Bianchini's Sundial - Santa Maria degli Angeli - Roma, Italy - Sundials on Waymarking.com

The floor was laid in the 18th century by Giuseppe Barbieri. The sundial in it was made by the astronomer, mathematician, archaeologist, historian and philosopher Francesco Bianchini.

Bianchini had been commissioned by Pope Clement XI to make it for the Holy Year of 1700. It took a bit longer; it was completed in 1703 with the assistance of the astronomer Giacomo Filippo Maraldi. Then it provided the standard for local Roman time until 1846, when it was replaced by a cannon being fired at noon from the Janiculum. This was controlled by an accurate chronometer.

Bianchini's sundial was built along the meridian that crosses Rome, at longitude 12° 30' E. At solar noon, which varies according to the equation of time from around 10:54 a.m. UTC in late October to 11.24 a.m. UTC in February (11:54 to 12:24 CET), the sun shines through a small hole in the wall (see our photo) to cast its light on this line each day. At the summer solstice, the sun appears highest, and its ray hits the meridian line at the point closest to the wall. At the winter solstice, the ray crosses the line at the point furthest from the wall. At either equinox, the sun touches the line between these two extremes. The longer the meridian line, the more accurately the observer can calculate the length of the year. The meridian line built here is 45 meters long and is composed of bronze, enclosed in yellow-white marble.

In addition to using the line to measure the sun's meridian crossing, Bianchini also added holes in the ceiling to mark the passage of stars. Inside the interior, darkened by covering the windows, Polaris, Arcturus and Sirius were observed through these holes with the aid of a telescope to determine their right ascensions and declinations.

On the floor around the sundial are several panels portraying signs of the zodiac.

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