1911 Encyclopædia Britannica/Armilla
|←Armidale||1911 Encyclopædia Britannica, Volume 2
|See also Armillary sphere on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.|
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.
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, Traité des instrumens de mathématique; also L’Usage des globes célestes; Sédillot, Mémoire sur les instrumens; J. B. Delambre, Histoire de l’astronomie ancienne; R. Grant, History of Physical Astronomy.(M. L. H.)