Observatory at Berlin. Fixing the value of each observation, and assuming that the average result was nearest the truth,

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