The Polar Sundial of
San Pietro refuge - Monte Calino
The polar sundial:
what it is and how it works
A polar sundial is a type of sundial that uses the position of the Sun to indicate the time of day. It works thanks to a rod, called a gnomon, which projects its shadow on a dial. This dial is usually a flat surface on which marks are drawn that correspond to the hours of the day. As the Sun moves across the sky, the gnomon's shadow moves across the signs, thus indicating the current time.
The peculiarity of the polar sundial is that the gnomon is oriented parallel to the earth's rotation axis, which means that it is inclined to the plane of the dial at an angle equal to the latitude of the place where the sundial is located. This configuration allows the polar sundial to be particularly accurate during the equinox, when the Sun is directly above the equator.
To build a polar sundial, you need to know the geographical coordinates of the place where it will be installed. This information is critical to correctly positioning the gnomon and designing the dial so that the time indications are accurate.
In summary, a polar sundial works by following these steps:
The gnomon is positioned parallel to the Earth's rotation axis.
The dial is designed with signs corresponding to the hours of the day, taking into account the latitude of the place.
The gnomon's shadow moves across the dial, indicating the time based on the position of the Sun in the sky.
How to read the THREE-DIMENSIONAL polar sundial of the San Pietro Refuge
The Sundial of the San Pietro Refuge has a peculiarity that makes it rare and - for the locality - unique: it is THREE-DIMENSIONAL.
This sundial is regulated by true local time (TVL) linked to the various positions assumed daily by the sun with respect to the location where the sundial is located.
In practice, the TVL time marked differs from the civil time time (indicated by modern clocks) by a certain variable value throughout the year. The daily value of the variation, referring to a given place, easily determinable with the gnomonic calculation, allows you to pass from the true local time to the time of our current civil time, in this case by adding a certain number of minutes to the time displayed on the sundial, according to the date on which the reading itself is performed.
GNOMONIC DATA
Latitude 45° 55′ 59′′ N Longitude 10° 50′ 17" E Azimuth clock orientation South 00'00" Time stamp: TVL (true local time)
How to calculate the time: example on March 1st
Time marked by the sundial 11:31.
Consulting the graph above, on the date of March 1st I note 29 minutes: these must be added to the time read on the sundial. The sum gives me the time 12:00, which will be the time shown on our clock.
Time in minutes to add to Solar Time to obtain the Average Time (our clock time) NB: with summer time in force, add one hour to the time indicated by the sundial.
The Artist, author of the Sundial
The Sundial of the San Pietro Refuge is the work of Mauro Rosà .
It is made of stainless steel, while the base on which the numbers are reflected by sunlight is in corten, a steel highly appreciated by architects and engineers, who choose it for its chromatic qualities and its technical excellence.
To support the entire structure, a slab of natural stone: it is tonalite from Adamello.
