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. 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. Aterini, B.: Geometric instruments for the orientation and measurement: the astrolabes. In: Advances in Historical Studies (AHS), 8, 58–78. Springer (2019b). http://www.scirp.org/journal/ahs. https://doi.org/10.4236/ahs.2019.81004

Obliquity: Earth's axial tilt or obliquity of the ecliptic is defined as the angle that its rotational axis makes with its orbital plane (ecliptic). The earth’s axial tilt is currently 23 o 26.2’ (23.44 o). Since the earth’s orbital plane is affected by the otherplanets in the solar system, its axial tilt varies over time. Data over the past 5 million years show that the obliquity varies from22.1 o and 24.5 o over about 41,000 years. Currently the earth’s obliquity is slowly decreasing towards the low values. There are other shorter-term variations in the obliquity, such as nutation, which is caused by the moon and varies over 18.6 years. Suspended within a ring the sphere sits in a cradle which is mounted on an elegant tripod stand. The position of the ring can be adjusted for latitude within the cradle. At the top of the stand is another ring representing the celestial horizon. As the sphere rotates, anything above the horizon ring at any one time will, in theory, be visible in the heavens; anything below, will be hidden. Art historian Jessica Stewart writes that in the 17th century, rings such as the above specimen were “used by astronomers to study and make calculations. These pieces of jewelry were considered tokens of knowledge. Inscriptions or zodiac symbols were often used as decorative elements on the bands.” Aterini, B.: The astrolabe: a mechanism for reading the stars. In: Zhang, B., Ceccarelli, M. (eds.) Explorations in the History and Heritage of Machines and Mechanisms. HMMS, vol. 37, pp. 227–242. Springer, Cham (2019a). https://doi.org/10.1007/978-3-030-03538-9_19

About it rotate a set of rings representing the heavens – the celestial sphere – with one complete revolution approximating 24 hours. Zodiac: a band 18 o wide that is centered on the ecliptic. The zodiac comes from the Greek word for "circle of animals" that form the various constellation of stars in the eclilptic. The constellation of animals aries (the ram), taurus (the bull), cancer (the crab), leo (the lion), scorpius (the scorpion), and capricornus (the capricorn) were named at least a thousand years before by Babylonian and Chaldean astronomers. Since Roman times we also have constellation of libra (scales) and the representation of people in the form of aquarius (the water carrier), gemini (the twins), virgo (the virgin), and sagittarius (the archer). Gnomon:represents the earth’s rotational or polar axis. The gnomon inclines to the horizon by an angle equal to the latitude of the observer. The metal rings, usually made of brass or bronze, are referred to as armilla, which is Latin for bracelet. Armilla were prestigious armbands awarded to Roman soldiers for distinguished gallantry. a b c d Editors of Encyclopædia Britannica. (16 November 2006). " Armillary Sphere." Encyclopædia Britannica. Accessed 14 October 2017.

This particular instrument is Italian, made in Rome in 1588, and is signed by Carlo Plato. It has a number of unusual features that make it rather special, such as the moon mounted on a separate ring within the sphere, and a number of stars represented by individual pointers. Very few examples of the instrument survive from earlier than the 16th century. This may be due to their fragile nature and the materials used to construct them. Delicate spheres composed of rings, even when these were made of brass, are less likely to have been preserved than flat instruments such as astrolabes. Each of the signs of the zodiac is engraved upon the ecliptic ring which is also calibrated with a calendar scale enabling the instrument to be used to model the apparent motion of the sun and the stars at any time of the year. The instrument could be used not only to model the movements and relative geometry of the heavens, but to carry out all sorts of calculations such as the times of sunrise and sunset, the length of a day, and the altitude of the sun or stars.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 center such a sphere is known as Ptolemaic; with the sun as center, as Copernican. [1] Elements of the general description incorporate text from the Encyclopædia Britannica First Edition (1771). a b Savage-Smith, Emilie (2017). "Of Making Celestial Globes There Seems No End" (PDF). Bulletin of the Scientific Instrument Society. No. 132: 1–9.

With the Earth as center, an armillary sphere is known as Ptolemaic. With the Sun as center, it is known as Copernican. [1] Further advances in this instrument were made by Danish astronomer Tycho Brahe (1546–1601), who constructed three large armillary spheres which he used for highly precise measurements of the positions of the stars and planets. They were described in his Astronomiae Instauratae Mechanica. [25]

De Rose, L.: La sfera armillare di Solunto, in XVIII Seminario di Archeoastronomia, 19–20 marzo, Sestri Ponente, Genova (2016)