Very fine brass astronomical equinoctial ring with two brass circles with engraved foliage decoration, signed "Jean-Baptiste-Nicolas Delure A Paris", on the hanger ring.

The meridian ring is engraved with a latitude scale 90-0-90 degrees, reverse with a scale for determining solar altitude and zennith distance, an equinoctial ring with obverse engraved with hour scale in Roman numerals, the central bridge with pin-hole sliding over calendar and zodiac scales, 100 mm diam.

Jean-Baptiste-Nicolas Delure (1695-1766) was a renowned French instrument, globe maker and horologist was given the title "Ingénieur du Roi, juré en 1721-1723" (under the license of the king" (Louis the XIV) in 1721-1723). He was a Master and Juror of the Corporation des Fondeurs and appointed Ingénieur du Roi to Louis XIV. He had two workshops, both on Il de la Cite, Paris. 

He was father-in-law to the fifth son of the eminent Parisian instrument maker Nicolas Bion.
Jean-Baptiste Nicolas Bion carried on both their businesses until around 1770.

The meridian ring is engraved with a latitude scale 90-0-90 degrees, the reverse with a scale for determining solar altitude and zenith distance, the quadrennials ring with obverse engraved with an hour scale in Roman numerals, the central bridge with pin-hole sliding over the calendar and zodiac scales, 100 mm diam.

This is a relatively large example of the classic pendant ring dial, useable anywhere on earth without a compass. Only the date and one's latitude are needed to determine the apparent solar time and even the North. the latitude can be determined using the quadrennial degree scale at the backside to measure the solar meridian altitude.

Nocturnal or noctules, sometimes called, are devices for telling the time of night. Their operation is based on the fact that the stars, while remaining fixed relative to one another, appear to rotate around the North star (Polaris)It is, of course, the earth that is rotating and the Polaris remains fixed because it lies along the earth´s axis of rotation. 

As the other stars appear to rotate, their position at any moment indicates the time. Sometimes called “horologium nocturnum” or “Nocturlabe” are related to the astrolabe and the sundials. 

The mention of a dedicated instrument for its measurement was not found before the Middle Ages. The earlier image presenting the use of a nocturnal is in a manuscript dating from the 12th century. 

Raymond Lull repeatedly described the use os a “Sphera horarum noctis”. With Martin Cortés de Albacar´s book “Arte de Navegar” published in 1551 the name and the instrument gained a larger popularity. 

Timekeeping was very important to navigators at sea. Precise time was needed to use tide tables to enter harbors safely and also to regulate work shifts aboard.. Thus, the navigators could tell the time at night if the weather was clear.

The problem was that when the sky was obscured in foul weather, they had no way of knowing the exact time except by running the sand-glasses. By the middle of the 18th century, more accurate clocks became available, known as chronometers and the nocturnals fell out of use. Nocturnals are really simple analog computers. The nocturnal is simpler to use than other instruments requiring mathematical tables and trigonometry.

Reference: A Butterfield ring dial of similar design but smaller, is illustrated in Sundial at Greenwich, Hester Highton, OUP 2001, page 200, catalogue no. 186.