Astronomical rings also known as Gemma's rings, are an early astronomical instrument. The instrument consists of three rings, the meridien, the equator, and the declination ring. It may be considered to be a simplified, portable armillary sphere

Armillary sphere

An armillary sphere is a model of objects in the sky , consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic...

, or a more complex form of astrolabe

Astrolabe

An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

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The Rings

The meridian

Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the...

ring, when in use, occupies a plane with one axis running north and south, and the other vertical. A sliding suspension shackle is attached to the top of this ring, from which the whole device can be suspended. When properly used, this points to the latitude at which the instrument is being used. This ring can function as the gnomon

Gnomon

The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

, when the rings are used as a sundial

Sundial

A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

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The equatorial ring

Equatorial ring

An equatorial ring was an astronomical instrument used in the Hellenistic world to determine the exact moment of the spring and autumn equinoxes...

occupies a plane parallel to the celestial equator, at right angles to the meridian. Often equipped with a graduated scale, it can be used to measure right ascension

Right ascension

Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

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The declination

Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

 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. This ring is often equipped with vanes and pinholes for use as the alidade of a dioptra

Dioptra

A dioptra is a classical astronomical and surveying instrument, dating from the 3rd century BCE. The dioptra was a sighting tube or, alternatively, a rod with a sight at both ends, attached to a stand...

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It can be used to measure declination

Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

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History

The parts of the instrument go back to instruments made and used by ancient Greek astronomers

Greek astronomy

Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras. It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the...

. Gemma Frisius

Gemma Frisius

Gemma Frisius , was a physician, mathematician, cartographer, philosopher, and instrument maker...

 combined several of the instruments into a small, portable instrument, first published in 1534, and in Petrus Apianus

Petrus Apianus

Petrus Apianus , also known as Peter Apian, was a German humanist, known for his works in mathematics, astronomy and cartography.The lunar crater Apianus and minor planet 19139 Apian are named in his honour....

's Cosmographia in 1539.