the most celebrated astronomer of antiquity, was a native of Nicea, a city of Bithynia. We have no means of discovering the exact date of his birth or of his death, but we find from Ptolemy that his observations of the heavenly bodies extended from B.C. 162 to 127. His observations were principally taken in the island of Rhodes; but we know nothing of his private history, excepting that he was remarkable for unrestrained industry and great love of truth. He seems to have first directed his attention to the rising and setting of stars; and whilst engaged on this subject he composed a commentary on the work of Aratus, and discovered many mistakes, which the latter, or rather Eudoxus, whom he copied, had made in respect to the position of stars. It was not, however, till a much later period that he discovered that the aspect of the heavens had undergone a considerable change since the observations of Eudoxus had been made. His anxiety to arrive at the truth induced him to subject the whole science of astronomy as then known to the strictest examination; and with this view deemed it a matter of the utmost consequence to the success of his ulterior objects, that he should fix with precision the equinoctial and solstitial points, as well as the time which the sun took to pass from one of these points to the other, or, in other words, the exact length of the year. He compared his own observations with those which had been made before his time, and he discovered that the period of 365 days six hours, which had hitherto been considered as the true length of the solar year, was too great by about five minutes. He was the first to perceive that the stars appeared to have a movement parallel to the ecliptic, and he established this doctrine in a work on the retrogradation of the equinoctial points. It was impossible in those early times to calculate with precision the exact quantity of this movement, as he had no observations with which to compare his own except those of Timocharis and Aristillus, who had lived only a short time before. He imagined, however, that it could not be more than 36° a year, though it is now known that it is in reality 50°. This important discovery would have been sufficient to have immortalized his name, but he possesses many other titles to the admiration of posterity. He was the first to discover the means of determining the inequality of the movement of the sun, or what is called the apparent eccentricity of the solar orbit. He observed that the four parts into which the year is divided by the solstices and equinoxes are by no means equal, the sun occupying ninety-four days and a half in passing from the vernal equinox to the summer solstice, and only ninety-two and a half from the same solstice to the autumnal equinox. It was evident, therefore, that the sun remained 187 days in that part of the ecliptic which lies between the equator and the north pole, and only about 178 in the other part. The only way in which he could account for this apparent inequality of the sun's motion, was by supposing that the earth was not placed exactly in the centre of the circular orbit of the sun, and that his distance from the earth is therefore subject to variation. The distance of the earth from the centre of the orbit is called the eccentricity; and it produces an equation between the real and apparent motions, which is called the equation of the centre. Hipparchus determined the magnitude of this equation in terms of the radius of the ecliptic, and fixed the position of the line of the apsides, or that which joins the two opposite points of the orbit at the greatest and least distance from the earth. With these data he constructed the first tables of the sun which are mentioned in the history of astronomy. With equal industry and acuteness Hipparchus laboured to bring the still more difficult theory of the moon to a more scientific form. By comparing a great number of observations of eclipses recorded by ancient writers with his own, he was enabled to determine more accurately the period of the moon's revolution relatively to the stars, to the sun, to her nodes, and to her apogee. He calculated with remarkable precision, for the time when he lived, the distance of the moon from the earth, as well as the size of the sun and moon.

The appearance of a new star in the heavens induced Hipparchus to undertake the formation of a catalogue of all the stars visible above his horizon, to fix their relative positions, and to mark their configurations, in order that posterity might be able to detect any changes which might take place in the aspect of the heavens. His catalogue contained 1080 stars. He also invented the planisphere, or method of representing the starry firmament on a plane surface, which afforded the means of solving the problems of spherical trigonometry in a manner often more exact and more commodious than the globe itself. He was the first who demonstrated the methods of calculating triangles, whether rectilineal or spherical; and he constructed a table of chords, from which he drew nearly the same advantages as we derive at present from the tables of sines. Geography is also indebted to him for the happy idea of fixing the position of places on the earth by means of their latitudes and longitudes, and he was the first who determined longitude by the eclipses of the moon. His writings have all unfortunately perished, except a commentary on the poem of Aratus, and a work entitled Asterismi. They were first published at Florence, 1567, and afterwards by Petavius in his Uranologium, with a Latin translation, Paris, 1630. See Delambre, Histoire de l'Astronomie Ancienne, tom. i. p. xxi.; Marcz, Astronomie solaire d'Hipparque soumise à une critique rigoureuse, et ensuite rendue à sa vérité primordiale, Paris, 1828. The views of Marcz are ably controverted by Letronne in the Journal des Savans, 1828 and 1829, January; also by Littrow in the Vienna Jahrbuch, 1830, January and March.