coordinates of the Basilica are:
41° 54' 11'' N, and 12° 29' 51'' E
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Milan Cathedral (Duomo di Milano) as seen from the square.
The solar meridian line near the main entrance of the cathedral.
The sun shines through a small hole in the southern wall to cast its light on the meridian line each day (aperture gnomon). The length of the meridian line is limited by the northern wall (at d=55.01 m).
The aperture gnomon.
The mean transit time (solar noon) is 11:23 UT, which varies according to the equation of time from around 11:00 UT to 11:37 UT.
The distance d of the disk of light along the meridian line and the size (horizontal and vertical) are increasing from summer solstice to winter solstice, whereas the time t to cross the meridian line remains nearly constant.
The diameter S of the disk of light is mainly determined by the apparent diameter α of the Sun:
S = D·tan α
The duration t (in seconds) of the meridian transit depends on the Sun's diameter α (arc seconds) and the declination δ:
The diameter a of the aperture and an amount due to diffraction (2·D·1.22·λ/a ≈4 mm) has to be added to S.
The diameter of the aperture is 7 mm
The image is of greatest sharpness if the distance D from the aperture (diameter a) to the surface is given) by (wavelength λ):
D = 22.3 m (for a=7 mm, λ=550 nm)
In Europe the Julian calendar was in use since the 4th century. The spring equinox was set to the date 21 March in the year 325 by the council of Nicea. By the year 1582, the spring equinox was occurring on March 11.
Pope Gregory XIII ordered that 10 days would be removed from the calendar so that the spring equinox would return to March 21. It was decided that the days October 5-14 of the year 1582 would be skipped during the transformation from the Julian to Gregorian calendar.
In the past centuries the obliquity of the ecliptic decreased by 0.013° per century. The result is a shift of the sun spot on the meridian line:
da Passano, Carlo Monti, Luigi Mussio: La meridiana solare del Duomo di
Milano, Veneranda Fabbrica del Duomo di Milano, 2001
J. L. Heilbron: The Sun in the Cathedral, Cathedrals as Solar Observatories,
Harvard University Press, 1999.
Frans and Margret Bruin: The Limits of Accuracy of Aperture-Gnomons,
in: Y. Maeyama and W. G. Saltzer (Editors): Primata, Naturwissenschaftliche Studien, Steiner, Wiesbaden 1977, pp.21-42.