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Astronomical Images : Method of observing a solar eclipse (camera obscura)

Daniel Santbech

Astronomical Images

<p style='text-align: justify;'>Daniel Santbech's <i>Problemata</i> is a miscellaneous work in mixed mathematics, including astronomy, gnomonics, perspective, surveying, gunnery and navigation. The unifying element in such a wide range of disciplines is the instrument called the geometric quadrant. According to Lynn Thorndike, Santbech showed appreciation both for specific aspects of Copernicus's work, such as lunar theory, and for the <i>Prutenic Tables</i>, but he ignored cosmological issues. Copernicus is mentioned many times. Virtually nothing is known about Santbech, apart from the fact that he published, in addition to this work, an edition of Regiomontanus's <i>De triangulis</i> (Basel, 1561). The title-page calls Santbech '<i>Noviomagus</i>', and this is generally interpreted as meaning 'of Nijmegen', but Andreas Kleinert reports that Santbech could be from Speyer of Neumagen, a small town near Trier. The volume included a dedicatory letter by Santbech to '<i>Domino Joanni Ludovico a Windeck</i>', dated 1561. The existence of a 1542 Basel edition is disputed by Ernst Zinner. The figures in this edition seem to have been copied from Oronce Fine's <i>Protomathesis</i>. In this passage, Santbech describes the use of a camera obscura to measure the extent of solar eclipses. A dark room with a small aperture in one wall is set up and the image of the Sun is thus projected onto the opposite wall. Santbech points out that the projected image will be inverted, so that if the lower part of the Sun is eclipsed, the top of the image on the wall will be missing. This is one of the many cases in early modern printing in which the text of an account does not quite match the relevant figure. In the textual account, the line representing the ray from the upper edge of the Sun to the lower edge of the projected image is called DF, which is clearly incorrect, since in the figure the ray in question in fact passes from D to R. We might therefore consider amending the text to read DR. However, Santbech also states in the text that the circumference of the projected image is GHIK. Since the ray described above had to pass by the edge of the projected image, we may correct the R in the image to K. R is mentioned later in the text, though, as the point at which the ray from the boundary between the dark and light parts of the Sun meets the corresponding boundary in the projected image. We may thus infer that R should in fact have been placed inside the projected circle at the intersection of HI and GK. It is difficult to know exactly what went wrong with the production of the image in this case. We may speculate that the person who cut the woodblock either did not have access to the textual account or did not understand it. Alternatively, the person who made the original drawing from which the block was copied may have erred. This highlights the extent of the division of labour required to produce printed figures in this period. As Santbech points out, Gemma Frisius had reported on his own use of the camera obscura in astronomical observation in his account of the solar eclipse that was visible from Louvain in 1544. The report of this observation was included in Frisius' <i>De radio astronomico et geometrico</i> (Antwerp, 1545), along with a figure of a set up very similar to the one shown here by Santbech.</p>

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