Readers of my blog know quite well how astrology was important in Rome. It was important for Latins, but much more in Renaissance and late Renaissance. There was almost nothing done without consulting the astrologers, even inside the same Catholic Church. I believe of some interest to give a deeper look to the famous sundial of Santa Maria degli Angeli, one of the most important Italian ones, built by the astronomer Francesco Bianchini in 1702 in the church designed by Michelangelo.
The sundial is called in Italian meridiana, because it is nothing else than a portion of a meridian of the place. The word comes from Latin, meridies, and means midday, because the Sun crosses it the Meridian at half of its path from rising to setting.
In the same way, the Sun enlightens a given point of the line painted in the floor exactly at midday.
This sundial was built in order to check the efficiency of the new Gregorian Calendar: Church was very interested in it, because Jews Passover, the one Jesus celebrated too, was
In the first month, on the fourteenth day of the month, – ie with the Full Moon– at twilight, is a Passover offering to the Lord ((23, 5))
When Christians moved Easter to Sunday, they put it in the following Sunday to the first Full Moon after the Spring Equinox.
But Julian Calendar was a little longer than the real duration of the year, so at the end of 1500 there were 10 days of difference between the real Equinox and the Calendar one, so it was necessary for the Catholic Church to line up the civil and the astronomical year. For this reason, because the Reform was necessary to calculate Easter, it was immediately adopted by Catholic countries, but not in the rest of the Europe.
For this reason, sundials are mostly built inside churches, because astronomy, astrology and religion were deeply linked, especially in Renaissance period.
This is how the sundial works. In the southern side there is an hole from which the Sun enters.
In Summer the Sun makes a greater arc- the famous arc of primary directions :)- and its rays are almost perpendicular to the Earth, in winter the arc is very short and its rays are almost parallel, so when the light of the Sun enters from the hole changes its inclination according the season, and enlightens the proper place in the Sundial
in the sundial we find fixed stars with their right ascension in 1702, and two data, the distance from zenith and the proportion per 100 degrees.
In particular the zenith distance is the angle between the ray and the wall where is the hole: the difference between the latitude and the zenith distance is the other coordinate, the declination.
Obviously we cannot forget the astrological signs.
The site of the church, http://www.santamariadegliangeliroma.it
Nicoletta Lanciano, Astronomia a Roma, Apeiron 2010
Girolamo Fantoni, La Grande Meridiana di Santa Maria degli Angeli a Roma, in “Atti dell’ Istituto Italiano della Navigazione”