The first woman whose name has come down to us, who deserved to be regarded as an astronomer, was most probably Aglaonice, the daughter of Hegetoris of Thessaly. By means of the lunar cycle known as the Saros, a period discovered by the Chaldean astronomers and embracing a little more than eighteen years, during which the eclipses of the moon and sun recur in nearly the same order as during the preceding period, this Greek woman was able to predict eclipses. The people among whom she lived regarded her as a sorceress; but she flouted them all, and declared that she was able to make the sun and moon disappear at will. The first woman, however, to attain eminence as an astronomer was undoubtedly Hypatia, that universal genius of the ancient world, who seemed equally at home in literature, philosophy and mathematics, and who may justly be regarded as one of the most highly gifted women that has ever lived. In Alexandria, where she was born and lived, this accomplished daughter of Theon taught not only philosophy, but also algebra, geometry and astronomy. One of her pupils, Synesius, who became Bishop of Ptolemais, informs us that she was the inventor of two important astronomical instruments: an astrolabe and a planisphere. In addition to two mathematical works, a _Treatise on the Conics of Apollonius_ and a _Commentary on the Arithmetic of Diophantus_, which was in reality a treatise on algebra, she was the author of an _Astronomical Canon_, which contained tables regarding the movements of the heavenly bodies. It is generally supposed that this was an original work; but there are some who think it was but a commentary on the tables of Ptolemy. In this latter case Hypatia's work may still exist in connection with that of her father, Theon, on the same subject.[134] If the works of Hypatia had not been destroyed by the ravages of time, they would undoubtedly prove that she fully merited all the encomiums bestowed on her by antiquity for her genius; and they would also prove, we may well believe, that she deserved to be ranked not only with the eminent mathematicians upon whose works she commented, but also with such masters of astronomic science as Ptolemy, Eratosthenes and Aristarchus. After the tragic death of Hypatia many centuries elapsed before any other woman attracted attention for her work in astronomy. Indeed, so neglected was the study of the heavens between the time of Hypatia and the Arab prince and astronomer, Albategni, who flourished during the latter part of the ninth century and the early part of the tenth, that only eight observations, it is asserted, were recorded during this long period. The works and observations of Albategni, it may be remarked, have a particular interest from the fact that they form a connecting link between those of the Alexandrine astronomers and those of modern Europe.