The instrument, from the Collina Sbaraglia donation [file 69], is typical of the best Flemish production of the XVIth century. Six interchangeable plates are lodged in the mater for the latitudes 32°-45°, 35°-37°, 39°-41°, 42°-43°, 49°-51°, and 33°-Horizontale Catholicum. Underneath the plates is depicted a Quadratum Nauticum. On the back - which carries another alidade in addition to the one on the front - is engraved a stereographic projection of the stars, showing the ecliptic and the most important stars. A small compass complete with brass cover is inserted in the lower part. At the top, underneath the suspension ring, is a throne with a male figure and a female next to him, in relief. At the bottom edge of the throne is written: Gualterus nepos Gemmae Frisij Louanij fecit anno 1565.
Gemma Frisius Reiner (1508-1555), astronomer and cosmographer at the University of Louvain, was the author of many astronomical treatises on the use of the astrolabe, including De Radio Astronomico and Medici ac Mathematici de astrolabio catholico liber quo latissime patientis instrumenti multiplex usus explicatur, both published at Antwerp in 1545 and 1556 respectively. He was founder of a large family of craftsmen among the most representative of the Flemish instrument school of the XVIth century. As well as the author of this astrolabe, Walter - maker of many other astronomical instruments, known also as Arsenius or, simply, as nepos Gemmae Frisij - there were also Reignier and Remigius, also nephews of Gemma working in Louvain around 1565-1574, and Ferdinand and Ambroise, brothers perhaps of Gemma, working in Antwerp between 1573 and 1618.
J.A. Bennett (1987).
A. Chapman (1988), p. 197.
F. Farinelli (1979), p. 190.
S. Gibbs, G. Saliba (1984).
D.A. King (1991a), p. 169.
H. Minow (1990).
G. L'E. Turner (1991), pp. 74, 77, 124.