It is expressly stated by Ptolemy that a celestial globe was constructed by Hipparchus, who is reputed to have been the founder of spherical trigonometry,[41] and Pliny tells us that Hipparchus was the inventor of the astrolabe,[42] which statement probably means that he greatly improved the simple armillae used at an earlier date as an instrument for astronomical calculations. Ptolemy, in his ‘Syntaxis,’ or ‘Almagest’ as it was called by the Arabs, devoted a chapter to the method of constructing, and to the use of the astrolabe, which must have closely resembled the armillary sphere, describing therein, in terms not altogether easy of comprehension, its several rings and cylinders, and the method of adjusting the same for purposes of determining the latitude and the longitude of celestial bodies. He tells us also how to construct a representation of the sphere of the fixed stars by means of a solid ball, how to place thereon the several constellations, and how to use the same in the study of astronomical problems. Such a globe, he says, “should be of a dark color, that it might resemble the night and not the day.” His description is detailed as to the proper method of procedure in marking the position of the celestial circles on this globe, in arranging the movable rings of “hard and well polished material,” in graduating the rings and adjusting them to move about an axis which is likewise an axis of the globe proper. In marking the position of the fixed stars, we are told that the proper method is to commence at some constant and invariable point of a certain constellation, and he suggests that the best starting point is the fixed star in Canis Major, that is, the so-called dog star, or Sirius. “The position of the other fixed stars, as they follow in the list, could easily be determined,” he says, “by making the globe to turn upon the poles of the zodiac, thus bringing the graduated circle to the proper point of each. The stars could be marked with yellow or with such other color as one might choose, having due regard for their brilliancy and magnitude. The outline of each of the constellations should be made as simple as possible, indicating with light strokes, differing but little in color from that of the surface of the globe, the figures which the stars in the several constellations represent, preserving in this manner the chief advantage of such representation, which should be to make the several stars very prominent without destroying, by a variety of color, the resemblance of the object to the truth. It will be easy to make and to retain a proper comparison of the stars if we represent upon the sphere the real appearance or magnitude of the several stars. While neither the equator nor the tropics can be represented on the globe, it will not be difficult to ascertain the proper position of these circles. The first could be thought of as passing through that point on the graduated meridian circle which is 90 degrees from the poles.