Astrolabe: detail inside mater showing contruction

Although several characteristics of this astrolabe are reminiscent of those by Gerard Mercator (1512-94), such as the throne and arrangement of some of the inscriptions, it is unlikely that he actually made this instrument. Mercator's three known astrolabes are very skilfully engraved in an elegant Italic script, but the labels on this one are all set in a miniscule Roman font and even though they are clearly the work of a first-class craftsman, they are not quite of the calibre of Mercator. It is possible that it could have been made by Gerard's son Arnold (1537-87), who was a highly competent maker of mathematical instruments. The date given on the instrument (1574) suggests that Arnold could have made it at Duisburgh, where he was engaged in making a large sundial for the Sint-Salvator church between 1572-77.

The throne comprises a vertical bar, cast as one piece with the limb and supported by two S-shaped brackets. A pelican crest and the motto 'Extra te nihil' is engraved on either side with the date: '1574', underneath. The mater is formed by a plate riveted to the limb, which has an inner circle forming a step in the hollow of the mater. Additional support is given by two brass bars set at right angles. There is one plate. On the back of the instrument is a universal orthographic projection. This type of projection was 'revived' by the Spaniard Juan De Rojas who described it in a treatise of 1550. Astrolabes with such arrangements are therefore sometimes referred to as being of the 'Rojas-type'.