Terrestrial and Celestial Globes Volume 2 / Their History and Construction Including a Consideration of their Value as Aids in the Study of Geography and Astronomy

by Edward Luther Stevenson

Fig. 118a. Terrestrial Globe of Johann Ludovicus Andreae, 1717.

Weigel, Castlemaine, Coronelli, and Treffler, as has been noted, represented a tendency in globe construction in their day which we have referred to as the ultrapractical. It was impossible that their ideas should find anything like a general acceptance and approval. A globe eleven or fifteen feet in diameter, in the better judgment of astronomers and geographers, could not be counted as possessing superior scientific value, and globes of such dimensions seem only to have won the praise of the novelty-loving contemporaries, and the same general criticism may be passed upon the smaller globes of Castlemaine and Treffler. Perhaps, however, one may well add that in all this a desire was expressing itself for improvement in globe construction.

In this connection attention may be called to a plan for reform in globe making proposed by Jean Dominique Cassini (1625-1712), one of the most famous astronomers of the period.[125] Cassini was a native of Périnaldo, Italy (Fig. [119]). Early in life he became interested in the study of astronomy, and at the age of twenty-five received an appointment as professor of this science at the University of Bologna. Recommended by Colbert as one worthy his royal master’s patronage, Cassini in 1669 accepted the invitation of Louis XIV to fill the chair of astronomy in the Collège de France, a position once held by Pierre Gassendi.[126] In 1671 he became the director of the Royal Observatory of Paris, a position held in succession by four generations of his family. To him we owe the determination of the rotation periods of Jupiter, Venus, and Mars, the discovery of four of Saturn’s satellites and the determination of their periods of revolution. He devoted much time and study to the problem of the obliquity of the ecliptic, to the precession of the equinoxes,[127] and to the determination of the latitude and longitude of places.[128] This precession, he found, could not be represented on a celestial globe such as hitherto had been constructed, and he set himself to the task of devising one on a new plan. The position of the constellations, as indicated on the ordinary celestial globe, he, as others, noted would soon be found to be inaccurate. What he proposed was a globe capable of such adjustment as to obviate this difficulty; in other words, he proposed the construction of a globe by means of which this perpetual change might be indicated, or one which would serve to indicate the position of the several constellations at any time, past, present or future.

Fig. 119. Portrait of Jean Dominique Cassini.

Fig. 123. Portrait of Nicolas Bion.

It was to Nicolas Bion, map and globe maker of Paris that the astronomer Cassini entrusted the manufacture of such an instrument, and it is from him that we have a brief description of its peculiar features.[129] He tells us that the sphere on which the several constellations were represented was enclosed within a number of armillae representing the celestial circles, that is, the colures, the ecliptic, the tropics, the equator, and the polar circles. This inner sphere was attached to a meridian circle at the poles of its equator, within which circle it turned as the ordinary sphere, and it was also attached to the same meridian at the poles of the ecliptic. Around this polar axis of the ecliptic the sphere, with the attached meridian, could be made to revolve, the pole of the equator in its revolution tracing a circle having a radius of twenty-three and a half degrees, a complete revolution being made to represent a period of twenty-five thousand two hundred years, or the time required for the complete precession of the equinox according to his reckoning. This pole in its circle of revolution could be immovably set at any desired point to represent any time past or future, and the sphere then revolved around the pole of the equator. The several stars or constellations could thus be represented in their proper position for the time selected. Bion’s reference to this globe seems to assure us that he completed its construction, yet no trace of it has been left, unless we have such in a record to be found in the history of the Royal Academy for the year 1727. In this we find that a globe constructed on the principle laid down by Cassini was presented to the Academy, in the year designated, by Outhier, a priest of Besançon.[130] This globe, which has disappeared, had the double movements, one about the axis of the equator and one about the axis of the ecliptic. It was a globe which would represent the daily and annual movements of the sun, the difference between the true and the mean time, the movements of the moon and its phases, the eclipses, and the passing of the several fixed stars across the meridian.