Abbey of San Leonardo: when architecture, astronomy and religion meet

Built in a Romanesque style, its exact date of foundation is unknown, although it is possible to associate it with the end of the 11th century and the beginning of the 12th. In fact, the first to settle in the abbey and inaugurate it as a place of worship were the Regular Canons of St. Augustine in 1127. On the left side of the abbey there is a vertical inscription in Latin, which most likely indicates the name of the founder or inspiration of the complex, the priest William.

The first half of the 13th century represented the most flourishing period for the abbey, while during the second half a quick decline began. In 1261 the abbey passes into the hands of the monks of the Teutonic Order until the second half of the 15th century, when its management was entrusted to the local diocese. Since the beginning of the 17th century the abbey was the home to the Franciscan Order until the end of the 18th century. In 1809 the hospital of St. Leonard was suppressed and the entire complex passed under the control of the hospital of Foggia.

The abbey was abandoned and during the years of the war it suffered a lot of damage. After the war, Don Silvestro Mastrobuoni started the reconstruction of the abbey, becoming its pastor. The abbey is currently in function as the parish of Siponto.

The importance of this abbey is not only linked to its historical period but also to the astronomical phenomena that takes place in it and that makes this abbey unique in the world.


A similar event to what happens in the abbey of St. Leonard can be found in the famous Gothic cathedral of Chartres, in France: here at noon of the solstice summer, on the 21st June, a beam of sunlight passes through a hole in a stained glass window and illuminates a tile metal embedded in the floor.

In the abbey of St. Leonard this “capture” of the sunlight, however, is much more fascinating and elaborate because there is no window! The beam penetrates through a hole in the ceiling of the church and is perfectly directed in the middle of the two pillars, facing the side entrance, which is the most important since it is enriched by a beautiful portal.

The mysterious performer chose first the point where the captured beam would drop and then he used his knowledge of astronomy to orient it towards the ceiling, as we would do pointing the telescope at a particular star (with the difference that we could easily correct direction and he couldn’t without punching the ceiling). After that he pre-punched hole inside a rose to eleven rays (recalling the rose window of the cathedral of Troy) so that the beam is focused to disperse and come to the floor. This masterpiece, at least from a technical point of view, is much more charming Chartres.


But what is the true meaning of this hole?

In many churches we find sundials or other instruments that served as watches or “calendars” to locate exactly on the day of the vernal equinox and determine when to celebrate Easter (the first Sunday after the full moon after the spring equinox).

However abbey sipontina we are faced with a “gnomonic hole” that only works at the summer solstice. Its operation is ensured from the point where the radius that must fall, as mentioned above, coincides with the half of the distance between the two columns, so even if the tile that affects is removed or it will ruin the entire floor, the distance between the columns will always be the same. The hole is said because the gnomonic projection beam of sunlight is centrografica, this means that at the exact moment a single imaginary axis joining Sun-hole-center of the earth.

In religious beliefs, the number eleven indicates the strength of the light, and then the attributes of Christ. In addition, the descent of the beam prepunched symbolizes the descent of the Holy Spirit, in fact it is customary to stand in the middle of the visitors making hit by this ray.


Another similar phenomenon takes place in the days of the equinox of spring and autumn, which we have already mentioned the importance. In the west facade there is a hole formed by ten petals through which passes the beam of light that passes through the aisle and hit the apse projecting an “almond” of sunlight.

At Chartres if something was wrong enough to redo a window. In San Leonardo if something went wrong you had to redo the whole abbey? I think so. I leave you to the final judgment of this comparison.

Article and photos by Vittorio Pagano