Well known since antiquity, the sundial is an instrument made up by a style casting a shadow on a quadrant variously shaped and inclined. Its functioning is based on the observation of the style's shadow, combined with the notion of the Sun as a time-keeper apparently revolving around the immobile Earth in a uniform daily motion.

The sundial of the Museo Galileo is made up by a large quadrant traced on the pavement and a huge bronze stele functioning as a gnomon.

This sundial was built in 2007 according to the design by Luise Schnabel and Filippo Camerota, with the contribution by Ente Cassa di Risparmio di Firenze. It has been restored in 2015 thanks to the generous support provided by Officine Panerai.

Three curves indicate the daily paths of the edge of the shadow on certain days of the year.

The curve farthest from the stele marks the path at the winter solstice, when the Sun moves at the lowest altitude above the horizon.

The intermediate curve shows the path at the spring equinox and fall equinox, when day and night are of equal length.

The curve nearest to the stele marks the path at the summer solstice, when the Sun travels highest above the horizon.

A second series of lines divides the three curves into uneven sections that are covered by the edge of the shadow in one hour each.

At morning, the shadow appears to tilt westward. During the first six hours of the day, the shadow increasingly shortens, reaching its minimum length at midday. It then lengthens in the following six hours until disappearing eastward at sunset.

From a careful observation of the shadow cast on the quadrant, we can therefore know not only the time of day, but also the time of year.

Date and time are shown by the shadow of the glass globe atop the large bronze gnomon.

The hours from 9:00am to 2:00pm are marked out by radial brass lines while the date is indicated by the travertine transversal lines which mark the Sun's diurnal course for various periods of the year - exactly when the Sun enters the signs of the Zodiac.

The shadow cast by the gnomon changes in length during the course of the days and seasons. It indicates the true solar time of the place, which is different from the time indicated by wristwatches. This last is known as the mean time.

In respect to true solar time, mean time has a periodic variation that can exceed a quarter of an hour during the course of a year.

The difference between the regular time indicated by wristwatches and the variable time shown by sundials is known as equation of time. The difference, given in minutes, is reported by a diagram showing the position of the Sun during the course of the year, measured at the same mean time.

The curve described in the sky by the position of the Sun at the same mean time is called analemma or lemniscàta.

During daylight saving time, in spring and summer, the difference between true and mean time must consider the setting of clocks one hour forward.

For example, in February the sundial would indicate true midday around 12:28 pm while in July, during daylight saving time, around 13:20 pm.

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 the hour line, you can read the half hour and quarter hour with close approximation, 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 months marked out along the meridian line.

The gnomon is made up by two large bronze stelae that symbolize the day and night. The stele representing the day, facing south, towards the Arno river, contains a vertical meridian line on which the shadow cast by the tail of a "liziper" - half-lizard, half-viper - indicate midday for each period of the year.

The stele representing the night, facing north, shows the representation of the two constellations that allow to identify the Pole Star: the Ursa Major and Ursa Minor.

The wind rose placed at the base of the gnomon shows the directions for geographic orientation.

After the sunset, the sundial continues to act as an attractive urban element thanks to the lighting underneath the base of the gnomon and the Zodiac signs.