Terrestrial and Celestial Globes (Vol. 1&2). Edward Luther Stevenson

      Fig. 7. Atlante Farnese, ca. 200 BC

      Fig. 8. Atlante Farnese Constellation Figures.

We learn from Cicero and from other early writers that Archimedes (ca. 287–212 BC), the celebrated geometrician of Syracuse, constructed a globe or contrivance for the purpose of demonstrating the movements of the heavenly bodies. Cicero’s statements imply that the work of Archimedes was well known in his day, yet he thought it merited a special word of commendation from himself. “I shall propose nothing new to you,” he says, “nor that which I have invented or discovered; but I remember C. Sulpicius Gallus, a very learned man, as you know, when this appearance (in the heavens) was spoken of, and he was, by chance, at the house of Marcellus, who had been consul with him, he described a globe among the spoils of that opulent and magnificent city of Syracuse, when captured, as the only thing among all the spoils which he ordered to be carried to his own house; about which globe I have often heard, on account of the fame of Archimedes, although the work itself was not very remarkable, for there was another far more beautiful and more honored by the common people, made by the same Archimedes, and placed in the Temple of Virtue by the same Marcellus. But afterward when Gallus began to explain scientifically the object of the machine, I thought there was more ingenuity in that Sicilian than human nature was capable of. For Gallus informed me that there was another ancient invention of a solid and elaborately formed globe which was made by Thales, the Milesian, to revolve. And afterward the same was, by Eudoxus of Cnidos, the disciple of Plato, adorned with the fixed stars of heaven, and with every ornament and embellishment, as described by Eudoxus, and was many years afterward celebrated by Aratus, not exactly in the scientific language of astronomy, but with the graces of poetry. This species of globe indeed, in which the sun and moon were made to revolve, and five of those stars which have been called travelers, and as it were wanderers, could not possibly be exhibited on that solid sphere. And more especially was that invention of Archimedes to be admired, for he had so contrived that one revolution of the machine served somehow to produce unequal and varied movements through their different paths. For when Gallus set the globe in motion, the moon succeeded the sun by as many turns of the brass wheel of the machine as days in the heavens, so that the globe represented in the heavens the same eclipse of the sun, when the moon arrived at a certain place or point, as the shadow of the earth did when the sun shone from the opposite region.”34

Lactantius’ allusion to Archimedes, at a later date, is perhaps derived from Cicero, but it is none the less interesting as indicating a belief that such a globe had existed. In his characteristic vein he refers to the mechanical device, finding therein a support for his theological arguments. “Was Archimedes of Sicily able to contrive a likeness and representation of the universe in hollow brass,” he inquires, “in which he so arranged the sun and moon, that they effected, as it were every day, motions unequal and resembling the revolutions of the heavens, and that sphere, while it revolved, exhibited not only the approaches and with drawings of the sun or the increase and waning of the moon, but also the unequal course of the stars, whether fixed or wandering? Was it then impossible for God to plan and create the original, when the skill of man was able to represent them by imitation? Would the stoic, therefore, if he should have seen the figures of the stars painted and fashioned in that brass, say that they moved by their own design, and not by the genius of the artificer?”35 Günther notes that at the beginning of the seventh book of the collection of Pappus, geometrician of Alexandria, may be found a reference to those skilled in mechanical devices in which it is stated that “Mechanicians are those who understand how to construct celestial globes and to represent the heavens and the course of the stars moving in circles by means of like circular movements of water.”36 It has been thought that in this passage we have a reference to a globe such as was probably constructed by Archimedes, although the reference is not to any particular example. It seems not improbable that the globe of Archimedes was made to revolve by an hydraulic contrivance, and that it resembled a planetarium or orrery.37 That the science of hydrostatics had been developed by Archimedes’ time to a high degree is very certain.

Instruments for measuring angles and distances were very early employed in the field of astronomy as well as in the field of geography. Of these instruments the Egyptian gnomon appears to have been the oldest.38 In its best form it consisted of a bowl having a perpendicular rod or staff erected at the central point of the inner curved surface. This rod cast a shadow upon the inner surface of the bowl, which had been graduated, giving a reading in degrees which furnished to the observer the information desired. Time brought improvements and variations in the construction of simple instruments of this character. As early as the third century before the Christian era, adjustable rings, or armillae, for example, were employed by astronomers to aid them in the solution of their problems, which instruments later developed, as noted below, into the more elaborate and complex armillary spheres. The simplest form of such an instrument appears to have been but a single graduated circle. To this, at a very early date, a second was added, thus providing an instrument in which one of the circles was regarded as fixed in the plane of the equator, the other, intersecting this at right angles, served as a meridian circle, being movable around an axis which could be called the world axis, the axis of the celestial sphere, or the axis of the universe. The position of a celestial body in declination could be determined on the meridian circle, and its right ascension on the fixed or horizon circle.39 It seems altogether probable that Eratosthenes made use of such an instrument in his efforts to measure the obliquity of the ecliptic. He tells us that in his time one of large dimensions hung in the portico of the academy of Alexandria.40 With the addition of other circles, and of an adjustable view-tube, that more accurate and detailed measurements might be made, this device, in Hipparchus’ day, came to be known as an astrolabe, and, after the addition of other rings in later years, to be known as an armillary sphere. Even in this last development it was not a true sphere on which could be represented the starry constellations, but an arrangement of circles forming a sort of imaginary sphere, the circles being intended to represent the relative position of the principal celestial circles. This instrument seems, at first, to have been suspended, when in use, but later was made to rest upon a base, the whole adjusted to revolve around an axis and within a graduated horizon circle. In the earliest examples, the earth at the center of the circles, it represented the Ptolemaic system (Fig. 9); in the later examples, having the sun at the center, it represented the Copernican system.

      Fig. 9. Armillary Sphere according to Ptolemy.

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