Jarosław Włodarczyk

SOLAR ECLIPSE OBSERVATIONS IN THE TIME OF COPERNICUS : TRADITION OR NOVELTY?

In Western astronomy before Tycho Brahe and Kepler, only a handful of astronomical observations were actually used to derive parameters in a theory or to test the structure of a theory. Those observations, usually quoted in scientific treatises, such as the Almagest or De revolutionibus, have been analysed by historians of astronomy who want to know how observations are to be tied to a theory. Not surprisingly, those records give us a very general picture of historical observational practice. However, there are still not fully explored series of observations that allow us to understand details of the observational methods used in the medieval and early modern period and the transmission of these methods. This paper will offer an examination of such a series of observations made in Frauenburg by Nicolaus Copernicus. The series was recorded by Copernicus in his copy of Johann Stoeffler’s Calendarium Romanum magnum (Oppenheim, 1518), and concerns four partial solar eclipses that occurred in 1530, 1536, 1540, and 1541. It will be argued that Copernicus employed the camera obscura (pinhole camera) to measure the magnitude of these eclipses. This conclusion will allow us to strengthen a thesis previously proposed by Ludwik A. Birkenmajer, that the astronomical use of images formed through an aperture, which spread among European astronomers in the second half of the sixteenth century, may have its source in eclipse measurements made by Copernicus during the later years of his scientific activity.

Edward Gresham, Copernican Cosmology, and Planetary Occultations in Pre-Telescopic Astronomy

This article introduces an understudied source in the history of astronomy, the Astrostereon or the Discourse of the Falling of the Planet (1603). Written by the English astrologer Edward Gresham, this text presents, among other things, the earliest known set of predicted planetary occultations (for 1603–1604) and the use of these phenomena to defend the Copernican cosmology. We analyse those predictions and then briefly survey all known pre-telescopic observations of reported planetary occulations and the motivations for such observations. These data suggest that for early observers, the greater the difference in apparent brightness between the two occulting bodies, the greater the angular separation could be for an occultation nonetheless to be reported. An appendix seeks to explain this finding by considering several factors known from modern experimental analyses of human visual performance.

The Solar Dial in the Olsztyn Castle: Its Construction and Relation to Copernicus

The article discusses the construction, function, and origin of the solar dial in the Olsztyn Castle, traditionally attributed to Copernicus. The dial, preserved partially on the wall of the cloister and presumably designed to determine the time of equinoxes, served as an astronomical instrument mapping the daily paths of the sun in the sky. The article provides a comprehensive mathematical model of the instrument, taking into account the astronomical and architectural factors which affected its functioning. The analysis allows to alienate the essential properties of the dial as an observational instrument and to contend that measurements were recorded indelibly on the wall and averaged by interpolation. Furthermore, it reconsiders the arguments which support the hypothesis ascribing the construction of the Olsztyn instrument to Copernicus. (Some mathematical appendices appear only in the online issue of the journal.)

Book Review: The Supposed Moon of Venus: The Moon That Wasn't: The Saga of Venus' Spurious SatelliteThe Moon That Wasn't: The Saga of Venus' Spurious Satellite. KraghHelge, with the assistance of Kurt Møller Pedersen (Science Networks: Historical Studies, xxxvii; Birkhäuser, Basel, 2008). Pp. xii + 199. €39.90. ISBN 978-3-7643-8908-6.

This book should appeal as much to historians of medieval intellectual history and of medieval astronomy. Its focus on diagrams is extremely welcome. Its orientation is novel in the history of early astronomy. Readers may well wish to argue with one or another interpretation in the book, but this will not diminish its importance in expanding our awareness of the significance of scientific diagrams. I should mention one noteworthy error in the book’s bibliography. Under the joint authorship of Eastwood/Graßhoff the reader will find only a preliminary version, under a different title (publ. Bern, 2000), rather than the full and final version, of Planetary diagrams for Roman astronomy in medieval Europe, ca. 800–1500 (Transactions of the American Philosophical Society, xciv/3 (Philadelphia, 2004)).