A detailed description of an as yet unrecorded astrolabe quadrant in a private collection is presented. A date between 1291 and 1310 is deduced from the calendrical data engraved on it. The characteristics of the newly recorded instrument have been compared with those of six other medieval astrolabe quadrants. The newly recorded instrument appears to present an early, if not the earliest, stage of development in the history of the astrolabe quadrant. In the comparison the newly recorded instrument is also (...) distinguished by the red and black colours applied to differentiate between the superposed scales of the astrolabe quadrant. The names and the positions of the stars on the instruments were used to establish a connection between two of the instruments studied, those in Rouen and Oxford, and a manuscript tradition on the construction of the astrolabe quadrant, exemplified by MS Oxford Ashmole 1522. The data on three other astrolabe quadrants are shown to stem from star lists in treatises on astrolabes. For two instruments, those dated earliest and latest, no such connection was found. Some star names on the newly recorded instrument have hitherto not been recorded. Finally, the competence of the later medieval artisans has been investigated by analysing the accuracy of these instruments. (shrink)

The Istituto e Museo di Storia della Scienza, Florence, Italy, possesses an astrolabe with five latitude plates that is now attributed to the Duisburg workshop of Gerard Mercator. Although it is known that Mercator made instruments, this is the first surviving example to be identified. Another latitude plate is shown to come from the workshop of the Florentine, Giovan Battista Giusti. A seventh plate, possibly engraved by Rumold Mercator, provides the only known Mercatorian polar stereographic projection. The role of Egnazio (...) Danti, cosmographer to the Grand Duke of Tuscany, in the acquisition of the astrolabe in about 1570 is considered. (shrink)

This paper examines the purport of epact tables encountered on scientific instruments, and explains their use. The epact is a valuable chronological aid for calculating the age of the moon. In handbooks of chronology, usually two types of epacts are distinguished: the epact used in medieval times, and the so-called Lilian epact used after 1582 in the Gregorian perpetual calendar. By examining the rules for calculating the age of the moon, it turns out that the Julian and Gregorian epacts encountered (...) on instruments must be distinguished from the medieval and Lilian epacts. It is shown that the Julian epact was already in use in 1478, and that, by adjusting for the shift of ten days in the date of the vernal equinox, the Gregorian epact was derived from it in 1582. The common association of the latter with the Lilian epact employed in the Gregorian perpetual calendar is incorrect. It is further shown that in contrast to the medieval and Lilian epacts, which served purely ecclesiastical purposes, the Julian and Gregorian epacts were mainly used to calculate the true age and zodiacal position of the moon. This knowledge was applied to secular interests such as ‘lunar astrology’, tidal computations, and the conversion of lunar into solar time. (shrink)

The impact on globe making of the change from a Ptolemaic to a Copernican world-view is examined. As well as showing a map of the Earth and the Heavens, the main use of globes originally was to demonstrate the natural phenomena as these are observed from a geocentric perspective. In the second half of the eighteenth century some belated attempts were made to construct so-called Copernican globes for this purpose. This late response did not stop the production and use of (...) the common Ptolemaic globe. It is argued that the technological developments of the nineteenth century made the role of the globe as a demonstration model superfluous and thus contributed more to the downfall of the common Ptolemaic globe than did any revolution in science. (shrink)

Early observations of the southern celestial sky were reported in many sixteenth-century books and compilations of voyages of discovery. Here we analyse these accounts in order to find out what was really seen and reported by the first navigators. Our analysis had resulted in new interpretations of the phenomena reported by Amerigo Vespucci and Andreas Corsali. Thus, a reassessment of the discovery of the Coalsack Nebula, the Magellanic Clouds, and the Southern Cross can be made. From a comparative review of (...) the observations of the latter constellation as published between 1500 and 1600, we demonstrate that only questionable records found their way to contemporary compilations of voyages of discovery, and that as a result public knowledge about this constellation at the end of the sixteenth century was entirely unreliable. Another problem we discuss is that although the stars of the Southern Cross were the first to be discovered, and were observed again and again by many navigators, it was not until 1678 that their proper positions were found in stellar atlases and star catalogues accessible to astronomers. We explain how negligence of and subsequently confidence in Ptolemy's astronomy by, respectively, the early navigators and cartographers, were at the root of this amazingly long-lasting gap in the knowledge of the southern celestial sky. (shrink)

Summary This paper describes a medieval instrument, a quadrans novus, which turned up during archaeological works in England. The invention of the instrument by Profacius in 1288 is discussed in terms of two other medieval instruments, the quadrans vetus and the common astrolabe. The characteristics of the present instrument are compared with those of the seven other known medieval quadrants. It is shown that the new quadrant was made in England for explicit use with the Sun.