[extropy-chat] The Sun in the Church

[Amara Graps]

Hello...

A five minute walk away from the main location in Rome where
buses, trains and the metro meet is a Renaissance church named
Basilica di Santa Maria degli Angeli, which was made out of the
core of the 4 AD Roman Baths of Diocletian with later (very
minor) interior modifications by  Michelangelo.

Because it was a five minute walk for me in one of the busiest
parts of Rome, and the structure from the outside looked like
'yet another ancient Roman ruin' (:-) ), I didn't spend the time
to go inside until a friend dragged me inside last November.
Inside the church, I discovered a solar observatory.

During much of the 16th, 17th, and 18th centuries, Roman Catholic
churches were the best solar observatories in the world.  Besides
being a place of worship, churches provided the only buildings
tall enough for such kinds of observatories.

The Great Meridian of the Basilica di Santa Maria degli Angeli
was used for 150 years to determine mid-day for the people of the
city of Rome, to obtain a correct calendar for determining the
date of Easter, and to have a suitable place for the Jesuit
astronomers to make their work.

The Great Meridian is kind of sundial. Light comes in through a
carefully placed opening ("gnomon") high up on the wall and falls
across a brass strip in the floor, the exact North-South meridian
of Rome. The Great Meridian in this church is actually a double
meridian system, one austral and boreal. The austral gnomon is
located 20 meters in height from the floor and tracks the Sun.
The boreal gnomon used to be possible to observe the North Star
but the hole was eliminated in the mid 1700s. The higher the
gnomic hole, the more accurate were the observations. The Great
Meridian can also tell you in which zodiacal sign the sun is
traveling, and when are the equinoxes.

These 'solar observatories' were higher in their precision than
any other astronomical tool, until the telescope (invented in
about 1600) surpassed the meridian's accuracy for timekeeping in
the mid-1800s or so.

The article below, gives this nice story about Cassini and the
Great Meridian in the San Petronio Church in Bologna.

----------------------------

<begin quote>
Among the best known of the observers was Giovanni Cassini, an
Italian astronomer who gained fame for discovering moons of
Saturn and the gaps in its rings that still bear his name, as
does a $3.4 billion spacecraft now speeding toward the planet.

Around 1655, Cassini persuaded the builders of the Basilica of
San Petronio that they should include a major upgrade of Danti's
old meridian line, making it larger and far more accurate, its
entry hole for daylight moved up to be some 90 feet high, atop a
lofty vault.

"Most illustrious nobles of Bologna," Cassini boasted in a flier
drawn up for the new observatory, "the kingdom of astronomy is
now yours."

The exaggeration turned out to have some merit as Cassini used
the observatory to investigate the "orbit" of the Sun, quietly
suggesting that it actually stood still while the Earth moved.

Cassini decided to use his observations to try to confirm the
theories of Johannes Kepler, the German astronomer who had
proposed in 1609 that the planets moved in elliptical orbits not
the circles that Copernicus had envisioned.

If true, that meant the Earth over the course of a year would
pull slightly closer and farther away from the Sun. At least in
theory, Cassini's observatory could test Kepler's idea, since the
Sun's projected disk on the cathedral floor would shrink slightly
as the distance grew and would expand as the gap lessened.

Such an experiment could also address whether there was any merit
to the ancient system of Ptolemy, some interpretations of which
had the Earth moving around the Sun in an eccentric circular
orbit. Ptolemy's Sun at its closest approach moved closer to the
Earth than Kepler's Sun did, in theory making the expected solar
image larger and the correctness of the rival theories easy to
distinguish.

For the experiment to succeed, Cassini could tolerate measurement
errors no greater than 0.3 inches in the Sun's projected face,
which ranged from 5 to 33 inches wide, depending on the time of
year.

No telescope of the day could achieve that precision.

The experiment was run around 1655, and after much trial and
error, succeeded. Cassini and his Jesuit allies confirmed
Kepler's version of the Copernican theory.

Between 1655 and 1736, astronomers used the solar observatory at
San Petronio to make 4,500 observations, aiding substantially the
tide of scientific advance.

<end quote>


References
------------
John L. Heilbron, _The Sun in the Church_ Harvard, 1999.

http://partners.nytimes.com/library/national/science/
101999sci-astronomy-cathedrals.html



***********************************************************************
Amara Graps, PhD        www.amara.com
Istituto di Fisica dello Spazio Interplanetario, CNR - ARTOV, 
Via del Fosso del Cavaliere, 100, I-00133 Roma, ITALIA
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"We came whirling out of Nothingness scattering stars like dust." 
--Rumi