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Florence: solstice in the Cathedral

by Nadia Fondelli – Two more events not to be missed, including one on the day of the summer solstice with the gnomon of the Cathedral of Florence, with its 90 meters is the highest in the world.

Between 12.30 and 13.30 on Friday 21 and 28 June, you can attend a phenomenon of great HGH beauty in the cathedral of Florence: the sun’s rays fall through Brunelleschi’s dome, forming the image of the solar disk that will to overlap perfectly with that of marble placed on the floor of the Chapel of the Cross, to the left of the High Altar.

And ‘since 1475 that this phenomenon is repeated in the Duomo of Florence every year, thanks to an astronomical instrument, the most ancient and widespread, said gnomon, from the greek “indicator”, designed to measure the position of the Sun in the sky and determine the duration the calendar year.

The initiative to be held even if the sky is covered with free entrance with access from the door of the Canons (in front of the rectory away) and is sponsored by the Santa Maria del Fiore with the Committee for the Popularization of Astronomy.

Had installed in the Cathedral presumably by the Florentine mathematician Paolo Dal Pozzo Toscanelli, as suggested some documents kept in the Opera di Santa Maria del Fiore, which is used to ups and downs for over 300 years, the gnomon of the Duomo in Florence, as mentioned, with its 90 meters high, is the largest in the world.
A gnomon and ‘usually a pole, column, or an obelisk whose shadow is used to measure the position of the Sun in the sky, but can be, as in the case of the Duomo of Florence, a hole on the wall of a darkened environment that produces, inside, the image of the solar disk. The image moves from west to east, due to the apparent motion of the sun from west to east. Its position with respect to appropriate references placed on the floor, it allows you to perform precise astronomical measurements and to determine the exact length of a solar year.

Knowledge of the exact duration of the year and the subsequent formulation of the calendar were, in fact, a problem for many centuries, from the classical world, and the instrument astronomical gnomon was a valuable aid until the eighteenth century, when it was replaced with the telescopes. The moment chosen for the comparison of images is that of the summer solstice, because it is the most appropriate for the measurement.

In the Cathedral of Florence and the gnomonic hole ‘was produced by a bronze tablet (the bush) containing a central opening of a few centimeters in diameter and placed horizontally in the south window of the drum of the dome, 90 meters of height from the floor.

The height of the gnomon and ‘that the Sun’s rays, passing through the hole, hit the floor of the church only from late May to late July and for a few minutes before and after noon.
In this period the solar image is formed on the floor of the Chapel of the Cross, to the left of the high altar, where they are, under the protection of brass plates, a meridian line finely graded circular and two marbles, one inside the other , which work by solstice marks. The major, with a diameter of about 90 centimeters, has the same size of the solar image on the summer solstice.

As early as the sixteenth century there has been only sporadic astronomical observations in the Duomo of Florence, sometimes even based on clear scientific objectives, and indeed, the predominant concern for any failure of the dome and for nearly 250 years, assuming the constancy of the height of the Sun to solstice, will be made improper use of the gnomon, and that is to monitor the stability of the church and found, year after year, the return of the solar image at the same points on the rim of marble solstice.

It is only in 1754 that returns to talk about astronomy in the Cathedral, when Leonardo Ximenes, grand-ducal mathematician and Jesuit, he obtained funding to calibrate the gnomon and wrap it with an appropriate meridian line. During the meticulous relief, also mentioned in the large plaque placed in the Chapel of the Cross, the researcher noticed that the floor was not perfectly horizontal so a strip had to be leveled in order to put the meridian line in bronze in the position where we still see it today . Through a series of measurements carried out from 1755 to 1782, compared with earlier in 1510, Ximenes was able to obtain a value of the secular variation of the obliquity of the ecliptic close to the one currently known.

The observation of 1756 can be considered as the beginning of astronomical operations of Ximenes and this date is conventionally traced the founding of the Observatory Ximenian within the complex of St. John Scolopi home at the time of the college and of ‘ Florentine Jesuit universities. However, after the death of Ximenes (1786) measurements were sparse and discontinuous, and the instrument fell into oblivion. In particular, during a restoration, was removed the bushing. When the then director of the Observatory, Giovanni Battista Donati obtained that the bush was reassembled (1865), was placed in a slightly different position from the original, thus interrupting any continuity with the measurements of Ximenes. In 1927-1928 Father Guido Alfani performed other measurements, demonstrating that the Dome is subject to minor fluctuations, seasonal and daily thermal nature. But now even the use of the gnomon was inadequate engineering, and the same Alfani took real seismographs to monitor, in a direct and continuous, even the smallest fluctuation of the building.

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