ARMILLA or ARMILLARY SPHERE, an astronomical model representing the great circles of the heavens, including in the complete instruments, the equator, meridian, ecliptic, and tropics. It is a skeleton celestial globe, with circles divided into degrees for angular measurement. In the 17th and 18th centuries such models, either suspended, rested on a stand, or affixed to a handle, were used to show the difference between the Ptolemaic theory of a central earth, and the Copernican theory of a central sun.
The earliest known complete armillary sphere with nine circles is believed to have been the meteoroskopion of the Alexandrine Greeks (c. A.D. 140, but earlier and simpler types of ring instru ments were also in general use. Ptolemy in the Almagest, enu merates at least three. The simplest of all was the Equinoctial Armilla, a ring of bronze fixed in the plane of the equator. At Rhodes and elsewhere the arrival of the equinoxes was noted by observing when the shadow of the upper half of the ring exactly covered the lower half. Similarly, the Solstitial Armilla, a double ring erected in the plane of the meridian with a rotating inner circle was used for measuring solar altitudes, and probably by Eratosthenes (276-196 B.c.) for measuring the obliquity of the ecliptic. Hipparchus (16o-12 5 B.c.) is stated to have used a sphere of four rings and in Ptolemy's instrument astrolabon (A.D. 139) there were diametrically disposed tubes upon the gradu ated circles, the instrument being kept vertical by a plumb line.
The Arabs employed similar instruments with diametric sight rules or alidades, and it is likely that those made and used in the 12th century by Moors in Spain were the prototypes of all later European armillary spheres. One large Chinese armillary sphere in Pekin is said to date from 1274, but another belongs to the period of the Jesuit astronomers in the r7th century.
BIBLIOGRAPHY.-Tycho Brahe, Astronomiae Instauratae Mechanica Bibliography.-Tycho Brahe, Astronomiae Instauratae Mechanica (I.98) ; N. Bion, L'usage des globes celestes (2nd ed. 1700) ; and Trait des instrumens de mathematique (Eng. trans. 1723) ; J. B. Delambre, Histoire de l'astronomie ancienne (1817) ; J. J. Sedillot, Traite des Instruments astronomiques des Arabes (1834) ; F. Nolte, Armillarsphiire (Erlangen, 1922), and G. R. Kaye, Hindu Astronomy (R. T. G.)