Evidentally Bernardus has so much material for his own new and very fine blog Swarf that he can afford to be sending this stuff to me. Be sure to check out his pics of the very first space launch, a nuclear sunrise and a very interesting article on Winthrop Wetherbee, translater and humanist.

By way of return, I thought I'd post some images that hopefully will appeal to him. They are images the eclipsed sun disc as projected onto the floors of various Catholic cathedrals in Italy. They were all taken during a total eclipse on the 11th of August, 1999.


The light is passing through a gnomon in the form of a hole in the roof of the cathedral which during the course of the day traces an arc across its floor. As the seasons change the position of this arc changes and passes through specific points marked on a meridian. These points represent specific times of the year such as the Vernal Equinox, an important date because Easter is calculated as the first Sunday following the full moon that follows the equinox. It was the locking down of dates such as these that led to the reform of the Julian calendar by Pope Gregory in 1582.

The interesting aspect here is that on normal days a circle is projected on the floor and it would be easy to assume that the shape of this circle was determined by the shape of the hole in the roof. But in fact the hole is so small that it actually acts like a pinhole lens and what is projected onto the floor is really an inverted image of the sun itself. The cathedral is working like a giant pinhole camera.

These gnomons first started to be installed in cathedrals in the 16th century but they got a renewed impetus in the 17th century when Cassini, the famous Italian-French astronomer, used the one at San Petronio in Bologna to try to prove Kepler's reformulation of the Ptolemaic system. Kepler, who had always been a vocal advocate of the Copernicus' heliocentric system recognised that in its original formulation it was actually a less accurate description of the motion of the planets than was Ptolemy's. Before he hit upon the — at the time pretty left-field — idea of planets moving around in elliptical orbits, Kepler described planetary motion in terms of a Ptolemaic concept known as the equant. Where Kepler differed from Ptolemy (apart from replacing the Earth with the Sun in his system) was in the estimation of the distance of this equant. With his access to vastly superior observational data, Kepler estimated that the distance to the equant point was only half that given by Ptolemy.

While Kepler was safe from prosecution by Rome for advocating Heliocentrism [1], Italian astronomers, particularly after the recent trial and abjuration of Galileo were all the more mindful of demonstrating the orthodoxy of their beliefs. Nevertheless Kepler's Ptolemaic argument could still be used in a Geocentric framework so Cassini piously went to work with his gnomons and meridians at San Petronio in order to bring the ancient system of the Almagest [2] into line with the latest astronomical data. For an interesting summary of this work, see this essay by J. L. Heilbron (and here) which excerpts from his book The Sun in the Church: Cathedrals as Solar Observatories. For a more detailed exposition of how this related to Ptolemy and Kepler see this very helpful mathematical supplement to Heilbron's book [3].

[1] safe from prosecution - although Kepler did have to live through the harrowing experience of seeing his mother put on trial for witchcraft.
[2] "the" Almagest - of course, the "the" should be considered redundant because the "al" in Almagest already means "the" in Arabic. Oh well, it seems to be the way the [sic] hoipolloi prefer to say it.
[3] some objections to Heilbron's thesis may be found here.