It is well known that Richard Hakluyt in a publication of 1581 congratulated Sir Walter Ralegh for employing Harriot to teach him and his many sea captains the sciences of navigation. Even more important, however, was the navigational research carried out by Harriot on behalf of Ralegh. He made important theoretical advances in map theory and in navigational astronomy, carried out the astronomical observations needed for a reform in navigational tables, and designed and himself tested at sea improved navigational instruments. (...) Harriot had many other responsibilities in connexion with Ralegh's enterprises. From August 1585 to June 1586 Harriot was in Virginia, and in 1589 he was listed as one of Ralegh's colonists in Munster. He collected intelligence concerning America for Ralegh, and his publication of 1588 was effective as propaganda for Virginia. Harriot was also entrusted with financial, and even political responsibilities by Ralegh. Instructing Ralegh and his captains in navigation was an important part of Harriot's work but it is more likely that he did this as the occasion demanded, rather than on a regular basis. (shrink)

The year 2009 marks the 400th anniversary of the publication of one of the most revolutionary scientific texts ever written. In this book, appropriately entitled, Astronomia nova, Johannes Kepler developed an astronomical theory which departs fundamentally from the systems of Ptolemy and Copernicus. One of the great innovations of this theory is its dependence on the science of optics. The declared goal of Kepler in his earlier publication, Paralipomena to Witelo whereby The Optical Part of Astronomy is Treated , was (...) to solve difficulties and expose illusions astronomers face when conducting astronomical observations with optical instruments. To avoid observational errors that had plagued the antiquated measuring techniques for calculating the apparent diameter and angular position of the luminaries, Kepler designed a novel device: the ecliptic instrument. In this paper we seek to shed light on the role optical instruments play in Kepler's scheme: they impose constraints on theory, but at the same time render astronomical knowledge secure. To get a comprehensive grasp of Kepler's astonishing achievements it is required to widen the approach to his writings and study Kepler not only as a mathematico-physical astronomer, but also as a designer of instruments and a practicing observer. (shrink)

A table in five columns for the radii of the Sun, the Moon, and the shadow is included in sets of astronomical tables from the fifteenth to the early seventeenth century, specifically in those by John of Gmunden (d. 1442), Peurbach (d. 1461), the second edition of the Alfonsine Tables (1492), Copernicus (d. 1543), Brahe (d. 1601), and Longomontanus (d. 1647). The arrangement is the same and the entries did not change much, despite many innovations in astronomical theories in this (...) time period. In other words, there is continuity in presentation and, from the point of view of the user of these tables, changes in the theory played no role. In general, the methods for computing the entries are not described and have to be reconstructed. In this paper, we focus on the users of these tables rather than on their compilers, but we refer to modern reconstructions where appropriate. A key issue is the treatment of the size of the Moon during a solar eclipse which was not properly understood by Tycho Brahe. Kepler’s solution and that of his predecessor, Levi ben Gerson (d. 1344), are discussed. (shrink)

Mathematical instruments in the early-modern period lay at the intersection of various knowledge traditions, both practical and scholarly. Scholars treated instrument-related questions in their works, while instrument makers and mathematical practitioners also put much energy into producing instrument books. Assessing the role of that literature in the exchange of knowledge between the different traditions is a complex task. Did it directly influence workshop practice? Here, I will examine instruments from a famous Louvain workshop ca. 1570, focussing on the role of (...) printed images. I will suggest that woodcuts did indeed inspire instrument makers; that images were sometimes more important than the text; and that the viewer’s appreciation of the images depended upon his familiarity with an instrument’s mathematical structure. -/- . (shrink)