A Critical Edition of Ibn al-Haytham's On the Shape of the Eclipse

The main content here is a translation and thorough reconstruction of Ibn al-Haytham’s On the Shape of the Eclipse, with Arabic text, from available manuscripts. At least as important, and occupying more pages, is the examination of this work as the first known scientific analysis of the camera obscura. That is, this a Tenth Century example of rigorous experiment with the camera obscura as the apparatus and a partial eclipse as the opportunity to assay fundamental optics. Reading of this circa 990 AD experiment stirred within me the same feelings of wonder at first-hand theoretical confirmation as reading of the 1919 solar eclipse; Sir Arthur Eddington’s opportunity for the first experimental test of relativity. Working a millennium before, al-Haytham did his own important work.

Three parts make up this work. Initially, the subject matter is stemmatological and codicological. That is, the process of reconstructing the genealogy of the text based on relations between the various extant versions to ascertain accuracy and derivation. The author discusses this approach generally, making for an interesting overview of textual sleuthing. Introductory material also includes basic optics and the geometry of the camera obscura with illustrative diagrams.

A reconstructed text, drawing on multiple sources, makes up the middle portion; English translation and diagrams verso, Arabic recto. The final part is the bulk of this work. It places On the Shape of the Eclipse into its historical context and frames for the reader its significance as methodical experimentalism in an antique era. As underscored here, al-Haytham confronted basic questions extant at his time, such as “Why does the Sun penetrating through quadrilaterals not form rectilinear shapes…” (This from some of the source material available to al-Haytham.) al-Haytham assayed such questions in a pre-lens era with the camera obscura as a ready laboratory for geometric optics. This is shown here to be an experimental method impressive for the age. The role as analytical experimenter shown in this treatise where the “methodological character is resolutely upheld” is detailed in eight cases of willful and decided experimentation singled out by the author. Again, this is the record of an experiment with a primitive optical apparatus done at an opportune time:

“Despite its title, On the Shape of the Eclipse is more an optical work than an astronomical treatise. The choice of studying a solar partial eclipse arises here essentially from optical considerations: The Sun provides a powerful light source; the partial eclipse breaks symmetry of the solar disc.

The lede is buried in the subtitle. The detailed geometric analysis and Spirograph-recalling exploration of the overlapping images that confronted al-Haytham is a ready undergraduate classroom capsule in geometry, optics, or astronomy. Despite emerging from the remote past, the synergistic intertwining of geometric analysis of light and experimental reasoning is supported by a systematic approach of varying all parameters: the topology of the aperture (considered with barleycorns), focal length, and (thanks to Ptolemaic scales) the distance to and scale of the celestial bodies. As the author summarizes, “al-Haytham freed himself from the context in which the camera obscura was commonly used … to engage in a thorough understanding of its operating … through a combination of mathematics and the experimental method, which was no less efficient in medieval Islam than in the modern era.”

Reading the first parts, I noticed that no date for the heavenly event was clearly stated. As explained, there is much more to ascertaining such a date, even within wide probability, and this is necessarily kept until the final third. Since al-Haytham did not date the partial eclipse or even locate the camera obscura for later readers, much consideration must be made to narrow down the possible dates. As with the introduction to stemmatology, the author offers a very general explanation of the approach. From the geography of candidate cities to the circularity of al-Haytham’s diagrams, the reader finds a gentle yet detailed overview on “dating of past astronomical events” made more difficult due to the frequency of partial eclipses compared to rarer totality. Finally, this study of al-Haytham and his disquisition includes an Arabic-English glossary with a profusion of plates and illustrations along with the more expected supplementary material.

For viewing the 2017 solar eclipse from Michigan, Tom Schulte built a box pinhole projector six-feet long for a quarter-sized image on a white label sticker surface.