R., W. The Great Gnomon of Florence Cathedral . Nature 73, 258–259 (1906). https://doi.org/10.1038/073258a0
https://www.nature.com/articles/073258a0
ALTHOUGH numerous Christian churches are either oriented or adorned with reference to some astronomical phenomenon, there are few of such direct interest to the astronomer as the magnificent cathedral of Florence, which contains a gigantic contrivance for determining. The advent of the summer solstice. We refer to the famous gnomon, placed in the dome of that cathedral by Paolo Toscanelli about the middle of the fifteenth century, and described and illustrated by Mr. W. A. Parr in the December number of Knowledge and Scientific News.
Lalande in 1765 referred to this instrument as ” la meridienne que I ‘on voit dans la Cathedrale de Florence est 1e plus grand monument d ‘Astronomie qu ‘il y ait aumonde, ” but at that time the astronomical significance of the Egyptian temples, or even of our own less pretentious Stonehenge, had not been demonstrated.
Sir Norman Lockyer has shown in “The Dawn of Astronomy ” that the enormous edifice at Karnak, the temple of Amen-Ra, was oriented for a similar purpose, so that at the setting of the sun on the day of the solstice, and a t that time only, the solar beam flashed along the darkened axis of the temple, some 500 yards long, and illuminated the ” holy of holies ” wherein the priest was ready to fulfil the rites of ” the Manifestation of Ra.” He has also shown that Stonehenge was erected for a similar purpose about the year 1680 B.C., but in this case the limb of the actual (rising) sun was observed, the avenue simply forming the sight-line to the point on the horizon where the sun-god would make his first appearance on the day of the solstice.
But although since Lalande made the statement quoted above it has been shown that the gnomon at Florence is not the largest monument to astronomy the world has ever seen, it is still one of the most interesting. As may be gathered from the accompanying illustration, the sunlight, passing through the southern window of the lantern, falls on to the gnomon, which is built into the marble sill of the window, and thence, through a circular orifice, on to a ” solstitial ” marble slab let into the cathedral floor some 300 feet below, where its correct position at the solstice is marked, or was intended to be marked.
This immense meridian instrument was probably designed by Toscanelli in order to provide exact observations for the correction ·of the Alphonsine tables which were then in u se, but which represented very inadequately the solar motion, more especially as regards the true length of the tropical year. Whether he also intended to observe the variation of the obliquity of the ecliptic is a much-discussed question, but it does not seem improbable. In either case his gnomon, probably built only in 1468 A.D., could not answer this purpose anything like so surely as do the Amen-Ra and Stonehenge structures, built many centuries earlier. Apart from this reason, the facts that the gnomon itself has been removed from its original position, and that the solstitial circle on the cathedral floor has been found to be erroneously placed, have destroyed forever the instrument’s utility in this direction.
It has been used, however, in order to detect any possible movement in the fabric of the cathedral, but, to the credit of Brunelleschi, who built the structure, no such movement has ever been demonstrated. In the light of the recent articles in NATURE (p. 153 ) concerning the fires, &c., by which the ancient British festivals were celebrated, it is interesting to note that Mr. Parr considers that the great display of fireworks, which to the modern Florentine forms the chief attraction of the Midsummer Day festival, is simply the analogue of the “St. John’s Fires ” kindled in former times to celebrate the advent of the summer solstice. On that day huge crowds of Florentines flock to the cathedral in order to celebrate the festival of their patron saint, St. Giovanni, and at night the great dome itself is illuminated. W. E. R