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 sphere is mounted at the celestial poles that define the axis of rotation, and its structure includes an equatorial ring and, parallel to this, two smaller rings representing the Tropics of Cancer and Capricorn to the north and south, respectively. Zenith: in the horizon coordinate system, it is the position directly overhead on the celestial sphere. The zenith’s altitude is 90 o.

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] 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. An armillary sphere is ideally suited as a sundial. With its hour face inscribed on the inner circular surface of the Equatorial ring—also known as the Equinoctial—the gnomon’s shadow sweeps uniformly along with time, allowing the hour markings to be equally spaced along the equinoctial ring. De Rose, L.: La sfera armillare di Solunto, in XVIII Seminario di Archeoastronomia, 19–20 marzo, Sestri Ponente, Genova (2016) 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.”

Inscriptions or zodiac symbols were often used as decorative elements on the bands.

The instrument survived throughout the medieval period into the early modern era, and in many respects came to symbolise the queen of the sciences, astronomy. It wasn’t until the middle of the sixteenth-century that the basis of the instrument – a geocentric concept of the Universe – was seriously challenged by the Polish mathematician, Nicolaus Copernicus. Even then, the instrument still continued to serve a useful purpose as a purely mathematical instrument. Savage-Smith, Emilie (1985), Islamicate Celestial Globes: Their History, Construction, and Use, Smithsonian Institution Press, Washington, D.C.

Further information: Planetarium and Antikythera mechanism Ptolemy with an armillary sphere model, by Joos van Ghent and Pedro Berruguete, 1476, Louvre, Paris 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.

For most of the second half of the 20th century this object was described by the Whipple Museum as having been made in about 1450. This would make it one of the earliest surviving demonstrational armillaries. Two other armillary spheres held in other museums date from this period, however this object is rather different from both of these. It is very difficult to date the armillary using the style of the lettering. The letter-shapes used to label the rings of the sphere suggest that it may have been produced at an even earlier date: perhaps in the 14th century. Solstitial colure: meridian or great circle that passes through the celestial poles and the two solstices (first point of Cancer and first point of Capricorn). Elements of the general description incorporate text from the Encyclopædia Britannica First Edition (1771).