CHAPTER IX. SCIENTIFIC KNOWLEDGE AMONG THE ROMANS.

Astronomical science received a great impulse from the school of Alexandria, and Eratosthenes had worthy successors in Aristarchus, Aristyllus, Apollonius. But the great light of this school was Hipparchus, whose lifetime extended from 190 to 120 years B.C. He laid the foundation of astronomy upon a scientific basis. "He determined," says Delambre, "the position of the stars by right ascensions and declinations; he was acquainted with the obliquity of the ecliptic. He determined the inequality of the sun, and the place of its apogee, as well as its mean motion; the mean motion of the moon, of its nodes and apogee; the equation of the moon's centre, and the inclination of its orbit; he likewise detected a second inequality, of which he could not, for want of proper observations, discover the period and the law. His commentary on Aratus shows that he had expounded, and given a geometrical demonstration of, the methods necessary to find out the right and oblique ascensions of the points of the ecliptic and of the stars, the east point and the culminating point of the ecliptic, and the angle of the east, which is now called the nonagesimal degree. He could calculate eclipses of the moon, and use them for the correction of his lunar tables, and he had an approximate knowledge of parallax." [Footnote: Delambre, Hist. de l'Astron. Anc., tom. i. p. 184.] His determination of the motions of the sun and moon, and method of predicting eclipses, evince great mathematical genius. But he combined, with this determination, a theory of epicycles and eccentrics, which modern astronomy discards. It was, however, a great thing to conceive of the earth as a solid sphere, and reduce the phenomena of the heavenly bodies to uniform motions in of circular orbits. "That Hipparchus should have succeeded in the first great steps of the resolution of the heavenly bodies into circular motions is a circumstance," says Whewell, "which gives him one of the most distinguished places in the roll of great astronomers." [Footnote: Hist. Ind. Science, vol. i. p. 181.] But he even did more than this. He discovered that apparent motion of the fixed stars round the axis of the ecliptic, which is called the Precession of the Equinoxes, one of the greatest discoveries in astronomy. He maintained that the precession was not greater than fifty- nine seconds, and not less than thirty-six seconds. Hipparchus framed a catalogue of the stars, and determined their places with reference to the ecliptic, by their latitudes and longitudes. Altogether, he seems to have been one of the greatest geniuses of antiquity, and his works imply a prodigious amount of calculation.

[Posidonius.]

[The Roman Calendar.]

Astronomy made no progress for three hundred years, although it was expounded by improved methods. Posidonius constructed an orrery, which exhibited the diurnal motions of the sun, moon, and five planets. Posidonius calculated the circumference of the earth to be two hundred and forty thousand stadia by a different method from Eratosthenes. The barrenness of discovery, from Hipparchus to Ptolemy, in spite of the patronage of the Ptolemies, was owing to the want of instruments for the accurate measure of time, like our clocks, to the imperfection of astronomical tables, and to the want of telescopes. Hence the great Greek astronomers were unable to realize their theories. Their theories were magnificent, and evinced great power of mathematical combination; but what could they do without that wondrous instrument by which the human eye indefinitely multiplies its power? - by which objects are distinctly seen, which, without it, would be invisible? Moreover, the ancients had no accurate almanacs, since the care of the calendar belonged to the priests rather than to the astronomers, who tampered with the computation of time for temporary and personal objects. The calendars of different communities differed. Hence Julius Caesar rendered a great service to science by the reform of the Roman calendar, which was exclusively under the control of the college of pontiffs. The Roman year consisted of three hundred and fifty-five days, and, in the time of Caesar, the calendar was in great confusion, being ninety days in advance, so that January was an autumn month. He inserted the regular intercalary month of twenty-three days, and two additional ones of sixty-seven days. These, together of ninety days, were added to three hundred and sixty-five days, making a year of transition of four hundred and forty-five days, by which January was brought back to the first month in the year after the winter solstice. And to prevent the repetition of the error, he directed that in future the year should consist of three hundred and sixty-five and one quarter days, which he effected by adding one day to the months of April, June, September, and November, and two days to the months of January, Sextilis, and December, making an addition of ten days to the old year of three hundred and fifty-five. And he provided for a uniform intercalation of one day in every fourth year, which accounted for the remaining quarter of a day. [Footnote: Suet., Caesar, 49; Plut.,Caesar, 59.]

  "Ille moras solis, quibus in sua signa rediret, 
  Traditur exactis disposuisse notis. 
  Is decies senos tercentum et quinque diebus 
  Junxit; et pleno tempora quarta die. 
  Hic anni modus est. In lustrum accedere debet 
  Quae consummatur partibus, una dies."