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Re: Galileo's gravity



Galileo on gravity (from Dialogue on the Great World Systems):

"All parts of the Earth cooperate to form the whole. It follows that
they have equal tendencies to come together from every direction so
as to unite in the closest possible way, which is spherically. Why
must we not then believe that the sun and moon and the other
celestial bodies are also round in shape by exactly such a concordant
instinct and natural cooperation of their component parts?"

Basically an Aristotelian sentiment, expressed in different terms --
a rock falls to earth because it has a moral obligation to do so.
Place it near the moon instead and it will fall that way. He never
speculated in print how the force was actually exerted and in Two New
Sciences, specifically declines to do so.

Regarding the motions of the planets, Galileo wrote in Narrative and
Demonstrations Concerning Sunspots:

"I do not deny the circular motions around the Earth or another
center (slightly) different from the Earth's, and even less other
circular motions entirely separate from the Earth. . .I am most
certain that there are circular motions described by eccentrics and
epicycles, but that to execute them nature really uses the farrago of
spheres and orbs the astronomers talk about -- this, I think, is
unnecessary to believe, since they are adopted in order to facilitate
astronomical computations. My own opinion lies between that of those
astronomers who assume not only that the planets move eccentrically
but that there are eccentric orbs and spheres that actually guide
them, and those philosophers who equally deny both the orbits and any
motions around centers other than the earth's."

I read this to mean that Galileo believed in the Copernican system as
Copernicus had given it, with the planets following epicyclic paths
because that is what planets do -- they do not need any mechanical
apparatus to guide them. Again, though, it sounds more nearly
Aristotelian than Newtonian.


I came across a quote in a text (Ralph, et al, _World Civilizations_,
Vol. 1) used in our World Cultures Course that is news to me:

"Galileo seems to have a broader conception of a universal force of
gravitation than Leonardo da Vinci, for he perceived that the power
which holds the moon in the vicinity of the earth and causes the
satellites of Jupiter to circulate around that planet is essentially
the same as the force which enables the earth to draw bodies to its
surface."

I was under the impression that it was Newton who first conceived of
the earth's gravity extending to the Moon and beyond. Galileo still
had some Aristotelian cobwebs in his cosmology in that he believed
the "natural state" of heavenly objects was uniform circular motion.
Does anyone know if Galileo really preempted Newton? Thanks.

Jeff Braun
University of Evansville
braun@charm.evansville.edu

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29528 variety in unity.
pjcamp@coastal.edu --Umberto Eco
pjcamp@postoffice.worldnet.att.net The Name of the Rose
(803)349-2227
fax: (803)349-2926