On Apr 18, 2025, at 11:43 AM, Dennis J Collyer (dennisjcollyer@gmail.com) nsaac@nsaac.org wrote:



All,

As you may be aware, the renowned Italian (and French) astronomer Cassini is celebrated for his meticulous observations of Saturn’s rings and moons. In fact, a NASA space mission is named after him.

(https://en.m.wikipedia.org/wiki/Cassini%E2%80%93Huygens)

During my recent trip to Bologna, Italy, I was pleasantly surprised when our tour guide mentioned that the church we were visiting contained “Cassini’s Meridian Line.”

Here’s a snippet from Wikipedia:
The church hosts also a marking in the form of a meridian line https://en.m.wikipedia.org/wiki/Sundial#Meridian_lines inlaid in the paving of the left aisle in 1655; it was calculated and designed by the astronomer Giovanni Domenico Cassini https://en.m.wikipedia.org/wiki/Giovanni_Domenico_Cassini, who was teaching astronomy at the University https://en.m.wikipedia.org/wiki/University_of_Bologna. A meridian line does not indicate the time: instead, with its length of 66.8 metres (219 ft) it is one of the largest astronomical instruments in the world, allowing measurements that were for the time uniquely precise. The sunlight, entering through a 27.07 mm (1.066 in) hole placed at a 27.07 m (88.8 ft) height in the church wall, projects an elliptical image of the sun, which at local noon falls exactly on the meridian line and every day is different as to position and size. The position of the projected image along the line allows to determine accurately the daily altitude of the sun at noon x-apple-data-detectors://6, from which Cassini was able to calculate with unprecedented precision astronomical parameters such as the obliquity of the ecliptic https://en.m.wikipedia.org/wiki/Obliquity_of_the_ecliptic, the duration of the tropical year https://en.m.wikipedia.org/wiki/Tropical_year and the timing of equinoxes https://en.m.wikipedia.org/wiki/Equinox and solstices https://en.m.wikipedia.org/wiki/Solstice. On the other hand, the size of the projected sun's image, and in particular its rate of variation during the year, allowed Cassini the first experimental verification of Kepler's laws of planetary motion https://en.m.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion.

I captured this photo of where the sunlight enters through the 1.066-inch hole in the ceiling.

Attachment(s):
image0.jpeg https://images.clubexpress.com/827459/attach/4137581_1_image0.jpeg (3.5 MB)
Video.mov https://images.clubexpress.com/827459/attach/4137581_2_Video.mov (3.3 MB)
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On Apr 18, 2025, at 11:43 AM, Dennis J Collyer (dennisjcollyer@gmail.com)
nsaac@nsaac.org wrote:



All,

As you may be aware, the renowned Italian (and French) astronomer Cassini
is celebrated for his meticulous observations of Saturn’s rings and moons.
In fact, a NASA space mission is named after him.

(https://en.m.wikipedia.org/wiki/Cassini%E2%80%93Huygens)

During my recent trip to Bologna, Italy, I was pleasantly surprised when
our tour guide mentioned that the church we were visiting contained
“Cassini’s Meridian Line.”

Here’s a snippet from Wikipedia:
The church hosts also a marking in the form of a meridian line
https://en.m.wikipedia.org/wiki/Sundial#Meridian_lines inlaid in the
paving of the left aisle in 1655; it was calculated and designed by the
astronomer Giovanni Domenico Cassini
https://en.m.wikipedia.org/wiki/Giovanni_Domenico_Cassini, who was
teaching astronomy at the University
https://en.m.wikipedia.org/wiki/University_of_Bologna. A meridian line
does not indicate the time: instead, with its length of 66.8 metres
(219 ft) it is one of the largest astronomical instruments in the world,
allowing measurements that were for the time uniquely precise. The *
sunlight, entering through a 27.07 mm (1.066 in) hole placed at a
27.07 m (88.8 ft) height in the church wall, projects an elliptical image
of the sun, which at local noon falls exactly on the meridian line and
every day is different as to position and size. The position of the
projected image along the line allows to determine accurately the daily
altitude of the sun at noon, from which Cassini was able to calculate
with unprecedented precision astronomical parameters such as the obliquity
of the ecliptic
https://en.m.wikipedia.org/wiki/Obliquity_of_the_ecliptic, the
duration of the tropical year
https://en.m.wikipedia.org/wiki/Tropical_year and the timing of *
equinoxes https://en.m.wikipedia.org/wiki/Equinox* and solstices
https://en.m.wikipedia.org/wiki/Solstice. On the other hand, the size
of the projected sun's image, and in particular its rate of variation
during the year, allowed Cassini the first experimental verification of Kepler's
laws of planetary motion
https://en.m.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion.*

I captured this photo of where the sunlight enters through the 1.066-inch
hole in the ceiling.

Attachment(s):
image0.jpeg
https://images.clubexpress.com/827459/attach/4137581_1_image0.jpeg (3.5
MB)
Video.mov
https://images.clubexpress.com/827459/attach/4137581_2_Video.mov (3.3
MB)
You have received this message as a member of: North Shore Amateur
Astronomy Club
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