ASTRONOMY OF THE ANCIENTS China.—Already, in the third millennium B.C., equinoxes and solstices were determined in China by means of culminating stars. This is known from the orders promulgated by the emperor Yao about 2300 B.C., as recorded in the the Shit King, a collection of documents antique in the time of Confucius (550-478 B.c.). And Yao was merely the renovator of a system long previously es tablished. The Shit King further relates the tragic fate of the official astronomers, Hsi and Ho, put to death for neglecting to perform the rites customary during an eclipse of the sun, identi fied by Professor S. E. Russell with a partial obscuration visible in northern China 2136 B.C. The date cannot be far wrong, and it is by far the earliest assignable to an event of the kind. There is, however, no certainty that the Chinese were then capable of predicting eclipses. They were, on the other hand, probably ac quainted, a couple of millenniums before Meton gave it his name, with the nineteen-year cycle, by which solar and lunar years were harmonized ; they immemorially made observations in the merid ian; regulated time by water-clocks, and used measuring instru ments of the nature of armillary spheres and quadrants. In or near i zoo B.C., Chou Kung, an able mathematician, determined with surprising accuracy the obliquity of the ecliptic; but his at tempts to estimate the sun's distance failed hopelessly as being grounded on belief in the flatness of the earth. From of old, in China, circles were divided into 365+ parts, so that the sun described daily one Chinese degree; and the equator began to be employed as a line of reference, concurrently with the ecliptic, probably in the second century B.C. Both circles, too, were mark ed by star-groups more or less clearly designated and defined. Cometary records of a vague kind go back in China to 2 296 B.C. ; they are intelligible and trustworthy from 611 B.C. onward. Two instruments constructed at the time of Kublai Khan's accession in 128o were still extant at Peking in 1881. They were provided with large graduated circles adapted for measurements of decli nation and right ascension, and prove the Chinese to have an ticipated by at least three centuries some of Tycho Brahe's most important inventions. The native astronomy was finally super seded in the r 7th century by the scientific teachings of Jesuit missionaries from Europe.
In the course of ages, Babylonian astronomy, purified from the astrological taint, adapted itself to meet the most refined needs of civil life. The decipherment and interpretation by the learned Jesuits, Fathers Epping and Strassmeier, of a number of clay tablets preserved in the British Museum, have supplied detailed knowledge of the methods practised in Mesopotamia in the 2nd century B.C. They show no trace of Greek influence, and were doubtless the improved outcome of an unbroken tra dition. How protracted it had been, can be in a measure esti mated from the length of the revolutionary cycles found for the planets. The Babylonian computers were not only aware that Venus returns in almost exactly eight years to a given starting point in the sky, but they had established similar periodic re lations in 46, 59, 79 and 83 years severally for Mercury, Saturn, Mars and Jupiter. They were accordingly able to fix in advance the approximate positions of these objects with reference to ecliptical stars which served as fiducial points for their deter mination. In the Ephemerides published year by year, the times of new moon were given, together with the calculated intervals to the first visibility of the crescent, from which the beginning of each month was reckoned; the dates and circumstances of solar and lunar eclipses were predicted; and due information was supplied as to the forthcoming heliacal risings and settings, con junctions and oppositions of the planets. The Babylonians knew of the inequality in the daily motion of the sun, but misplaced by io° the perigee of his orbit. Their sidereal year was 41m too long, and they kept the ecliptic stationary among the stars, mak ing no allowance for the shifting of the equinoxes. The striking dis covery, on the other hand, has been made by the Rev. F. X. Kug ler that the various periods underlying their lunar predictions were identical with those heretofore believed to have been reached independently by Hipparchus, who accordingly must be held to have borrowed from Chaldaea the lengths of the synodic, sidereal, anomalistic and draconitic months.
Greece.--A steady flow of knowledge from East to West be gan in the seventh century B.C. A Babylonian sage founded a school about 64o B.C. in the island of Cos, and perhaps counted Thales of Miletus (c. 639-548) among his pupils. The famous "eclipse of Thales" in 585 B.C. has not, it is true, been au thenticated by modern research; yet the story told by Herodotus appears to intimate that a knowledge of the Saros, and of the fore casting facilities connected with it, was possessed by the Ionian sage. Pythagoras of Samos (fl. 540-510 B.c.) learned on his travels in Egypt and the East to identify the morning and eve ning stars, to recognize the obliquity of the ecliptic, and to regard the earth as a sphere freely poised in space. The tenet of its axial movement was held by many of his followers—in an ob scure form by Philolaus of Crotona after the middle of the 5th century B.c., and more explicitly by Ecphantus and Hicetas of Syracuse (4th century B.c.), and by Heraclides of Pontus. Hera clides, who became a disciple of Plato in 36o B.C., taught in ad dition that the sun, while circulating round the earth, was the centre of revolution to Venus and Mercury. A genuine heliocen tric system, developed by Aristarchus of Samos (fl. 280-264 B.c.), was described by Archimedes in his Arenarius, only to be set aside with disapproval. The long-lived conception of a series of crystal spheres, acting as the vehicles of the heavenly bodies, and attuned to divine harmonies, seems to have originated with Pythagoras himself.
The first mathematical theory of celestial appearances was devised by Eudoxus of Cnidus (408-355 B.c.). The problem he attempted to solve was so to combine uniform circular move ments as to produce the resultant effects actually observed. The sun and moon and the five planets were, with this end in view, accommodated each with a set of variously revolving spheres, to the total number of 27. The Eudoxian or "homocentric" system, after it had been further elaborated by Callippus and Aristotle, was modified by Apollonius of Perga (fl. 250-220 B.c.) into the hypothesis of deferents and epicycles, which held the field for 1,80o years as the characteristic embodiment of Greek ideas in astronomy. Eudoxus further wrote two works descriptive of the heavens, the Enoptron and Phaenomena, which, substantially pre served in the Phaenomena of Aratus (fl. 27o B.c.), provided all the leading features of modern stellar nomenclature.
Greek astronomy culminated in the school of Alexandria. It was, soon after its foundation, illustrated by the labours of Aris tyllus and Timocharis (c. 320-260 B.C.), who constructed the first catalogue giving star-positions as measured from a ref erence point in the sky. This fundamental advance rendered inevitable the detection of precessional effects. Aristarchus of Samos ob served at Alexandria 280-264 B.C. His treatise on the magnitudes and distances of the sun and moon, edited by John Wallis in 1688, describes a theoretically valid method for determining the relative distances of the sun and moon by measuring the angle between their centres when half the lunar disc is illuminated; but the time of dichotomy being widely indeterminate, no useful result was thus obtainable. Aristarchus in fact concluded the sun to be not more than twenty times, while it is really four hundred times farther off than our satellite. His general conception of the universe was comprehensive beyond that of any of his predeces sors.
Eratosthenes (276-196 B.c.) , a native of Cyrene, was sum moned from Athens to Alexandria by Ptolemy Euergetes to take charge of the royal library. He invented, or improved armillary spheres, the chief .implements of ancient astrometry, determined the obliquity of the ecliptic at 23° 51' (a value 5' too great), and. introduced an effective mode of arc-measurement. Knowing Alexandria and Syene to be situated 5,000 stadia apart on the same meridian, he found the sun to be 7° 12' south of the zenith at the northern extremity of this arc when it was vertically over head at the southern extremity, and he hence inferred a value of 252,000 stadia for the entire circumference of the globe. This is a very close approximation to the truth, if the length of the unit employed has been correctly assigned.
Among the astronomers of antiquity, two great men stand out with unchallenged pre-eminence. Hipparchus and Ptolemy enter tained the same large organic designs ; they worked on similar methods ; and, as the outcome, their performances fitted so ac curately together that between them they re-made celestial science. Hipparchus fixed the chief data of astronomy—the lengths of the tropical and sidereal years, of the various months, and of the synodic periods of the five planets ; determined the obliquity of the ecliptic and of the moon's path, the place of the sun's apogee, the eccentricity of his orbit, and the moon's hori zontal parallax; all with approximate accuracy. His borrowings from Chaldaean experts appear, indeed, to have been numerous; but were doubtless independently verified. His supreme merit, however, consisted in the establishment of astronomy on a sound geometrical basis. His acquaintance with trigonometry, a branch of science initiated by him, together with his invention of the planisphere, enabled him to solve a number of elementary prob lems ; and he was thus led to bestow especial attention upon the position of the equinox, as being the common point of origin for measures both in right ascension and longitude. Its steady retro gression among the stars became manifest to him in 130 B.C., on comparing his own observations with those made by Timo charis a century and a half earlier; and he estimated at not less than 36" (the true value being 50") the annual amount of "pre cession." The choice made by Hipparchus of the geocentric theory of the universe decided the future of Greek astronomy. He further elaborated it by the introduction of "eccentrics," which accounted for the changes in orbital velocity of the sun and moon by a dis placement of the earth, to a corresponding extent, from the centre of the circles they were assumed to describe. This gave the elliptic inequality known as the "equation of the centre," and no other was at that time obvious. He attempted no detailed dis cussion of planetary theory; but his catalogue of 1,080 stars, divided into six classes of brightness, or "magnitudes," is one of the finest monuments of antique astronomy. It is substantially embodied in Ptolemy's Almagest (see PTOLEMY).
An interval of 25o years elapsed before the constructive labours of Hipparchus obtained completion at Alexandria. His observa tions were largely, and somewhat arbitrarily, employed by Ptolemy. Professor Newcomb, who compiled a very instructive table of the equinoxes severally observed by Hipparchus and Ptolemy, with their errors deduced from Leverrier's solar tables, found palpable evidence that the discrepancies between the two series were artificially reconciled on the basis of a year 6m too long, adopted by Ptolemy on trust from his predecessor. He neverthe less held the process to have been one that implied no fraudu lent intention.
The Ptolemaic system was, in a geometrical sense, defensible; it harmonized fairly well with appearances, and physical reason ings had not then been extended to the heavens. To the ignorant it was recommended by its conformity to crude common sense; to the learned, by the wealth of ingenuity expended in bringing it to perfection. The Alntagest was the consummation of Greek astronomy. Ptolemy had no successor; he found only commen tators, among the more noteworthy of whom were Theon of Alexandria (fl. A.D. 40o) and his daughter, Hypatia (37o-415)• Arabia.—With the capture of Alexandria by Omar in 641, the last glimmer of its scientific light became extinct, to be re kindled, a century and a half later, on the banks of the Tigris. The first Arabic translation of the Almagest was made by order of Harun al-Rashid about the year Boo; others followed, and the Caliph al-Mamun built in 829 a grand observatory at Baghdad. Here Albumazar (805-885) watched the skies and cast horoscopes; here Tobit ben Korra (836-901) developed his long unquestioned, yet misleading theory of the "trepidation" of the equinoxes; Abd-ar-rahman al-Sufi (903-986) revised at first hand the cata logue of Ptolemy; and Abulwefa (939-998), like al-Sflfi, a native of Persia, made continuous planetary observations, but did not (as alleged by L. Sedillot) anticipate Tycho Brake's discovery of the moon's variation. Ibn Junis (c. 950-1008), although the scene of his activity was in Egypt, falls into line with the astrono mers of Baghdad. He compiled the Hakimite Tables of the planets, and observed at Cairo, in 977 and 978, two solar eclipses which, as being the first recorded with scientific accuracy, were made available in fixing the amount of lunar acceleration. Nasir ud din (1201-1274) drew up the Ilkhanic Tables, and determined the constant of precession at 51". He directed an observatory established by Hulagu Khan (d. 1265) at Maraga in Persia, and equipped with a mural quadrant of 12 f t. radius, besides altitude and azimuth instruments. Ulugh Beg , a grandson of Tamerlane, was the illustrious personification of Tatar astronomy. He founded about 142c) a splendid observatory at Samarkand, in which he re-determined nearly all Ptolemy's stars, while the Tables published by him held the primacy for two centuries.