The cathedral as observatory

We add some complication to an often oversimplified story.

Our astronomer has just finished rereading a book dealing with some aspects of astronomy in the sixteenth through eighteenth centuries, concentrating on Italy and France and so closely bound up with the activities of the Roman Catholic Church.  The story is often told as the fearless proponent of science and truth, Galileo Galilei, being crushed by the obscurantist Church sensing a challenge to its power and authority.  Subsequently, science south of the Alps withers under intellectual censorship, while in the Protestant North it flourishes.  Often the narrative is simplified to an eternal and irreconcilable conflict between religion and science.  The actual story is different.

We start with the date of Easter.  This was very early defined in terms of three things: the time of year (just after the spring equinox), the phase of the Moon (the months of the Jewish calendar depended on the actual Moon, as ours do not) and the day of the week (Easter must be on a Sunday).  Unfortunately, there aren’t an even number of days in a year, nor days in a month, nor months in a year, so any simple rule you make up will fail after a long enough time.  By the sixteenth century it was clear that the rules the Church had adopted centuries before were no longer working.

So the Church funded astronomical research.  To get a figure for the length of the year, one thing you need to know, you have to time the solstices.  That had been done for millennia by sticking something (a gnomon) in the ground and measuring its shadow (sometimes it was a wooden stake, sometimes an enormous stone block).  But if you seek a more accurate answer you have to be more careful.  You want a big gnomon, so it casts a big shadow, so you can measure it easily; you want the gnomon and the measuring-area to be strong and stable and carefully calibrated, made out of something like stone and maybe with inlaid brass scales; and you want it somewhere out of the weather, so it doesn’t get disturbed or worn away.  A perfect answer was to cut a small hole high in the wall of a cathedral and lay out a scale where the image of the Sun would fall throughout the year.  Beginning in the late sixteenth century, this was being done in several places in Italy (and eventually France).

The remarkable thing to us now is how precise these instruments could be.  Without telescopes or decent clocks or any of the refinements our navigator has used, the astronomers of the time were measuring tiny angles.  They found that they had to be concerned with such subtle effects as the refraction of air and the dip of the horizon.  We would also remark on the fact that most of the observers, including the best ones, were in Holy Orders and in fact held offices in the Church.

All was not easy going, of course.  The lamentable Galileo affair meant that Church astronomers were forced to use some circumlocution like “assuming as an hypothesis” when making calculations in a Sun-centered Solar System.  The irony is that their measurements were finally accurate enough to prove Copernicus right.

(The Sun in the Church, J. L. Heilbron: Harvard University Press, 1999)