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The Heliocentric Theory: Copernicus, Galileo,
Kepler, Newton
by Tom Irvine, February 17, 2006
Introduction
The conclusion that the "Earth circles the Sun," was
reached and publicized by Copernicus, Galileo, Kepler, Newton,
and Halley. This is the "heliocentric theory." To appreciate
the work of these men, one must also consider the role of ancient
Greek philosophers and the Roman Catholic Church.
Aristotle
Aristotle (384 - 322 BC), a Greek philosopher, taught that the Earth was the center of the Universe. He thus taught that the Sun, the Moon, and the planets thus orbited the Earth. This is the "geocentric theory." Aristotle set forth this theory in a book called, "On the Heavens."
On the other hand, another Greek named Aristarchus of Samos in
the 3rd century BC placed the Earth and other planets in motion
around the central Sun. This idea, however, was rejected by the
people in favor of Aristotle's geocentric theory.
Aristarchus of Samos
Aristarchus (310 - 230 BC) was both a mathematician and astronomer. He the first to propose a sun-centered universe. Archimedes (287 - 212 BC) critized this theory in The Sand-Reckoner.
Ptolemy
Claudius Ptolemy (85-165 AD) was a later philosopher who lived in Alexandria.
He wrote that the Earth was motionless because constant gales
would sweep across it if it were in motion. Ptolemy also devised
a complex system of "epicycles" to account for the
apparent retrograde motion of the planets. Retrograde means backwards.
Consider two objects rotating about a common point with the inner
object traveling at a faster angular speed than the outer object.
As the inner objects passes by the outer object, the outer object
will appear to move backwards. This effect is an optical illusion.
In Ptolemy's epicycle system, each planet revolved around the
earth in a large circle by making a series of smaller circles,
to account for this effect.
Ptolemy wrote these theories in book called "Almagest,"
around 140 AD.
Thomas Aquinas
The theories of Aristotle and Ptolemy were adopted by the Roman Catholic Church, which played an important role in presenting these theories to the Europeans.
Thomas Aquinas (1225-1274), a priest and philosopher, used the writings of these men to show that the heavens were "God-ordained and man-centered." The Church leaders felt that they had the duty of teaching the people about the universe, which declared the glory of God.
Nicholas Copernicus
Nicholas Copernicus (1473 - 1543) studied to become a priest in the University of Bologna in 1497. His religious training included astronomy, since one of his duties would be to warn the people about the future by what happened in the heavens.
Domenico Maria Novara was an astronomy professor who taught Copernicus.
Domenico and Copernicus spent many nights studying the heavens
together. They freely discussed ways for improving and simplifying
Ptolemy's system of planetary motion.
Copernicus set out to improve the system devised by Ptolemy.
He realized that the rising and setting of the Sun, Moon, and
stars could be accounted for by a daily revolution of the Earth.
Also, he found that if he put the Sun at the center of the planet's
orbits he could simplify the number of epicycles from 80 in Ptolemy's
system to a mere 34.
Although epicycles do not exist, Copernicus' intuition was correct.
His idea that the Earth and planets orbited about the sun became
know as the "heliocentric theory." He wrote about it
in his book "De Revolutionibus, " which translates
to "Concerning the Revolutions."
Copernicus defended his placement of the Sun at the center of
the Universe by asking, "For who would place this lamp of
a very beautiful temple in another or better place than this,
wherefrom it can illuminate everything at the same time?"
Martin Luther, Philipp Melancthon, Andrew Osiander, Tommasco
Caccini and other religious leaders were quick to denounce Copernicus'
heliocentric theory.
Galileo Galilei
Galileo Galilei (1564 - 1642) lived in Italy. He used the newly-invented telescope to make his own observations. He studied mountains and craters on the Earth's moon, the phases of Venus, and the moons of Jupiter. Particularly he noted that Venus at times appears to be a crescent, just as the Earth's moon does. All of these findings supported Copernicus' heliocentric theory.
Galileo wrote about his observations and thus angered the Roman
Catholic Church. The Church eventually placed him under house
arrest. The Inquisition was the tribunal of the Roman Catholic
Church at this time. The Inquisition made Galileo kneel before
them and confess that the heliocentric theory was false.
A modern author name Preserved Smith wrote, "Though the
implications of the new science were not worked out immediately,
it began to be suspected that if the theories were true, man
had lost his birthright as the creature for whose sake all else
existed, and had been reduced to the position of a puny and local
spectator of infinite forces unresponsive to his wishes and unmindful
of his purposes."
Tycho Brahe
Tycho Brahe (1546-1601) was a Danish astronomer who made measurements of the planet and stars. His measurements were the most accurate that had yet been made.
Tycho began his observations in Denmark but later moved to Prague to continue his work.
Tycho proposed a system in which all of the planets except for Earth orbited about the Sun. He claimed that the Sun still orbited about the Earth, however.
Johannes Kepler
Johannes Kepler (1571 - 1630) was a German astronomer. Kepler was invited to live in Prague by Tycho Brahe. Tycho died a year after Kepler's arrival. Kepler inherited a wealth of astronomical data from Tycho. Kepler used this data to draw conclusions about the orbits of the planets.
Kepler's Three Laws can be used to describe the motion of the Planets:
1. The Planets move in orbits about the Sun that are ellipses
2 . The planets move such that the line between the Sun and the Planet sweeps out the same area in the same area in the same time no matter where in the orbit.
3. The square of the period of the orbit of a planet is proportional to the mean distance from the Sun cubed.
Isaac Newton
Isaac Newton (1642 - 1727) lived in England. Newton derived the law of gravitation between two masses. Since the Sun was the most massive object in the planetary system, all of the planets would naturally be attracted to it and revolve around it, in the same manner as the Moon revolves around the Earth.
Newton eventually wrote about gravitation and the heliocentric theory in Principia Mathematica in 1687, at the prompting of another famous astronomer, Edmund Halley (1656-1742) .
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