1543: Heliocentric Cosmology
Very few revolutions of thought shook the foundations of humanity, as did the
change from a terracentric cosmology — an immoveable Earth, with all other
celestial bodies, including the Sun, revolving around it — to a
heliocentric one. The heliocentric model, finally arrived at by the combined
work of Copernicus, Galileo, and Kepler, was the single greatest scientific
achievement prior to the 20th century. It affected not only science,
which was just being born about that time, but theology & philosophy as
well. It changed the way humanity related to the universe; humanity no longer
stood at the center of creation.
Intuition: Aristotle & Ptolemy
Prior to 1543, the conventional view — accepted by everyone as truth
— was that the Earth was the center of the universe. Everything else moved
around it. This is only natural. It is the obvious, intuitive conclusion that
one comes to, when one views the sky. The Sun, Moon, planets & stars all seem
to move around the Earth. This was so intuitive and obvious that no one
questioned terracentrism, not even great thinkers such as Aristotle.
Yes, the ancient Greeks, at least, & possibly the Babylonians &
Egyptians before them, had toyed with the notion that the Sun — or some
other random object or location — might be the center of the universe. But
two things brought a halt to this sort of speculation:
- Parallax: Aristotle pointed out, quite correctly, that
if, indeed, the Earth moved through the sky, then the positions of celestial
objects would shift, over time, because the Earth would be at a different
location & the object would appear to have shifted. Greek astronomers,
however, had not noticed any parallax.
- Ptolemy: The ancient astronomer Claudius Ptolemy (a
Greek living in Alexandria, Egypt) had come up with a very good model of the
Solar System, which explained the occasional apparent retrograde motions of
the planets — they moved in tiny circles (called epicycles) around a
circle of orbit, thus appearing to move "backwards" on occasion
when viewed from Earth. His model worked so well, & predicted the motion
of celestial bodies so closely, that it was widely assumed to be correct.
Together with the fact that it is such an obvious conclusion, both of these
pretty much eliminated anything but a terracentric cosmology.
Copernicus's Solar System
Nikolas Kopernig, better known as Copernicus, a Polish canon (Church lawyer)
came up with a new concept — that the Sun was the center of the universe,
& all other bodies, including Earth, revolved around it, in perfect circles.
His model, however, was not as precise as Ptolemy's, & remained in doubt for
some time. This is because he assumed the planets moved in perfect circles,
which they don't, so his model didn't predict their locations as precisely as
Ptolemy's did.
Oddly enough, Copernicus was not the first person to create a heliocentric
model of the Solar System. In the 4th century BCE, perhaps a generation after
Aristotle, the Greek scholar Aristarchus of Samos came up with a heliocentric
model. However, his work appears to have been abandoned at some point, & was
lost when the great library at Alexandria burned down.
Copernicus worked out his model many years before releasing it publicly, in
1543. He appears to have been intimidated by his own conclusion, & feared
that the Church might come down on him (many Bible passages specify a
terracentric universe) & he probably also feared that he would be ridiculed
by other academics, for proposing something that seemed so counter-intuitive. He
died the same year that his work went public (although a few people he'd
corresponded with had known about it a few years before that).
Galileo Confirms Copernicus
The famed scholar Galileo Galilei accomplished many things in the sciences,
especially in the field of mechanics. But he is equally famous for his work in
astronomy, thanks to his many observations through his telescope. He was able
to confirm Copernicus's theory in a number of ways, but primarily via the four
moons he discovered revolving around Jupiter — which had phases, like our
own Moon does, as would be predicted by Copernicus's heliocentric Solar System.
He was, however, unable to explain why Ptolemy's terracentric model with its
planetary epicycles, coincided more precisely with celestial observations.
Kepler Works The Data
Johannes Kepler was a mathematician (& mystic) who studied in Prague along
with Tycho Brahe. Brahe had cataloged a vast amount of extremely precise
astronomical data — the best observational data to date, in fact —
without benefit of a telescope. Brahe's work was precise enough to show that Ptolemy's model was, in
fact, not entirely accurate. Brahe had worked out a model of the Solar System
with the Earth at center, with the Sun going around it, but with all the rest of
the planets orbiting the Sun. There were flaws, however, even in this
"compromise" model.
Kepler determined to make the data reveal the model — which until then
was the exact opposite of how previous models had been worked out (they began
with a structural concept & then confirmed it by observed data). This was
probably because of his mystical beliefs in mathematics — that all of
"natural philosophy" (i.e. science) could be distilled down to
mathematical expressions.
Over a period of time, probably going through a number of different, revised
models along the way, Kepler concluded that the planets — & the Earth
— revolved around the Sun, but not in perfect circles. Rather, they had
"eccentric" orbits, which were elliptical, & they did not move at
uniform speed. He worked out equations to describe the orbits of each of the
planets.
Not only did Kepler's work fit in with Galileo's observations, it precisely
predicted the motion of celestial bodies, to a degree surpassing even Ptolemy's
model. Kepler settled, once & for all, the most fundamental issue of
cosmology. The Earth & all the other planets revolved around the Sun.
Newton's Laws
Kepler had worked the data, creating formulae which fit Brahe's observations
& which precisely predicted the planets' future motion. However, the rhyme
& reason for these formulae remained a mystery, until Sir Isaac Newton, the
greatest classical physicist & one of the most brilliant intellectuals of
all time, worked out a theory of gravity. His theory explained why the planets
moved in elliptical orbits of varying speed.
Using Newton's "laws" of gravity, physicists could now look at
Kepler's formulae, & determine the masses of the planets & the Sun &
Moon, and work out their distances. Things became somewhat complicated when
planets neared each other & their mutual gravity caused perturbations in
their expected orbits, but these were all worked out & overall, Kepler's
work held up under scrutiny.
Working Out The Bugs
As astronomy progressed, with ever-better telescopy & the accumulation of
more & more observations over time, even Kepler's model of the Solar System
showed a few minute flaws. Perturbations Saturn's orbit were particularly
troublesome. Finally, in the 17th century, William Herschel discovered Uranus,
another "gas giant" beyond Saturn's orbit; its gravitational influence
had caused the disturbances. Once Uranus was discovered, using Newtonian
physics & Kepler's model, its orbit was worked out.
Despite the fact that Kepler had pretty much proven that the Sun, not the
Earth, was at the center of the Solar System, parallax (see
above) still bothered many astronomers. Specifically, astronomers had failed
to observe any stellar parallaxes. Although Copernicus & others had argued
that, if the stars were sufficiently far away, there might be no observable
parallax, many still hedged their bets. But in 1838, Friedrich Bessel finally
put this to rest, when he revealed parallax measurements for the star 61 Cygni.
Improved observations showed that even Uranus's orbit had perturbations
— astronomers used this to calculate, according to celestial mechanics,
that there should be another planet further out from Uranus, & sure enough,
Neptune was discovered, in 1846. (It had been noted previously, by Galileo, in
fact, however, it was believed to be a star & not a planet.)
Impact of the New Cosmology
The discovery that Earth was not, in fact, the center of the universe — it
is, after all, just a planet circling a star, one of billions in a galaxy, which
is, in turn, one of billions of galaxies, was humbling. Humanity was no longer
the center of the universe. In fact, it's highly probable that we aren't even
the only intelligent race in the universe; there are very likely billions of
worlds with life on them & millions of those have, or will, develop
intelligent life.
It's little wonder that early pioneers of this change in thought were targets
of derision, & in the case of Galileo, of oppression & persecution. The
underpinnings of many beliefs were pulled away, & many other beliefs were
now "fair game" for analysis & examination.
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