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Astronomy

Science of determining the places of the heavenly bodies

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* Published by Encyclopedia Press, 1913.


Astronomy (from Greek astron, star; nemein, to distribute), a science of prehistoric antiquity, originating in the elementary needs of mankind. It is divided into two main branches, distinguished as astrometry and astrophysics; the former concerned with determining the places of the heavenly bodies, the latter, with the investigation of their chemical and physical nature. But the division is of quite recent date. The possibilities of antique science stopped short at fixing the apparent positions of objects on the sphere. Nor was any attempt made to rationalize the observed facts until the Greeks laboriously built up a speculative system, which was finally displaced by the vast fabric of gravitational theory. Descriptive astronomy, meanwhile, took its rise from the invention of the telescope, and the facilities thus afforded for the close scrutiny of the denizens of the sky; while practical astronomy gained continually in refinement with the improvement of optical and mechanical arts. At the present time, astrophysics may be said to have absorbed descriptive astronomy, and astrometry necessarily includes practical research. But mathematical astronomy, grounded on the law of gravitation, keeps its place apart, though depending for the perfecting of its theories and the widening of its scope upon advances along the old, and explorations in new, directions.

PREHISTORIC ASTRONOMY.—Formal systems of astronomical knowledge were early established by the Chinese, Indians, Egyptians, and Babylonians. The Chinese were acquainted, probably in the third millennium B.C., with the cycle of nineteen years (rediscovered in 632 B.C. by Meton at Athens), by which, since it comprised just 235 lunations, the solar and lunar years were harmonized; they recorded cometary apparitions, observed eclipses, and employed effective measuring apparatus. European methods were introduced at Pekin by Jesuit missionaries in the seventeenth century. Indian astronomy contained few original elements. It assigned particular prominence to the lunar zodiac, called the nakshatras, or mansions of the moon, variously reckoned at twenty-seven or twenty-eight; and these, which were probably a loan from Chaldea, served mainly for superstitious purposes. In Egypt, on the other hand, considerable technical skill was attained, and a peculiar constellational system of obscure derivation, came into use. The Babylonians alone, among the nations of the foretime, succeeded in laying the foundations of a progressive science. Through the medium of the Greeks, they transmitted to the West their entire scheme of uranography, our familiar constellations having been substantially designed on the plain of Shinar about 2800 B.C. Here, too, at a remote epoch, the "Saros" became known. This is a cycle of eighteen years and ten or eleven days, which affords the means of predicting the recurrence of eclipses. The changing situations of the planets among the stars were, moreover, diligently recorded, and accurate acquaintance was secured with the movements of the sun and moon. The interpretation in 1889, by Fathers Epping and Strassmaier, of a collection of inscribed tablets preserved in the British Museum vividly illuminated the methods of official Babylonian astronomy in the second century B.C. They were perfectly effectual for the purpose chiefly in view, which was the preparation of yearly ephemerides announcing expected celestial events, and tracing in advance the paths of the heavenly bodies. Further analysis in 1899 by Father Kugler, S.J., of the tabulated data employed in computing the moon's place, disclosed the striking fact that the four lunar periods—the synodic, sidereal, anomalistic, and draconitic months—were substantially adopted by Hipparchus from his Chaldean predecessors.

GREEK ASTRONOMY.—Astronomy, however, no sooner became a distinctively Greek science than it underwent a memorable transformation. Attempts began to be made to render the appearances of the sky intelligible. They were, indeed, greatly hampered by the assumption that movement in space must be conducted uniformly in circles, round an immobile earth; yet the problem was ostensibly solved by Apollonius of Perga (250-220 B.C.), and his solution, applied by Hipparchus to explain the movements of the sun and moon, was extended by Claudius Ptolemaeus (Ptolemy) to the planets. This was the celebrated theory of eccentrics and epicycles, which, by the ingenuity of its elaboration, held its own among civilized men during fourteen centuries. Hipparchus, the greatest of ancient astronomers, observed at Rhodes (146-126 B.C.), but is considered as belonging to the Alexandrian school. He invented trigonometry, and constructed a catalogue of 1080 stars, incited, according to Pliny's statement, by a temporary stellar outburst in Scorpio (134 B.C.). Comparing, as the work progressed, his own results with those obtained 150 years earlier by Timocharis and Aristyllus, he detected the slow retrogression among the stars of the point of intersection of the celestial equator with the ecliptic, which constitutes the phenomenon of the precession of the equinoxes. The circuit is completed in 25,800 years; hence the tropical year, by which the seasons are regulated, is shorter than the sidereal year by just twenty-one minutes, the equinox shifting backward to meet the sun by the annual amount of 501". Greek astronomy was embodied in Ptolemy's "Almagest" (the name is of mixed Greek and Arabic derivation), composed at Alexandria about the middle of the second century

A.D. It was based upon the geocentric principle. The starry sphere, with its contents, was supposed to revolve, once in twenty-four hours, about the fixed terrestrial globe, while the sun and moon, and the five planets, besides sharing the common movement, described variously conditioned orbits round the same center. The body of doctrine it inculcated made part of the universal stock of knowledge until the sixteenth century. The formidable task of demonstrating its falsity, and of replacing it with a system corresponding to the true relations of the world, was undertaken by an active and exemplary ecclesiastic, Nicholas Copernicus, Canon of Frauenburg (1473-1543). The treatise in which it was accomplished, entitled "De Revolutionibus Orbium Coelestium", saw the light only when its author lay dying; but a dedication to Pope Paul III bespoke the protection of the Holy See for the new and philosophically subversive views which it propounded. Denounced as impious by Luther and Melanchthon, they were, in fact, favorably received at Rome until theological discredit was brought upon them by the wild speculations of Giordano Bruno (1548-1600), and the imprudent utterances of Galileo Galilei (1564-1642).

DESCRIPTIVE ASTRONOMY.—Descriptive astronomy may be said to have originated with the invention of the telescope by Hans Lippershey in 1608, Its application to the scrutiny of the heavenly bodies, by Galileo and others, led at once to a crowd of striking discoveries. Jupiter's satellites, the phases of Venus, the mountains of the moon, the spots on the sun, Saturn's unique appendages, all descried with a little instrument resembling a uniocular opera-glass, formed, each in its way, a significant and surprising revelation; and the perception of the stellar composition of the Milky Way represented the first step in sidereal exploration. Johann Kepler (1571-1630) invented in 1611, and Father Scheiner of Ingolstadt (1575-1650) first employed, the modern refracting telescope; and the farther course of discovery corresponded closely to the development of its powers. Christian Huygens (1629-95) resolved, in 1656, the ante of Saturn into a ring, divided into two by Giovanni Domenico Cassini (1625-1712) in 1675. Titan, the largest of Saturn's moons, was detected by Huygens in 1655, and four additional members of the family by Cassini before 1684. The Andromeda nebula was brought to notice by Simon Marius in 1612, the Orion nebula by J. B. Cysatus, a Swiss Jesuit, in 1618; and some few variable and multiple stars were recognized.

THEORETICAL ASTRONOMY.—The theoretical, however, far outweighed the practical achievements of the seventeenth century. Kepler published the first two of his "Three Laws" in 1609, the third in 1619. The import of these great generalizations is: (I) that the planets describe ellipses of which the sun occupies one focus; (2) that the straight line joining each planet with the sun (its radius vector) sweeps out equal areas in equal times; (3) that the squares of the planetary periods are severally proportional to the cubes of their mean distances from the sun. The geometrical plan of movement in the solar system was thus laid down with marvelous intuition. But it was reserved for Sir Isaac Newton (1643-1727) to expound its significance by showing that the same uniformly acting force regulates celestial revolutions, and compels heavy bodies to fall towards the earth's surface. The law of gravity, published in 1687 in "Philosophiae Naturalis Principia Mathematica" is to the following effect: every particle of matter attracts every other with a force directly proportional to their masses, and inversely proportional to the squares of their distances apart. Its validity was tested by comparing the amount of the moon's orbital deflection in a second with the rate at which an apple (say) drops in an orchard. Allowance being made for the distance of the moon, the two velocities proved to tally perfectly; and the identity of terrestrial gravity with the force controlling the revolutions of the heavenly bodies was definitively established. But this was only a beginning. The colossal work remained to be accomplished of calculating the consequences of the law, in the minute details of its working, and of comparing them with the heavens. It was carried forward, first by Newton himself, and in the ensuing century, by Euler, Clairaut, d'Alembert, Lagrange, and Laplace. Urbain Leverrier (1811-77) inherited from these men of genius a task never likely to be completed; and the intricacies of lunar theory have been shown, by the researches of John Couch Adams (1819-92), of Hansen and Delaunay, of Professors Hill and Newcomb, and many more, to be fraught with issues of unexpected and varied interest.

DISCOVERIES IN THE SOLAR SYSTEM.—The extraordinary improvement of reflecting telescopes by Sir William Herschel (1738-1822) opened a fresh epoch of discovery. His recognition of the planet Uranus (March 13, 1781) as a non-stellar object marked the first enlargement of the bounds assigned of old to the solar system; two Uranian moons, Oberon and Titania, were detected by him January 11, 1787, and the innermost Saturnian pair, Enceladus and Mimas, August 28 and September 17 of the same year. Saturn was, in 1906, known to possess ten satellites. Hyperion was descried by W. C. Bond at the observatory of Harvard College September 16, 1848, and Professor W. H. Pickering, of the same establishment, discovered by laborious photographic researches, Phcebe in 1898, and Themis in 1905. In point of fact, an indefinite number of satellites are agglomerated in the rings of Saturn. Their constitution by separately revolving, small bodies, theoretically demonstrated by J. Clerk Maxwell in 1857, was spectroscopically confirmed by the late Professor Keeler in 1895. The system includes a dusky inner member, detected by Bond, November 15, 1850. The discovery of the planet Neptune, September 23, 1846, was a mathematical, not an observational feat. Leverrier and Adams independently divined the existence of a massive body, revolving outside Uranus, and exercising over its movements disturbances the analysis of which led to its capture. Its solitary moon was noted by William Lassell of Liverpool in October, 1846; and he added, in 1851, two inner satellites to the remarkable system of Uranus. With the great Washington refractor, 26 inches in aperture, Professor Asaph Hall discerned, 16 and August 17, 1877, Deimos and Phobos, the swiftly circling moonlets of Mars; the Lick 36-inch enabled Professor Barnard to perceive, September 9, 1892, the evasive inner satellite of Jupiter; and two exterior attendants on the same planet were photographically detected by Professor Perrine in 1904-05. The distances of the planets are visibly regulated by a method. They increase by an ordered progression, announced by Titius of Wittenberg in 1772, and since designated as "Bode's Law". But their succession was quickly seen to be interrupted by a huge gap between the orbits of Mars and Jupiter; and the conjecture was hazarded that here a new planet might be found to revolve. It was verified by the discovery of an army of asteroids. Ceres, their leader, was captured at Palermo, January 1, 1801, by Giuseppe Piazzi, a Theatine monk (1746-1826); Pallas, in 1802 by Olbers (1758-1840), and Juno and Vesta in 1804 and 1807, by Harding and Olbers respectively. The original quartette of minor planets began in 1845 to be reinforced with companions, the known number of which now approximates to 600, and may be indefinitely increased. Their discovery has been immensely facilitated by Professor Max Wolf's introduction, in 1891, of the photographic method of discriminating them from stars through the effects of their motion on sensitive plates.

The solar system, as at present known, consists of four interior planets, Mercury, Venus, the Earth, and Mars; four exterior, and relatively colossal planets, Jupiter, Saturn, Uranus, and Neptune, the diffuse crowd of pygmy globes called asteroids, or minor planets, and an outlying array of comets with their attendant meteor-systems. All the planets rotate on their axes, though in very different periods. That of Mercury was determined by Signor Schiaparelli of Milan in 1889 to be 88 days, the identical time of his revolution round the sun, and Venus was, in the following year, shown by him to be, in all likelihood, similarly conditioned, the common period of rotation and circulation being, in her case, 225 days. This implies that both planets keep the same hemisphere always turned towards the sun, as the moon does towards the earth; nor can we doubt that the friction of tidal waves was, on the three bodies, the agency by which the observed synchronism was brought about. All the planets travel round the sun from west to east, or counter clockwise, and most of the satellites move in the same direction round their primaries. But there are exceptions. Phoebe, Saturn's remotest moon, circulates oppositely to the other members of the system; the four moons of Uranus are retrograde, their plane of movement being inclined at more than a right angle to the ecliptic; and the satellite of Neptune travels quite definitely backward. These anomalies are of profound import to theories of planetary origin. The "canals" of Mars were recognized by Schiaparelli in August, 1877, and he caught sight of some of them duplicated two years later. Their photographic registration at the Lowell observatory in 1905 proves them to be no optical illusion, but their nature remains enigmatical.

COMETS AND METEORS.—The predicted return of Halley's comet in 1759 afforded the first proof that bodies of the kind are permanently attached to the sun. They accompany its march through space, traversing, in either direction indifferently, highly eccentric orbits inclined at all possible angles to the ecliptic. They are accordingly subject to violent, even subversive disturbances from the great planets. Jupiter, in particular, sways the movements of a group of over thirty "captured" comets, which have had their periods curtailed, and their primitive velocities reduced by his influence. Schiaparelli announced in 1866 that the August shooting-stars, or Perseids, pursue the same orbit with a bright comet visible in 1862; and equally striking accordances of movement between three other comets and the Leonid, Lyraid, and Andromede meteor-swarms were soon afterwards established by Leverrier and Weiss. The obvious inference is that meteors are the disintegration-products of their cometary fellow-travellers. A theory of comets' tails, based upon the varying efficacy of electrical repulsion upon chemically different kinds of matter, was announced by Theodor Bredikhine of Moscow in 1882, and gave a satisfactory account of the appearances it was invented to explain. Latterly, however, the authority of Arrhenius of Stockholm has lent vogue to a "light-pressure" hypothesis, according to which, cometary appendages are formed of particles driven from the sun by the mechanical stress of his radiations. But the singular and rapid changes photographically disclosed as taking place in the tails of comets, remain unassociated with any known cause.

SIDEREAL ASTRONOMY.—Sir William Herschel's discovery, in 1802, of binary stars, imperfectly anticipated by Father Christian Mayer in 1778, was one of far-reaching scope. It virtually proved the realm of gravity to include sidereal regions; and the relations it intimated have since proved to be much more widely prevalent than could have been imagined beforehand. Mutually circling stars exist in such profusion as probably to amount to one in three or four of those unaccompanied. They are of limitless variety, some of the systems formed by them being exceedingly close and rapid, while others describe, in millennial periods, vastly extended orbits. Many, too, comprise three or more members; and the multiple stars thus constituted merge, by progressive increments of complexity, into actual clusters, globular and irregular. The latter class is exemplified by the Pleiades and the Hyades, by the Beehive cluster in Cancer, just visible to the naked eye, and by the double cluster in Perseus, which makes a splendid show with an opera-glass. Globular clusters are compressed "balls" of minute stars, of which more than one hundred have been catalogued. The scale on which these marvellous systems are constructed remains conjectural, since their distances from the earth are entirely unknown. Variable stars are met with in the utmost diversity. Some are temporary apparitions, which spring up from invisibility often to an astonishing pitch of splendor, then sink back more slowly to quasiextinction. Nova Persei, which blazed February 22, 1901, and was photographically studied by Father Sidgreaves at Stonyhurst, is the most noteworthy recent instance of the phenomenon. Stars, the vicissitudes of which are comprised in cycles of seven to twenty months, or more, are called "long-period variables". About 400 had been recorded down to 1906. They not uncommonly attain, at maximum, to 1,000 times their minimum brightness. Mira, the "wonderful" star in the Whale, discovered by David Fabricius in 1596, is the exemplar of the class. The fluctuations of "short-period variables" take place in a few days or hours, and with far more punctuality. A certain proportion of them are "eclipsing stars" (about 35 have so far been recognized as such), which owe their regularly recurring failures of light to the interposition of large satellites. Algol in Perseus, the variations of which were perceived by Montanari in 1669, is the best-known specimen. Hundreds of rapid variables have been recently detected among the components of globular clusters; but their course of change is of a totally different nature from that of eclipsing stars. Edmund Halley (1656-1742), the second Astronomer Royal, announced in 1718 that the stars, far from being fixed, move onward, each on its own account, across the sky. He arrived at this conclusion by comparing modern with antique observations; and stellar "proper motions" now constitute a wide and expansive field of research. A preliminary attempt to regularize them was made by Herschel's determination, in 1783, of the sun's line of travel. His success depended upon the fact that the apparent displacements of the stars include a common element, transferred by perspective from the solar advance. Their individual, or "peculiar" movements, however, show no certain trace of method. A good many stars, too, have been ascertained to travel at rates probably uncontrollable by the gravitational power of the entire sidereal system. Arcturus, with its portentous velocity of 250 miles a second, is one of these "runaway" stars. The sun's pace of about 12 miles a second, seems, by comparison, extremely sedate; and it is probably only half the average stellar speed. The apex of the sun's way, or the point towards which its movement at present tends, is located by the best recent investigations near the bright star Vega.

DISTANCES OF THE SUN AND STARS.—The distances of the heavenly bodies can only be determined (speaking generally) by measuring their parallaxes, in other words, their apparent changes of position when seen from different points of view. That of the sun is simply the angle subtended at his distance by the earth's semidiameter. Efforts were made with indifferent success to fix its value by the aid of the transits of Venus in the eighteenth and nineteenth centuries. The asteroids have proved more efficient auxiliaries; and through the mediation of Iris, Sappho, and Victoria, in 1888-89, Sir David Gill assigned to the great unit of space a length of 92,800,000 miles, which the photographic measures of Eros, in 1900-01, bid fair to ratify. The stars, however, are so vastly remote that the only chance of detecting their perspective displacements is by observing them at intervals of six months, from opposite extremities of a baseline nearly 186,000 miles in extent. Thus, the annual parallax of a star means the angle under which the semidiameter of the earth's orbit would be seen if viewed from its situation. This angle is in all cases, extremely minute, and in most cases, altogether evanescent; so that, from only about eighty stars (as at present known), the terrestrial orbit would appear to have sensible dimensions. Our nearest stellar neighbor is the splendid southern binary, a Centauri; yet its distance is such that light needs four and one-third years to perform the journey thence. Thomas Henderson (1798-1844) announced his detection of its parallax in 1839, just after Bessel of Konigsberg (1784-1846) had obtained a similar, but smaller result for an insignificant double star designated 61 Cygni.

CELESTIAL PHOTOGRAPHY.—The second half of the nineteenth century was signalized by a revolutionary change in the methods and purposes of astronomy. Experiments in lunar photography, begun in 1840 by J. W. Draper of New York, were continued in the fifties by W. C. Bond, Warren de la Rue, and Lewis M. Rutherfurd. The first daguerreotype of the sun was secured at Paris in 1845, and traces of the solar corona appeared on a sensitized plate exposed at Konigsberg during the total eclipse of July 28, 1851. But the epoch of effective solar photography opened with the Spanish eclipse of July 18, 1860, when the pictures successively obtained by Father Angelo Secchi, S.J., and Warren de la Rue demonstrated the solar status of the crimson protuberances by rendering manifest the advance of the moon in front of them. At subsequent eclipses, the leading task of the camera has been the portrayal of the corona; and its importance was enhanced when A. C. Ranyard pointed out, in 1879, the correspondence of changes in its form with the alternations of solar disturbance. The eleven-year periodicity of sunspots was published in 1851 by Schwabe of Dessau; and among the numerous associated phenomena of change, none are better ascertained than those affecting the shape of the silvery aureola seen to encompass the sun when the moon cuts off the glare of direct sunlight. At spot maxima the aureola spreads its beamy radiance round the disc. But at times of minimum, it consists mainly of two great wings, extended in the sun's equatorial plane. A multitude of photographs, taken during the eclipses of 1898, 1900, 1901, and 1905, attest with certainty the punctual recurrence of these unexplained vicissitudes. The fundamental condition for the progress of sidereal photography is the use of long exposures; since most of the objects to be delineated emit light so feebly that its chemical effects must accumulate before they become sensible. But long exposures were impracticable until Sir William Huggins, in 1876, adopted the dry-plate process; and this date, accordingly, marks the beginning of the wide-spreading serviceableness of the camera to astronomy. In nebular investigations above all, it far outranges the telescope, Halley described in 1716 six nebulae, which he held to be composed of a lucid medium collected from space. The Abbe Lacaille (1713-62) brought back with him from the Cape, in 1754, a list of forty-two such objects; and Charles Messier (1730-1817) enumerated, in 1781, 103 nebulae and clusters. But this harvest was scanty indeed compared with the lavish yield of Herschel's explorations. Between 1786 and 1802 he communicated to the Royal Society catalogues of 2,500 nebulae; he distinguished their special forms, classified them in order of brightness, and elaborated a theory of stellar development from nebulae, illustrated by selected instances of progressive condensation. The next considerable step towards a closer acquaintance with nebulie was made by Lord Rosse in 1845, when the prodigious light-grasp of his six-foot reflector afforded him the discovery of the great "Whirlpool" structure in Canes Venatici. It proved to be typical of the entire class of spiral nebulie, the large prevalence of which has been one of the revelations of photography. The superiority in nebula-portraiture of the chemical to the eye-and-hand method was strikingly manifested in a photograph of the Orion nebula taken by Dr. A. A. Common, January 30, 1883. Its efficacy for discovery became evident through the disclosure, on plates exposed by Paul and Prosper Henry, and by Isaac Roberts in 1885-86, of complex nebulous formations in the Pleiades, almost wholly invisible optically. Professor Keeler (1857-1900) estimated at 120,000 the number of nebulae which the Crossley reflector of the Lick observatory would be capable of recording in both hemispheres with an hour's exposure, while telescopically constructed catalogues include less than 10,000. But it is through the combination of photography with spectroscopy, constituting the spectrographic mode of research, that astrophysics has achieved its most signal triumphs.

ASTROPHYSICS.—The fundamental principle of spectrum analysis, enunciated by Gustav Kirchhoff (1824-87) in 1859, depends upon the equivalence of emission and absorption. This means that, if white light be transmitted through glowing vapours, they arrest just those minute sections of it with which they themselves shine. And if the source of the white light be hotter than the arresting vapour, there results a prismatic spectrum, interrupted by dark lines, distinctive of the chemical nature of the substance originating them. Now this is exactly the case of the sun and stars. The white radiance emanating from their photospheres is found, when dispersed into a spectrum, to be crossed by numerous dusky rays indicating absorption by gaseous strata, to the composition of which Kirchhoff's principle supplies the clue. Kirchhoff himself identified in 1861, as prominent solar constituents, sodium, iron magnesium, calcium, and chromium; hydrogen was recognized by A. J. Angstrom (1814-74); helium by Sir Norman Lockyer in 1868; and about forty elementary substances are now known with approximate certainty to be common to the earth and sun. The chemistry of the stars is strictly analogous to that of the sun, although their spectra exhibit diversities symptomatic of a considerable variety in physical state. Father Angelo Secchi, S.J. (1818-78), based on these diversities in 1863-67 a classification of the stars into four orders, still regarded as fundamental, and supplied by Dr. Vogel in 1874 with an evolutionary interpretation, according to which, differences of spectral type are associated with various stages of progress from a tenuous and inchoate towards a compact condition. Since 1879, when Sir William Huggins secured impressions of an extended range of ultraviolet white star light, stellar spectra have been mostly studied photographically, the results being, not only precise and permanent, but also more complete than those obtainable by visual means. The same eminent investigator discovered, in 1864, the bright-line spectra of certain classes of nebulae, by which they were known to be of gaseous composition, and recognized, as of carbonaceous origin, the typical colored bands of the cometary spectrum, noted four years previously, though without specific identification, by G. B. Donati (1827-73) at Florence.

Doppler's principle, by which light alters in refrangibility through the end-on motion of its source, was first made effective for astronomical research by Huggins in 1868. The criterion of velocity, whether of recession or approach, is afforded by the shifting of spectral lines from their standard places; and the method was raised to a high grade of accuracy through Dr. Vogel's adaptation, in 1888, of photography to its requirements. It has since proved extraordinarily fruitful. Its employment enabled Dr. Vogel to demonstrate the reality of Algol's eclipses, by showing that the star revolved round an obscure companion in the identical period of light-change; and the first discoveries of non-eclipsing spectroscopic binaries were made at Harvard College in 1889. These interesting systems cannot be sharply distinguished from telescopic double stars, which are, indeed, believed to have developed from them under the influence of tidal friction; their periods vary from a few hours to several months; and their components are often of such unequal luminosity that only one leaves any legible impression on the sensitive plate. Their known number amounted, in 1905, to 140; and it may be indefinitely augmented. It probably includes all short-period variables, even those that escape eclipses; though the connection between their duplicity and luminous variations remains unexplained. The photography in daylight of solar prominences was attempted by Professor Young of Princeton in 1870, and the subject was prosecuted by Dr. Braun, S.J., in 1872. No genuine success was, however, achieved until 1891, when Professor Hale of Chicago and M. Deslandres at Paris independently built up pictures of those objects out of the calcium-ray in their dispersed light, sifted through a double slit on to moving photographic plates. Professor Hale's invention of the "spectroheliograph" enables him, moreover, to delineate the sun's disc in any selected quality of its light, with the result of disclosing vast masses of calcium and hydrogen flocculi, piled up at various heights above the solar surface.

SIDEREAL CONSTRUCTION.—The investigation of the structure of the sidereal heavens was the leading object of William Herschel's career. The magnitude of the task, however, which he attempted single-handed grows more apparent with every fresh attempt to grapple with it; and it now engages the combined efforts of many astronomers, using methods refined and comprehensive to a degree unimagined by Herschel. An immense stock of materials for the purpose will be provided by the international photographic survey, at present advancing towards completion at eighteen observatories in both hemispheres. About thirty million stars will, it is estimated, appear on the chart-plates; and those precisely catalogued are unlikely to fall short of four millions. The labor of discussing these multitudinous data must be severe, but will be animated by the hope of laying bare some hidden springs of the sidereal mechanism. The prospect is indeed remote that the whole of its intricacies will ever be penetrated by science. We only perceive that the stars form a collection of prodigious, but limited, extent, showing strongly concentrative tendencies towards the plane of the Milky Way. Nor can the nebula be supposed to form a separate scheme. The closeness of their relations, physical and geometrical, with stars excludes that supposition. Stars and nebula belong to the same system, if such the sidereal world may properly be called in the absence of any sufficient evidence of its being in a state of dynamical equilibrium. We cannot be sure that it has yet reached the definitive term appointed for it by its Creator. Suggestive hints, on the contrary, of instability and evanescence help us to realize that the heavens are, in very truth, the changing vesture of Him whose "years cannot fail."

ASTRONOMY IN THE BIBLE.—No systematic observations of the heavenly bodies were made by the Jews. Astral worship was rife in Palestine, and they could hardly have attended closely to its objects without yielding to its seductions. Astronomy was, under these circumstances, inseparable from astrolatry, and the anathemas of the prophets were not carelessly uttered. As the most glorious works of the Almighty, the celestial luminaries were indeed celebrated in the Scriptures in passages thrilling with rapture; but the appeal to them for practical purposes was reduced to a minimum. Even the regulation of times and seasons was largely empirical. The Jews used a lunar year. It began, for religious purposes, with the new moon next after the spring equinox, and consisted normally of twelve months, or 354 days. The Jewish calendar, however, depended upon the course of the sun, since the festivals it appointed were in part agricultural celebrations. Some process of adjustment had then to be resorted to, and the obvious one was chosen of adding a thirteenth, or intercalary, month whenever the discrepancy between the ripening of the crops and the fixed dates of the commemorative feasts became glaringly apparent. Before the time of Solomon, the Jews appear to have begun their year in the autumn; and the custom, revived for civil purposes about the fifth century B.C., was adopted in the systematized religious calendar of the fourth century of our era.

Both the ritual and the civil day commenced in the evening, about half an hour after sunset. Its subdivisions were left indeterminate. The Old Testament makes no mention of what we call hours; and it refers to the measurement of time, if at all, only in the narrative of the miracle wrought by Isaias in connection with the sundial of Achaz (IV Kings, xx, 9-11). In the New Testament, the Roman practice of counting four night-watches has superseded the antique triple division, and the day, as among the Greeks, consists of twelve equal parts. These are the "temporary hours" which still survive in the liturgy of the Church. Since they spanned the interval from sunrise to sunset, their length varied with the season of the year, from 49 to 71 minutes. Corresponding nocturnal hours, too, seem to have been partially used in the time of the Apostles (Acts, xxiii, 23).

As might have been expected, the Sacred Books convey no theory of celestial appearances. The descriptive phrases used in them are conformed to the elementary ideas naturally presenting themselves to a primitive people. Thus, the earth figures as an indefinitely extended circular disk, lying between the realm of light above and the abyss of darkness beneath. The word firmamentum, by which the Hebrew rakia is translated in the Vulgate, expressed the notion of a solid, transparent vault, dividing the "upper waters" from the seas, springs, and rivers far below. Through the agency of the floodgates, however, the waters sustained by the firmament were, in due measure, distributed over the earth. The first visibility after sunset of the crescent moon determined the beginning of each month; and this was the only appeal to the skies made for the purposes of the Jewish ritual. Eclipses of the sun and moon are perhaps vaguely referred to among the signs of doom enumerated by the Prophets Joel and Amos, who may easily have enhanced their imagery from personal experience, since modern calculations show solar totalities to have been visible in Palestine in the years 831, 824, and 763 B.C., and the moon reddened by immersion in the earth's shadow is not an uncommon sight in any part of the world. But the passages in question cannot be literally associated with mere passing phenomena. The prophets aimed at something higher than intimidation. An express warning against ignoble panic was indeed uttered by Jeremias in the words: "Be not afraid of the signs of heaven, which the heathens fear" (x, 2). The stellar vault, conceived to be situated above the firmament, is compared by Isaias to a tent stretched out by the Most High. The "host of heaven", a frequently recurring Scriptural expression, has both a general and a specific meaning. It designates, in some passages, the entire array of stars; in others it particularly applies to the sun, moon, planets, and certain selected stars, the worship of which was introduced from Babylonia under the later kings of Israel. Venus and Saturn are the only planets expressly mentioned in the Old Testament. Isaias (xiv, 12) apostrophizes the Babylonian Empire under the unmistakable type of Helal (Lucifer in the Vulgate), "son of the morning"; and Saturn is no less certainly represented by the star Kaiwan, adored by the reprobate Israelites in the desert (Amos, v, 26). The same word (interpreted to mean "steadfast") frequently designates, in the Babylonian inscriptions, the slowest-moving planet; while $akkuth, the divinity associated with the star by the prophet, is an alternative appellation for Ninib, who, as a Babylonian planet-god, was merged with Saturn. The ancient Syrians and Arabs, too, called Saturn Kaiwan, the corresponding term in the Zoroastrian Bundahish being Kevan. The other planets are individualized in the Bible only by implication. The worship of gods connected with them is denounced, but without any manifest intention of referring to the heavenly bodies. Thus, Gad and Meni (Isaias, lxv, 11) are, no doubt, the "greater and the lesser Fortune" typified throughout the East by Jupiter and Venus; Neba, the tutelary deity of Borsippa (Isaias, xlvi, 1), shone in the sky as Mercury, and Nergal, transplanted from Assyria to Kutha (IV Kings, xvii, 30), as Mars.

The uranography of the Jews is fraught with perplexity. Some half-dozen star-groups are named in the Scriptures, but authorities differ widely as to their identity. In a striking passage the Prophet Amos (v, 8) glorifies the Creator as "Him that made Kimah and Kesil", rendered in the Vulgate as Arcturus and Orion. Now Kimah certainly does not mean Arcturus. The word, which occurs twice in the Book of Job (ix, 9; xxxviii, 31), is treated in the Septuagint version as equivalent to Pleiades. This, also, is the meaning given to it in the Talmud and throughout Syrian literature; it is supported by etymological evidences, the Hebrew term being obviously related to the Arabic root kum (accumulate), and to the Assyrian kamu (to bind); while the "chains of Kimah" referred to in the sacred text, not inaptly figure the coercive power imparting unity to a multiple object. The associated constellation Kesil is doubtless no other than our Orion. Yet, in the first of the passages in Job where it figures, the Septuagint gives Herper; in the second, the Vulgate quite irrelevantly inserts Arcturus; Karstens Niebuhr (1733-1815) understood Kesil to mean Sirius; Thomas Hyde (1636-1703) held that it indicated Canopus. Now kesil signifies in Hebrew "foolish", or "impious", adjectives expressive of the stupid criminality which belongs to the legendary character of giants; and the stars of Orion irresistibly suggest a huge figure striding across the sky. The Arabs accordingly named the constellation Al-gebbar, "the giant", the Syriac equivalent being Gabbara, "a strong man"; and Kesil is actually translated Gabbara in the old Syriac version of the Bible known as the Peshitta. We may then safely admit that Kimah and Kesil did actually designate the Pleiades and Orion. But further interpretations are considerably more obscure. In the Book of Job—the most distinctively astronomical part of the Bible—mention is made, with other stars, of Ash and Ayish, almost certainly divergent forms of the same word. Its signification remains an enigma. The Vulgate and Septuagint inconsistently render it "Arcturus" and "Hesperus". Abenezra (1092-1167), however, the learned Rabbi of Toledo, gave such strong reasons for holding Ash, or Ayish, to mean the Great Bear, that the opinion, though probably erroneous, is still prevalent. It was chiefly grounded on the phonetic resemblance between ash and the Arabic na 'ash, "a bier", applied to the four stars of the Wain, the three in front figuring as mourners, under the title of Bendt na 'ash, "daughters of the bier". But Job, too, speaks of the "children of Ayish", and the inference seems irresistible that the same star-group was similarly referred to in both cases. Yet there is large room for doubt. Modern philologists do not admit the alleged connection of Ayish with na 'ash, nor is any funereal association apparent in the Book of Job. On the other hand, Professor Schiaparelli draws attention to the fact that ash denotes "moth" in the Old Testament, and that the folded wings of the insect are closely imitated in their triangular shape by the doubly aligned stars of the Hyades. Now Ayish in the Peshitta is translated Iyutha, a constellation mentioned by St. Ephrem and other Syriac writers, and Schiaparelli's learned consideration of the various indications afforded by Arabic and Syriac literature makes it reasonably certain that Iyutha authentically signifies Aldebaran, the great red star in the head of the Bull, with its children, the rainy Hyades. It is true that Hyde, Ewald, and other scholars have adopted Capella and the Kids as representative of Iyutha, and therefore of "Ayish and her children"; but the view involves many incongruities. The glories of the sky adverted to in the Book of Job include a sidereal landscape vaguely described as "the chambers [i. e. penetralia] of the south". The phrase, according to Schiaparelli, refers to some assemblage of brilliant stars, rising 20 degrees at most above the southern horizon in Palestine about the year 750 B.C. (assumed as the date of the Patriarch Job), and, taking account of the changes due to precession, he points out that the stellar pageant formed by the Ship, the Cross, and the Centaur meets the required conditions. Sirius, although at the date in question it culminated at an altitude of 41 degrees, may possibly have been thought of as belonging to the "chambers of the south"; otherwise, this splendid object would appear to be ignored in the Bible. Job opposes to the "chambers of the south", as the source of cold, an asterism named Mezarim (xxxvii, 9). Both the Vulgate and the Septuagint render this word by Arcturus, evidently in mistake (the blunder is not uncommon) for Arctos. The Great Bear circled in those days much more closely round the pole than it now does; its typical northern character survives in the Latin word septentrio (from septem triones, the seven stars of the Wain); and Schiaparelli concludes, from the dual form of mezarim, that the Jews, like the Phoenicians, were acquainted with the Little, as well as with the Great, Bear. He identifies the word as the plural, or dual, of mizreh, "a winnowing-fan", an instrument figured by the seven stars of the Wain, quite as accurately as the Ladle of the Chinese or the Dipper of popular American parlance.

Perhaps the most baffling riddle in Biblical star-nomenclature is that presented by the word Mazzaroth, or Mazzaloth (Job, xxxviii, 31, 32; IV Kings, xxiii, 5), usually, though not unanimously, admitted to be phonetic variants. As to their signification, opinions are hopelessly divergent. The authors of the Septuagint transcribed, without translating, the ambiguous expression; the Vulgate gives for its equivalent Lucifer in Job, the Signs of the Zodiac in the Book of Kings. St. John Chrysostom adopted the latter meaning, noting, however, that many of his contemporaries interpreted Mazzaroth as Sirius. But this idea soon lost vogue, while the zodiacal explanation gained wide currency. It is, indeed, at first sight, extremely plausible. Long before the Exodus the Twelve Signs were established in Euphratean regions much as we know them now. Although never worshipped in a primary sense, they may well have been held sacred as the abodes of deities. The Assyrian manzaltu (sometimes written manzazu), "station", occurs in the Babylonian Creation tablets with the import "mansions of the gods"; and the word appears to be etymologically akin to Mazzaloth, which in rabbinical Hebrew signifies primarily the Signs of the Zodiac, secondarily the planets. The lunar Zodiac, too, suggests itself in this connection. The twenty-eight "mansions of the moon" (menazil al-kamar) were the leading feature of Arabic skylore, and they subserved astrological purposes among many Oriental peoples. They might, accordingly, have belonged to the apparatus of superstition used by the soothsayers who were extirpated in Judah, together with the worship of the Mazzaroth, by King Josias, about 621 B.C. Yet no such explanation can be made to fit in with the form of expression met with in the Book of Job (xxxviii, 32). Speaking in the person of the Almighty, the Patriarch asks, "Canst thou bring forth Mazzaroth in its time?"—clearly in allusion to a periodical phenomenon, such as the brilliant visibility of Lucifer, or Hesperus. Professor Schiaparelli then recurs to the Vulgate rendering of this passage. He recognizes in Mazzaroth the planet Venus in her double aspect of morning and evening star, pointing out that the luminary designated in the Book of Kings, with the sun and moon, and the "host of heaven", must evidently be next in brightness to the chief light-givers. Further, the sun, moon, and Venus constitute the great astronomical triad of Babylonia, the sculptured representations of which frequently include the "host of heaven" typified by a crowd of fantastic animal-divinities. And since the astral worship anathematized by the prophets of Israel was unquestionably of Euphratean origin, the designation of Mazzaroth as the third member of the Babylonian triad is a valuable link in the evidence. Still, the case remains one of extreme difficulty. Notwithstanding the scepticism of recent commentators, it appears fairly certain that the "fugitive serpent" of Job, xxvi, 13 (coluber tortuosus in the Vulgate) does really stand for the circumpolar reptile. The Euphratean constellation Draco is of hoary antiquity, and would quite probably have been familiar to Job. On the other hand, Rahab (Job, ix, 13; xxvi, 12), translated "whale" in the Septuagint, is probably of legendary or symbolical import.

The subjoined list gives (largely on Schiaparelli's authority) the best-warranted interpretations of biblical star-names: Kimah, the Pleiades; Kesil, Orion; Ash, or Ayish, the Hyades; Mezarim, the Bears (Great and Little); Mazzaroth, Venus (Lucifer and Hesperus); Hadre theman—" the chambers of the south "—Canopus, the Southern Cross, and a Centauri; Nachash, Draco.

The New Testament is virtually devoid of astronomical allusions. The "Star of the Magi" can scarcely be regarded as an objective phenomenon; it was, at least, inconspicuous to ordinary notice. Kepler, however, advanced, in 1606, the hypothesis that a remarkable conjunction of Jupiter and Saturn, which occurred in May of the year 7 B.C., was the celestial sign followed by the Wise Men. Revived in 1821 by Dr. Minter, the Lutheran Bishop of Zealand, this opinion was strongly advocated in 1826 by C. L. Ideler (Handbuch der Chronologie, II, 399). But the late Dr. Pritchard's investigation (Smith's Dict. of the Bible, Memoirs Roy. Astr. Society, XXV, 119) demonstrated its inadequacy to fulfil the requirements of the Gospel narrative.


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