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Motion of Sun, Earth, and Mars according to heliocentrism (left) and to geocentrism (right), before the Copernican-Galilean-Newtonian revolution. Note the retrograde motion of Mars on the right. Yellow dot, Sun; blue, Earth; red, Mars.
(In order to get a smooth animation, it is assumed that the period of revolution of Mars is exactly 2 years, instead of the actual value, 1.88 years). The orbits are assumed to be circular, in the heliocentric case.

The Copernican Revolution was the paradigm shift from the Ptolemaic model of the heavens, which described the cosmos as having Earth stationary at the center of the galaxy, to the heliocentric model with the Sun at the center of the Solar System. Beginning with the publication of Nicolaus Copernicus’s De revolutionibus orbium coelestium, contributions to the “revolution” continued until finally ending with Isaac Newton’s work over 200 years later.

Historical overview

The Copernican Revolution started with the publishing of the book De revolutionibus orbium coelestium by Nicolaus Copernicus. His book proposed a heliocentric system verses the widely accepted geocentric system of that time. Then next astronomer to further the revolution would be Tycho Brahe. Tycho challenged the Aristotelian model for he observed a comet that went through the region of the planets. This region was said to only have uniform circular motion on solid spheres, which meant that it would be impossible for a comet to enter into the area.^[1] Johannes Kepler followed Tycho and developed the three laws of planetary motion. Kepler would not have been able to produce his laws without the observations of Tycho, because they allowed Kepler to prove that planets traveled in ellipses, and that the sun does not sit directly in the center of an orbit but off to the side. Galileo Galilei came after Kepler and developed his own telescope with enough magnification to allow him to study Venus and discover that it has phases like a moon. The discovery of the phases of Venus was one of the more influential reasons for the transition from geocentrism to heliocentrism. ^[2] Sir Isaac Newton was the character who put the missing parts in they system that led to the end of the Copernican Revolution. The end was with the development of his laws of planetary motion and universal gravitation which explain observational motion related to the heavens and gravitational force of attraction on two objects.^[3]

Nicolaus Copernicus

Aware of the recent criticism of the Ptolemaic model, led by the arguments made by Arabic astronomer Averoes,^[4] Copernicus developed a detailed heliocentric model, which he summarized his short work Commentariolus. He claimed it described the physical reality of the cosmos, something the Ptolemaic model was believed to lack. He removed Earth from the center of the universe, set the heavenly bodies in rotation around the sun, and introduced Earth’s daily rotation on its axis.^[5]

He continued to refine his system until publishing his larger work, On the Revolutions of the Heavenly Spheres (1543), which contained detailed diagrams and tables.^[5]

While Nicolaus’s work sparked the Copernican revolution, it did not mark its end. In fact, Copernicus’s own system had multiple shortcomings that would have to be amended by later astronomers. Much of these were due to an inability to make new, more extensive observations. The instruments needed to make them simply did not yet exist and Copernicus had to trust the observations made by ancient astronomers.^[6]

Tycho Brahe

Tycho Brahe (1546-1601) was a Danish nobleman who was well known as an astronomer in his time. Further advancement in the understanding of the cosmos would require new, more accurate observations than those that Nicolaus Copernicus relied on and Tycho made great strides in this area.

In 1572, Tycho Brahe observed a new star in the constellation Cassiopeia. For eighteen months, it shown brightly in the sky with no visible parallax, indicating it was part of the heavenly region of stars according to Aristotle's model. However, according to that model, no change could take place in the heavens so Tycho’s observation was a major discredit to Aristotle’s theories. In 1577, Tycho observed a great comet in the sky. Based on his parallax observations, the comet passed through the region of the planets. According to Aristotelian theory, only uniform circular motion on solid spheres existed in this region, making it impossible for a comet to enter this region. Tycho concluded there were no such spheres, raising the question of what kept planets in orbit.^[1]

With the patronage of the King of Denmark, Tycho Brahe established Uraniborg, an observatory in Hven.^[7] For 20 years, Tycho and his team of astronomers compiled astronomical observations that were vastly more accurate than those made before. These observations would prove vital in future astronomical breakthroughs.

Tycho also formulated his own astronomical system, claiming it to be superior to those of Ptolemy and Copernicus. Although Tycho appreciated the advantages of Copernicus’s system, he could not accept the movement of the earth and settled on geoheliocentrism, meaning the sun moved around the earth while the planets orbited the sun. ^[1]

Johannes Kepler

Kepler's Platonic solid model of the Solar system from *Mysterium Cosmographicum*

Johannes Kepler was a German scientist who is largely remembered for his work in astronomy. Kepler found employment as an assistant to Tycho Brahe and, upon Brahe’s unexpected death, replaced him as imperial mathematician of Emperor Rudolph II. He was then able to use Brahe’s extensive observations to make remarkable breakthroughs in astronomy.

In 1596, Kepler published his first book, the Mysterium cosmographicum, which was the first to openly endorse Copernican cosmology by an astronomer since 1540.^[1] The book described his model that used Pythagorean mathematics and the five Platonic solids to explain the number of planets, their proportions, and their order. The book garnered enough respect from Tycho Brahe to invite Kepler to Prague and serve as his assistant.

In 1600, Kepler set to work on the planet Mars and its orbit, the most eccentric of the six known planets at that time. This work was the basis of his next book, the Astronomia nova, which he published in 1609. The book argued heliocentrism and ellipses for planetary orbits instead of circles modified by epicycles. This book contains the first two of his eponymous three laws of planetary motion. In 1619 Kepler published his third and final law which showed the relationship between two planets instead of single planet movement.

Johannes Kepler’s work in astronomy was truly revolutionary. Unlike those who came before him, he discarded the assumption that planets moved in uniform circular motion, replacing it with elliptical motion. Also, like Copernicus, he asserted the physical reality of a heliocentric model as opposed to a geocentric one. Yet, despite all of his breakthroughs, Kepler could not explain the physics that would keep a planet in its elliptical orbit.

Kepler's laws of planetary motion

1. The Law of Ellipses: All planets move in elliptical orbits, with the sun at one foci.

2. The Law of Equal Areas in Equal Time: A line that connects a planet to the sun sweeps out equal areas in equal times.

3. The Law of Harmony:The time required for a planet to orbit the sun, called its period, is proportional to half the long axis of the ellipse raised to the 3/2 power. The constant of proportionality is the same for all the planets.

Galileo Galilei

The phases of Venus, observed by Galileo in 1610

Galileo Galilei was an Italian scientist who is sometimes referred to as the “father of modern observational astronomy”.^[8] His improvements to the telescope, impressive astronomical observations, and support for Copernicanism were all integral to the Copernican Revolution.

Based on the designs of Hans Lippershey, Galileo designed his own telescope which, in the following year, he had improved to 30x magnification.^[9] Using this new instrument, Galileo made a number of astronomical observations which he published in the Sidereus Nuncius in 1610. In this book, he described the surface of the Moon as rough, uneven, and imperfect. He also noted that “the boundary dividing the bright from the dark part does not form a uniformly oval line, as would happen in a perfectly spherical ^[10] These observations challenged Aristotle’s claim that the moon was a perfect sphere and the larger idea that the heavens were perfect and unchanging.

Galileo’s next astronomical discovery would prove to be an astonishing one. While observing Jupiter over the course of several days, he noticed four stars close to Jupiter whose positions were changing in a way that would be impossible if they were fixed stars. After much observation, he concluded these four stars were orbiting the planet Jupiter and were in fact moons, not stars.^[11] This was a radical discovery because, according to Aristotelian cosmology, all heavenly bodies revolve around Earth and a planet with moons obviously contradicted that popular belief.^[12] While contradicting Aristotelian belief, it supported Copernican cosmology which stated earth is planet like all others.^[13]

In 1610, Galileo observed that Venus had a full set of phases, similar to the phases of the moon we can observe from Earth. This was explainable by the Copernican system which said that all phases of Venus would be visible due to the nature of its orbit around the Sun, unlike the Ptolemaic system which stated only some of Venus’s phases would be visible. Due to Galileo’s observations of Venus, Ptolemy’s system became highly suspect and the majority of leading astronomers subsequently converted to various heliocentric models, making his discovery one of the most influential in the transition from geocentrism to heliocentrism.^[2]

Sir Isaac Newton

Newton was a well known English physicist and mathematician who was known for his book Philosophiæ Naturalis Principia Mathematica.^[14] He was a main figure in the Scientific Revolution for his laws of motion and universal gravitation. The laws of Newton is said to be the ending point of the Copernican Revolution. Without Newton's laws we would not have an explanation for gravity, or how we observe motion related to the skies.

Newton used Kepler's laws of planetary motion to derive his law of universal gravitation. Newton's law of universal gravitation was his first law he developed and proposed in his book Principia. The law states that any two objects will exert a gravitational force of attraction on to each other. The magnitude of the force is proportional to the product of the gravitational masses of the objects, and inversely proportional to the square of the distance between them.^[15] Along with Newton's law of universal gravitation the Principia also presents his three laws of motion. The three laws explain inertia, acceleration, action and reaction when a net force is applied or not applied.

Three laws of motion

1. The law of Inertia: Every object will remain at rest or in a uniform motion unless acted on by an external force.

2. F=MA: The acceleration of a body is directly proportional to the net force acting on the body, and inversely proportional to its mass

3 Action & Reaction: For every action there is an equal and opposite reaction.

References

^ ^a ^b ^c ^d Osler (2010), p.53
^ ^a ^b Thoren (1989), p. 8
^ Cohen, Isaac Newton. A new transl. by I. Bernard (1999). The Principia (2. print. ed.). Berkeley: University of California Press. ISBN 0-520-08817-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Osler (2010), p.42
^ ^a ^b Osler (2010), p.44
^ Osler (2010), p.45
^ J J O'Connor and E F Robertson. Tycho Brahe biography. April 2003. Retrieved 2008-09-28
^ Singer (1941), p.217
^ Drake (1990), pp.133-134
^ Galileo, Helden (1989), p.40
^ Drake (1978), p.152
^ Drake (1978), p. 157
^ Osler (2010), p. 63
^ See the Principia on line at Andrew Motte Translation
^ Cohen, Isaac Newton. A new transl. by I. Bernard (1999). The Principia (2. print. ed.). Berkeley: University of California Press. ISBN 0-520-08817-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)