Picture Caption in Francis Maddison, Medieval Scientific Instruments
An armillary sphere representing the Ptolemaic system, ?c.1425, of brass and gilt brass, overall height: 290mm; diam. of base: 135mm. Within the rings, representing the equator (divided into the 24 hours, with sub-divisions down to 4 minute intervals), the tropics, the polar circles, and the ecliptic (divided into the 12 zodiacal signs, each subdivided into 30°), there is a plain wooden globe on the polar axis. The sphere can be placed in the horizon circle of the stand to represent the heavens as seen from any latitude; the presence of a notch in the meridian ring, 45° from the Pole, and of two diametrically opposite slots in the horizon ring, suggest that a semi-circular band may have been used to lock the sphere in a position suitable for use in latitude 45°. On the base of the stand is engraved an eccentric zodiac/calendar scale (0° Aries = 11 March) with a rotatable index; inset in the base is a small compass, showing only the four cardinal points, with a south-pointing needle (see Taylor, ‘The South-Pointing Needle’, cited in n.44). The vertical support, linking the edge of the base to the horizon ring, is provided with the means of suspending and aligning a plumb-line and bob. The positions of five named stars (the names of Cornu ari[etis] and Aldebaran can be seen on the photograph) are carefully marked on the zodiac band of the sphere, and the longitudes assigned to these stars conform closely with those of star catalogues of the fourteenth and the beginning of the fifteenth centuries. There is no doubt that the maker was attempting an accuracy of better than half a degree; and, therefore, if we were to assume that he was using tables not more than a quarter of a century out of date, and admit a possible error of measurement of a third of a degree (equivalent to a precessional movement of around 25 years), we should arrive at a probable date of the first quarter of the fifteenth century. This date, however, is somewhat earlier than that suggested by the position of the vernal point in the zodiac/calendar scale, but is not inconsistent with the style of the instrument, which is one of the earliest surviving armillary spheres. A groove cut centrally in the zodiac band may have held a rotatable collar bearing an index (or ‘sight’) adjustable to the position of one of the named stars or to the solar declination. This would have enabled the sphere to have been used for simple observational purposes (e.g. time-telling), but like most European armillary spheres its prime use was certainly demonstrational.
[Caption to Figure 2 (colour plate) in Francis Maddison, Medieval Scientific Instruments (Coimbra, 1969), the technical/astronomical analysis acknowledged to John North; transcribed with typographical corrections; latitude 45° is suitable for northern Italy]