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Modern editions of John Philoponus' treatise on the astrolabe are De usu astrolabii eiusque constructione libellus (On the Use and Construction of the Astrolabe), ed. Heinrich Hase, Bonn: E. Weber, 1839, OCLC 165707441 (or id. Rheinisches Museum für Philologie 6 (1839): 127–71); repr. and translated into French by Alain Philippe Segonds, Jean Philopon, traité de l'astrolabe, Paris: Librairie Alain Brieux, 1981, OCLC 10467740; and translated into English by H.W. Green in R.T. Gunther, The Astrolabes of the World, Vol. 1/2, Oxford, 1932, OL 18840299M repr. London: Holland Press, 1976, OL 14132393M pp. 61–81. This ring is also often marked with the zodiac signs and twenty-five stars, similar to the astrolabe. There is no evidence for the Hellenistic origin of the spherical astrolabe, but rather evidence so far available suggests that it may have been an early but distinctly Islamic development with no Greek antecedents." Dirk L. Couprie, Robert Hahn, Gerard Naddaf: Anaximander in Context: New Studies in the Origins of Greek Philosophy, 2003, ISBN 978-0-7914-5537-1, p. 179

It can be used as a sun dial to tell time, if the approximate latitude and season is known, or to tell latitude, if the time is known or observed (at solar noon). It may be considered to be a simplified, portable armillary sphere, or a more complex form of astrolabe. The Greek astronomer Hipparchus ( c. 190– c. 120 BC) credited Eratosthenes (276–194 BC) as the inventor of the armillary sphere. [11] [12] [13] [14] [15] Names of this device in Greek include ἀστρολάβος astrolabos and κρικωτὴ σφαῖρα krikōtē sphaira "ringed sphere". [16] The English name of this device comes ultimately from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth. Before the advent of the European telescope in the 17th century, the armillary sphere was the prime instrument of all astronomers in determining celestial positions.

Brashear, Ronald (May 1999). "Astronomiæ instauratæ mechanica by Tycho Brahe: Introduction". Special Collections Department. Smithsonian Institution Libraries . Retrieved July 11, 2020. The tropic of Cancer C, touching the ecliptic at the beginning of Cancer in e, and the tropic of Capricorn D, touching the ecliptic at the beginning of Capricorn in f; each 23 1⁄ 2 degrees from the equinoctial circle. The equatorial ring occupies a plane parallel to the celestial equator, at right angles to the meridian. It is aligned by

Lindberg, David C.; Shank, Michael H. (7 October 2013). The Cambridge History of Science: Volume 2, Medieval Science. Cambridge University Press. p.173. ISBN 978-1-316-02547-5 . Retrieved 15 May 2018.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 positions, in combinations with angular divisions. When several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere. [1] a b c d Editors of Encyclopædia Britannica. (16 November 2006). " Armillary Sphere." Encyclopædia Britannica. Accessed 14 October 2017. A representation of an armillary sphere is present in the modern flag of Portugal and has been a national symbol since the reign of Manuel I. Further information: Planetarium and Antikythera mechanism Ptolemy with an armillary sphere model, by Joos van Ghent and Pedro Berruguete, 1476, Louvre, Paris In 1610, Edward Wright created the sea ring, which mounted a universal ring dial over a magnetic compass. This permitted mariners to determine the time and magnetic variation in a single step. [4] These are also called "sundial compasses".

Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices. Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the zenith of wisdom and knowledge. This ring is often equipped with vanes and pinholes for use as the alidade of a dioptra (see image). It can be used to measure declination. Since ancient times, astronomers around the world have used models of the sky to make calculations. With the advent of the armillary sphere, stargazers were given a physical model to better visualize the lines of celestial longitude and latitude. Created independently in ancient Greece and ancient China, these armillary spheres consisted of spherical rings centered on either the Earth or the Sun. During the 16th and 17th centuries, these astronomy tools were sized down to become fashionable finger rings that moved just like regular armillary spheres. The meridional ring can function as the gnomon, when the rings are used as a sundial. A horizontal line aligned on a meridian with a gnomon facing the noon-sun is termed a meridian line and does not indicate the time, but instead the day of the year. Historically they were used to accurately determine the length of the solar year. A fixed meridional ring on its own can be used as an analemma calendar sundial, which can be read only at noon. The whole fabric is supported on a pedestal N, and may be elevated or depressed upon the joint O, to any number of degrees from 0 to 90, by means of the arc P, which is fixed in the strong brass arm Q, and slides in the upright piece R, in which is a screw at r, to fix it at any proper elevation.The equinoctial A, which is divided into 360 degrees (beginning at its intersection with the ecliptic in Aries) for showing the sun's right ascension in degrees; and also into 24 hours, for showing its right ascension in time. With the Earth as center, an armillary sphere is known as Ptolemaic. With the Sun as center, it is known as Copernican. [1] The declination ring is moveable, and rotates on pivots set in the meridian ring. An imaginary line connecting these pivots is parallel to the Earth's axis. The declination "ring" of the traveller's sundial above is not a ring at all, but an oblong loop with a slider for setting the season. The angle between the vertical and the light beam gives the solar elevation. The solar elevation is a function of latitude, time of day, and season. Any one of these variables can be determined using astronomical rings, if the other two are known. The meridian ring is placed vertically, then rotated (relative to the celestial object) until it is parallel to the local north-south line. The whole ring is thus parallel to the circle of longitude passing through the place where the user is standing.