MONUMENTAL SUNDIAL . MUSEO GALILEO . FLORENCE

The Museo Galileo‘s Monumental Sundial was built as a mathematical ornament in 2007. The slender bronze column (stele) is in fact formed from two matching columns closely aligned, symbolising day and night. The (mid)day stele faces south, with a vertical meridian line on which the shadow is cast by a lizard’s tail (actually, an imaginary half-lizard, half-viper). The night stele faces north and signifies the constellations Ursa Major and Ursa Minor that enable the Pole Star to be identified.

The encircled quadrant design on the pavement at the base of the bronze columns indicates the geographic orientation. This glass base of the gnomon, and also the Zodiac signs in the meridian line (below), are up-lit after dark.

The Museo explains the meridian line in helpfully simple terms: A travertine and brass meridian line is drawn on the pavement, flanked with glass and marble signs of the Zodiac. The meridian line extends for about 15 metres from the museum entrance, where the winter solstice is marked, to the base of the gnomon, where the summer solstice is marked. The travertine curves crossing the meridian line indicate the date. The brass radial lines forming a grid with the two solstitial curves indicate the hours.

The seasons and the four elements are symbolised by the choice of materials: travertine for the earth and autumn; glass for the water and winter; grey stone for the air and spring; bronze for the fire and summer.

LIZARD / VIPER GNOMON ON THE SOUTH FACE OF THE COLUMN

This extraordinary sundial stands by the Arno with the Ponte Vecchio (which itself has a wonderful dial LINK) close by to the west. For anyone with even a minuscule interest in or curiosity about the gradual development of scientific instruments and techniques from medieval times onwards, pay a visit to the excellent online gallery LINK. Look in particular for the two astronomical telescopes made by Galileo himself.

The North American Sundial Society has very good online information about this unique dial. You can watch a short animation of how this gnomonic sundial works here LINK

For those interested in finer details of the way the dial works, the museum’s detailed account is included at the end of this article.

Museo Galileo – Istituto e Museo di Storia della Scienza

Museo Galileo – Monumental Sundial

NASS (North American Sundial Society) Video: Filippo Camerota, Luise Schnabel, Giorgio Strano

How the Sundial works

The shadow cast by the glass polyhedron atop the large bronze gnomon indicates the date and time. The hours from 9:00 AM to 2:00 PM are marked out by radial brass lines. The date is indicated by the travertine traversal lines which mark the Sun’s diurnal course for various periods of the year – precisely when the Star enters the signs of the Zodiac. The shadow cast by the gnomon changes in length during the course of the days and seasons, and indicates true solar time for the place where it is located, which is a different time than that of our wristwatches, known as mean time. In respect to mean time, true solar time has a periodic variation that can exceed a quarter of an hour.

Moreover, during daylight saving time, the hands of a clock are moved forward one hour. For example, true midday in the month of February would be indicated by the sundial around 12:28 AM while in the month of July it would be indicated around 1:20 PM daylight saving time.

To read the hour and date, you have to identify the hour lines and the calendrical lines closest to the gnomon’s shadow. When the shadow does not fall exactly on a hour line, you can read the half-hours and quarters with close approximation by ideally subdividing the space between two hour lines in two or four parts. The date can also be read by referring to the Zodiac signs and the start of the months marked out along the meridian line.

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