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Astronomical Ring Dial
Attributed to Johannes Motter
circa 1550; Flemish
Brass; 115 mm in diameter

This is an astronomical ring of the Gemma Frisius type. The outer (meridian) ring is engraved in the quadrant with a co-latitude scale of degrees of 0 to 90 reading to one degree, and carries in the suspension piece. This is composed of a suspension ring carried in the eye of a rotatable shackle set at the centre of the base strip. This strip is curved to fit round the ring to which it is attached by two screws passing through a slot cut in the strip. These screws may be placed in any pair of six holes drilled at 10? intervals from 70? to 20?, while final adjustment is made by sliding the strip until a fiducial mark at its centre corresponds with the value required. This adjustment may be made over a range of ten degrees. When the instrument is open, a second 90? scale is visible the remainder of this face being blank. The reverse of the ring is blank except for a 90? scale on the reverse of the quadrant diametrically opposite the co-latitude scale. This scale also is only visible when the instrument is open.

Pivoted from the point where the two degree quadrants meet is the second ring, marked on its outer edge of the ring (with 0? Aries at 21 March). The calendar scale is fully graduated to days, the zodiac scale is not divided. On one side of the second ring is a scale of [0] to 12 to [0] numbered by groups of three and divided to fifths. The division are radial from the centre of the ring, but form a quadrant which begins at 45? from the pivots. This scale is equivalent to the shadow square on an astrolabe. The two quadrants opposite are engraved with a scale 1 to 12 divided to 15 in groups of three. The other side of this ring is engraved with a similar hour scale. When the instrument is closed only the shadow square scale is visible.

The third ring is pivoted in the meridian ring so that it may be turned within the second ring. One side is engraved with a double hour scale (1 to 12 ? 2) reading to four minutes, the other with a degree scale 90 to 0 to 90 (reading from the pivots) divided to 1?. Disposed symmetrically around the point opposite the 0? point of this scale is a solar declination scale. The outer edge of the ring is engraved with an unequally spaced series of numbered divisions and with star names. The inner edge of the third ring is provided by a separate flat ring of very exact fit carrying two sight vanes with a rib raised at the centre of the back of each to act as an index against the side of the mother, third ring. The separate sight vane ring may be rotated within the third ring so that the position of the vanes may be adjusted against either the solar declination scale or the degree scales engraved on the side of the mother ring. Each sight vane has two pinnules, one of which is set on the centre line of the vane running through the fiducial edge of the index on the back of the vane.

The present instrument is virtually identical in size, layout and script with an astronomical ring preserved in the Mus?es Royaux d'Art et d'Histoire, Brussels which is signed 'Fr Ioannes motter Alias mueter'. Comparison of the two instruments shows them to be of virtually the same size (Brussels 125 mm, Florence 127 mm) and they share the same layout of scales and construction of sight vanes. There are also extremely close parallels in all of the lettering, so close indeed that it is virtually identical, including the characteristic upper case 'E' composed of two letters 'C' set one above the other with the upper and lower arcs looped. See also three other ring dials by Motter in the Collection Real Universita, Florence.

Anthony J. Turner

Istituto e Museo di Storia della Scienza, Firenze
Inventory no. 2453

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