In the time of Harun Al-Rashid (800 A.D.), and his son, the Caliphate of Bagdad was the center of Arab science. Mathematics and astronomy were especially cultivated; an observatory was established; and the work of translation was systematically carried on by a sort of institute of translators, who rendered the writings of Aristotle, Hippocrates, Galen, Euclid, Ptolemy, and other Greek scientists, into Arabic. The names of the great Arab astronomers and mathematicians are not popularly known to us; their influence is greater than their fame. One of them describes the method pursued by him in the ninth century in taking measure of the circumference of the earth. A second developed a trigonometry of sines to replace the Ptolemaic trigonometry of chords. A third made use of the so-called Arabic (really Hindu) system of numerals, and wrote the first work on Algebra under that name. In this the writer did not aim at the mental discipline of students, but sought to confine himself to what is easiest and most useful in calculation, "such as men constantly require in cases of inheritance, legacies, partition, law-suits, and trade, and in all their dealings with one another, or where the measuring of lands, the digging of canals, geometrical computation, and other objects of various sorts and kinds are concerned." In the following centuries Arab institutions of higher learning were widely distributed and the flood-tide of Arab science was borne farther west. At Cairo about the close of the tenth century the first accurate records of eclipses were made, and tables were constructed of the motions of the sun, moon, and planets. Here as elsewhere the Arabs displayed ingenuity in the making of scientific apparatus, celestial globes, sextants of large size, quadrants of various sorts, and contrivances from which in the course of time were developed modern surveying instruments for measuring horizontal and vertical angles. Before the end of the eleventh century an Arab born at Cordova, the capital of Moorish Spain, constructed the Toletan Tables. These were followed in 1252 by the publication of the Alphonsine Tables, an event which astronomers regard as marking the dawn of European science.