ARMILLA

armilla, armil or armillary sphere (from the lat. _armilla_, a bracelet), an instrument used in astronomy. in its simplest form, consisting of a ring fixed in the plane of the equator, the _armilla_ is one of the most ancient of astronomical instruments. slightly developed, it was crossed by another ring fixed in the plane of the meridian. the first was an equinoctial, the second a solstitial armilla. shadows were used as indices of the sun's position, in combination with angular divisions. when several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere. armillae are said to have been in early use in china. eratosthenes (276-196 b.c.) used most probably a solstitial armilla for measuring the obliquity of the ecliptic. hipparchus (160-125 b.c.) probably used an armillary sphere of four rings. ptolemy (c. a.d. 107-161) describes his instrument in the _syntaxis_ (book v. chap, i.), and it is of great interest as an example of the armillary sphere passing into the spherical astrolabe. it consisted of a graduated circle inside which another could slide, carrying two small tubes diametrically opposite, the instrument being kept vertical by a plumb-line. [illustration: from m. blundeville's _treatise of the first principles of cosmography and specially of the spheare._ armillary sphere. a.d. 1636.] no material advance was made on ptolemy's instrument until tycho brahe, whose elaborate armillary spheres passing into astrolabes are figured in his _astronomiae instauratae mechanica_. the armillary sphere survives as useful for teaching, and may be described as a skeleton celestial globe, the series of rings representing the great circles of the heavens, and revolving on an axis within a horizon. with the earth as centre such a sphere is known as ptolemaic; with the sun as centre, as copernican. the designer of the instrument shown no doubt thought that the north pole might suitably have the same ornament as was used to mark n. on the compass card, and so surmounted it with the _fleur-de-lys_, traditionally chosen for that purpose on the compass by flavio gioja in honour of charles of anjou, king of sicily and naples. armillary spheres occur in many old sculptures, paintings and engravings; and from these sources we know that they were made for suspension, for resting on the ground or on a table, for holding by a short handle, or either for holding or for resting on a stand. authorities.--tycho brahe, _astronomiae instauratae mechanica; m. blundeville, his exercises_; n. bion, _traite des instrumens de mathematique_; also _l'usage des globes celestes_; sedillot, _memoire sur les instrumens_; j.b. delambre, _histoire de l'astronomie ancienne_; r. grant, _history of physical astronomy_. (m. l. h.)