GNOMONICS

The history, art and science of sundials

A brief history of sundials

Sundials have ancient origins. To mention some of the more noteworthy examples, there were the Roman semicircles, hemispherical cavities that showed the time on the inside; the Tower of Winds in Athens; Augustus's clock in Rome, possibly the largest dial plate of ali time, only recently discovered underground and partially restored following research based on the written testimony of Plinio il Vecchio. Time, then measured in temporary hours, subdivided the day from sunrise to sunset into twelve equal parts, regardless of the season.

Later on, temporary hours, which have the drawback of being longer in summer than in winter, were abandoned in favour of Italic hours, a system of timekeeping that took sunset as its starting point. The 24th hour was sunset itself, the 23rd indicated one hour to sunset, the 22nd two hours to sunset and so on. This new system was a considerable improvement on that of temporary hours in that there were twenty-four hours in the day that remained virtually equal throughout the year. At that time, another similar system was introduced elsewhere which marked the 24th hour as being half an hour after sunset, as opposed to sunset itself.

The Italic system was subsequently also dismissed in favour of another that came from France which, although it also divided the day into 24 equal parts, began counting the hours as from midnight.

Nearly all the sundials built in the last century use this last system of timekeeping which, however, shows only local time. This means that a sundial in Florence wilI show a different time to one located in Turin.

In the last few centuries sundials were so common that there was not a single area of town where there was not one to be found on the wall of some building or other.

With the advent of the mechanical clock and the introduction of mean time and time zones, sundials lost importance and began to fall into disuse, first in the cities and finally even in villages in the countryside, so that after centuries of use, this historic timekeeping device was completely abandoned.

In Italy in the last twenty years or so, there has been a renewed upsurge of interest in sundials and in Gnomonics, the science of sundials. In our country, land of great gnomonists, especially from the 1400s to the 1700s, a new generation of students of this discipline is easily formed. At the beginning of the eighties we were few and far between, about ten to be exact. Nowadays there are over a hundred of us and we are continually growing in number.

Many books have been written and many interesting articles published on the subject. A national census of sundials is currently being carried out, having been initiated a few years ago by a group of volunteers and now taken up by the Union of Italian Astrophiles. Besides numerous beautiful, traditional sundials, other types of sundial have also been built - monumental sundials, sundials with curved surfaces designed with the help of the computer.

There are many computer programs nowadays which allow even novices to calculate the plate of a dial with relative case, leaving one free to use one's imagination as to the design and execution.

I myself have produced numerous programs, some of which generate a video image of the sundial whilst others furnish the Cartesian co-ordinates and other necessary data for the construction of the sundial. Some of these programmes are now at the disposition of the interested user in the software page of this site.