Philosophy, alchemy, and mathematics were not the only branches of learning in which the Arabs had excelled. The appearance in English of such words as _azimuth_, _nadir_, and _zenith_ towards the end of the fourteenth century suggests among other things that the thinking of this Syrian race contributed in no small degree to the rise in Europe of the new astronomy. These three Arabic words (two of them for the first time in English) are to be found in Chaucer’s _Treatise on the Astrolabe_, written in 1391 for the instruction of his little son, “Lowis”; and this interesting document contains many other words also for which the _Oxford Dictionary_ does not give any earlier quotation, such as _almanac_, _ecliptic_, _equinox_, _equator_, _horizon_, _latitude_, _longitude_, _meridian_, _minute_ (meaning one-sixtieth of a degree), while _zodiac_ was used by Gower a few years before. Such words show us that the Europe of the Dark Ages had been experiencing once more what the ancient scientists had known. Its learned men had been marking down recurrences of natural phenomena and orientating themselves on the earth by dividing its face up into imaginary rings and segments. For such purposes they had found Latin and Greek terms ready to their hand, and the survival of the Greek _zodiac_ reminds us that they had, moreover, adopted the ancient system of mapping out the heavens into twelve “signs.” When, therefore, we find three Arabic words among these relics of classical wisdom, we need not be surprised to see that they express something which the ancients had, apparently, never felt the need of expressing—that is, an abstracted geometrical way of mapping out the visible heavens. These are conceived of as a vast sphere encircling the earth; the _zenith_ and the _nadir_ are its poles, while the _azimuths_ are meridians of celestial longitude.