The planispheric astrolabe is the most important of all early scientific instruments. Invented in the Hellenistic world, it was handed down to central Europe by learned Muslims via medieval Syria and Andalusia in a continuous tradition of design and construction. It was the most popular instrument of medieval and Renaissance astronomers. Its popularity declined in Europe only after 1600 when a new generation of laboratory and optical instruments started to emerge.

An astrolabe is a two dimensional representation of the Ptolemaic cosmos, with the earth at its centre, showing the movement of the celestial sphere around the pole and allowing the relative position of stars to be determined at any given moment for one particular latitude. It is, as it were, a model of the universe that can be held in the hand.

Reduced to its mechanical essentials, the astrolabe consists of two main parts: the celestial star map, or ‘rete’, which rotates over a terrestrial grid of reference co-ordinates, or ‘latitude plate’.
The rete usually constitutes the most delicate and elegant part of the instrument, composed of a number of ornate bands to which are attached decorative star pointers. A wide circular band is always present at the centre representing the ecliptic (the apparent annual path of the sun against the background of fixed stars), marked with the signs of the zodiac. Stars are indicated by pointers which are named on the strapwork. The rete is cut-out, so that the reference lines on the latitude plate can be seen through it.

The pattern of curves which constitute the latitude plate represent a stereographic projection: the projection of the three-dimensional curves of a sphere onto a flat surface. Astrolabes are usually equipped with a selection of latitude plates each engraved for a different latitude on each side, for example, for latitudes 44°/46°, 48°/50° and 52°/54°. As a consequence, the usability of any one astrolabe was limited by its selection of latitude plates, something that frustrated medieval travellers considerably.
Astrolabes were used for observational purposes, as computing devices and for demonstrational purposes. They could, for example, be used to measure the angular position of celestial bodies in relation to the horizon, the meridian, and the zenith. Their most important applications were related to the discipline of astrology, in connection with plotting horoscopes, which depend on the position of the different constellations in the ‘houses’ of the heavens. Astrolabes could also be used for time-telling, by day or by night, as long as the sun or some recognisable star present on the rete was visible.

In most cases astrolabes are a successful and stunning symbiosis of applied mathematics and applied art. As such they provide us with an unusual synthesis of historical evidence. The theoretical information that constitutes them places them in the history of scientific development, while at the same time they are the product of craftsmen, placing them within the tradition of the history of art.

The earliest European astrolabes were made around the turn of the first millennium. Medieval astrolabes are relatively crude in execution and present many problems for historians who wish to associate them with particular masters or centres of production. The places where they were constructed coincide with the intellectual centres of the period: monasteries and cathedral schools. It is not until the beginning of the fifteenth century that the first workshops with a systematic production make an appearance, when instruments were produced and exported from leading commercial cities, such as Paris, Florence and Vienna.
By the end of the fifteenth century the art of instrument-making had become established as a discipline in its own right. Again there is a close connection with centres of learning, which by this period had moved to the universities. Skills in the working of materials and engraving were a prerequisite for constructing astrolabes: by 1500 the two South German cities of Nuremberg and Augsburg formed important centres for their production, as a result of their traditions of metal-working coupled with their position as centres of commerce and trade.

Between circa 1520 and 1580 the university city of Louvain became Europe’s most important instrument-making centre. Scholars of Renaissance instruments agree that Louvain instruments are of exceptional quality, exceeding those of other contemporary workshops. This high reputation is due in many ways to their unique combination of beauty and precision. The decorative aspects of the instruments took their inspiration from contemporary manuals by Antwerp artists, while their functional parts embodied the most up-to-date knowledge of the time as distilled by contemporary mathematical practitioners. It was this perfect harmony of aesthetics and science that made the Louvain instruments so sought-after in the European market.
Indeed, from 1500 onwards the University of Louvain enjoyed fame for its mathematical tradition, attracting students and professors alike and from 1520 this intellectual reputation was accompanied by an equal material one, initiated by the production of terrestrial globes, which were made to accompany geographical treatises.

A crucial event in Louvain’s instrument-making history was the arrival around 1530 of two young men of exceptional skill and learning who would lay the foundations of the ‘Louvain School of Instrument Makers’: Gemma Frisius (1508-1555) and Gerard Mercator (1512-1594). Frisius was a professor of medicine who taught astronomy in private and published extensively on the construction and application of instruments; Mercator a renowned cartographer who also had considerable talents for producing fine instruments. In his treatise on the construction of astrolabes of 1556, Frisius described three features that were included on almost all Louvain examples, including the astrolabe in question: a quadratum nauticum (for use in connection with navigation), a horizontale catholicum (a set of horizon lines from 0° till 90° which allowed the limited use of the astrolabe at any latitude without the need to change latitude plates), and a magnetic compass (used to orient the astrolabe in the horizontal plane for terrestrial observations).

The presence and support of the Habsburg court in Brussels was an important impulse for the establishment of an instrument making trade in Louvain. In 1541, on the recommendation of Nicholas Granvelle, Gerard Mercator made a set of scientific instruments for Emperor Charles, described as being of ‘exquisite workmanship’.

The Arsenius astrolabe of 1559

The maker of the astrolabe presented here was an apprentice of Mercator: Gualterus Arsenius (c.1530-1580). Arsenius dominated Louvain instrument making during the third quarter of the sixteenth century. At present forty-three instruments from his workshop have been recorded, all of them characterised by great refinement and luxury. He produced a wide range of other instrument types, including armillary spheres, astronomical rings, cross-staffs, theodolites and sundials.

Arsenius’s career as an instrument maker can be divided into two parts: from 1554 to 1559; and from 1559 until circa 1575. During the first period he was still mastering the necessary skills for constructing instruments and seeking a clientele and patrons. In both aspects he relied heavily on the legacy of Frisius and Mercator.

This astrolabe is among his finest and best-preserved instruments. It heralds a shift in Arsenius’ career: from 1559 onwards he begins to make a larger variety of instruments of greater complexity and refinement. It is his earliest instrument to display all the characteristics of his mature style. It is also unique in incorporating the earliest known geographical plate. Not least, it is impressive by virtue of its size and completeness, incorporating all the typical components of a Louvain astrolabe, including the two faun-like figures flanking the central shield, or ‘throne’. The tradition of including mythological characters in art objects stems from Antwerp, and Arsenius probably imported the idea (as well as the figures themselves) from that city.

The exceptional geographical plate is on the reverse of the plate for the latitude of 51°. It consists of a circular world map, stereographically projected, with the north pole as the centre and the Tropic of the Capricorn as the outer limit. Only four circular world maps on Renaissance astrolabes have been recorded, this example being the earliest and largest. The use of this projection is rarely found in printed terrestrial maps of the 16th century. Arsenius was inspired by the circular world map first published by Peter Apian in Landshut (Germany) as part of his important treatise Cosmographicus Liber of 1524. Gemma Frisius later re-edited this book in Louvain in 1529, with another edition in 1544 still incorporating the circular map with the north pole as centre and extending to the Tropic of Capricorn, but with more up-to-date geographical information. It seems that Arsenius used the 1544 engraving as a model for the plate on this astrolabe, as suggested by the similarities between names and the form of the coastlines. The large sizes of Cuba and ‘Hispaniola’, for example, as well as the inadequate delineation of the Red Sea, Arabia and Persian Gulf, reflect the 1544 engraving. The addition of Japan shows that Arsenius was not a slavish copier however, since this island is absent from Gemma Frisius’s map. All the names of rivers, oceans, seas, countries and cities are engraved in a very elegant and sloping Italic script with long ascenders and descenders. Coastlines and mountains are indicated with hatching, oceans and seas by stippling. The line for longitude 0° passes through the Canary islands. Every 5° of longitude is indicated and every 10° of latitude. Near the ‘Mountains of the Moon’ in Africa is an engraved elephant and near East India is a whale-like creature.