The Monte Generoso sundial

The vertical sundial, declined 10º 8’ towards the west, shows the true time by the sun, corrected for longitude. The correction involves shifting the hour lines, and therefore also the midday line, by 23 minutes and 56 seconds behind the true sun time. This is known as Local Mean Time. It is due to the difference of 5º 59’ between the longitude of Monte Generoso (9º 1’ E) and that of the point on the prime meridian located 15º E of Greenwich which defines the mean time of our time zone, i.e. the time shown on our clocks and watches. The sundial is constructed of a vertical aluminium panel (180 x 150 cm) set on a stainless steel pedestal.
The position of the plane was based on an idea of a group of students from Mendrisio secondary school and their teacher, Enzo Pfister. The students’ idea was inspired by the Golden Ratio.

The Golden Ratio

In the arts and mathematics, the Golden Ratio is a geometrical proportion based on a specific ratio, whereby the whole (i.e. the sum of the two parts) is to the larger part as the larger part is to the smaller part.
Plato is generally considered to be the father of studies on the Golden Ratio. His definition of the Golden Ratio can be found in the treatise on the elements written by the Greek mathematician Euclid in the third century B.C.
Recognised as an aesthetically pleasing proportion, the Golden Ratio has been used as a basis for the composition of many elements, be they pictorial (Piero della Francesca) or architectonical (the Parthenon).
In actual fact, various experiments suggest than human perception tends to show a natural preference for proportions in accordance with the Golden Ratio; it would seem that artists, almost subconsciously, tend to arrange the elements of a composition on the basis of this ratio.
The position of the style (i.e. the part of the sundial that casts the shadow) and the inclination of the green-coloured equatorial line stem from an iterative application of the definition of the Golden Ratio by a class of students from Mendrisio secondary school.
The sundial shows the true sun time corrected by longitude
On the dial, the hour lines, including the midday line, have been shifted by 23 minutes and 56 seconds to represent Local Mean Time. This is on account of the difference of 5° 51’ between the local longitude (9° 1’ E) and 15°, i.e. the longitude of the meridian situated at the centre of the Central European time zone, the first to the east of the Greenwich prime meridian.

How can we tell the correct clock time from the position of the shadow?

1. Carefully read the time as indicated by the shadow of the style
2. Add one hour (only applicable during legal summer time)
3. Add or deduct the minutes according to the equation of time.

The correct clock time can be worked out by following these simple instructions

The luminous circlet projected unto the dial by the hole at the tip of the style overlaps exactly with and runs rights across the green line during the spring (21 March) and autumn (23 September) equinoxes. The dial also shows the hyperbolae which record the entry of the sun into the different signs of the zodiac.
When the shadow of the style overlaps with the vertical line, it is exactly 12 noon, i.e. noon according to the true local solar time.

The equation of time:

1. In its apparent movement around the earth, the sun traces an ecliptic line every day. Its projection on the celestial equator is a variable magnitude, represented by a double sinusoid in the course of the year.
2. According to Kepler’s second law, the earth orbits around the sun at variable velocity, travelling faster when closest to the sun.
   

The sum of the two variables, day after day, produces the equation of time shown on the sundial.

Planned and designed by Luigi Ferioli, Elia Cozzi and Enzo Pfister.
Realised thanks to the contribution of the management of Ferrovia Monte Generoso S.A.