Nicolaus Copernicus, 1543
Johannes Kepler, 1630
Nicolaus Copernicus (19 February 1473 – 24 May 1543) was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe.
Copernicus’ epochal book, De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), published just before his death in 1543, is often regarded as the starting point of modern astronomy and the defining epiphany that began the scientific revolution. His heliocentric model, with the Sun at the center of the universe, demonstrated that the observed motions of celestial objects can be explained without putting Earth at rest in the center of the universe. His work stimulated further scientific investigations, becoming a landmark in the history of science that is often referred to as the Copernican Revolution.
Among the great polymaths of the Renaissance, Copernicus was a mathematician, astronomer, physician, quadrilingual polyglot, classical scholar, translator, artist, Catholic cleric, jurist, governor, military leader, diplomat and economist. Among his many responsibilities, astronomy figured as little more than an avocation – yet it was in that field that he made his mark upon the world.
At original publication, Copernicus’ epoch-making book caused only mild controversy, and provoked no fierce sermons about contradicting Holy Scripture. It was only three years later, in 1546, that a Dominican, Giovanni Maria Tolosani, denounced the theory in an appendix to a work defending the absolute truth of Scripture. He also noted that the Master of the Sacred Palace (i.e., the Catholic Church’s chief censor), Bartolomeo Spina, a friend and fellow Dominican, had planned to condemn De revolutionibus but had been prevented from doing so by his illness and death.
It was not until six decades after Spina and Tolosani’s attacks on Copernicus’s work that the Catholic Church took any official action against it. In March 1616, in connection with the Galileo affair, the Roman Catholic Church’s Congregation of the Index issued a decree suspending De revolutionibus until it could be “corrected,” on the grounds that the supposedly Pythagorean doctrine that the Earth moves and the Sun does not was “false and altogether opposed to Holy Scripture.” The same decree also prohibited any work that defended the mobility of the Earth or the immobility of the Sun, or that attempted to reconcile these assertions with Scripture. The corrections to De revolutionibus, which omitted or altered nine sentences, were issued four years later, in 1620. This censorship was not lifted until 1835.
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Johannes Kepler (December 27, 1571 – November 15, 1630) was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican Astronomy. They also provided one of the foundations for Isaac Newton’s theory of universal gravitation.
During his career, Kepler was a mathematics teacher at a seminary school in Graz, Austria, an assistant to astronomer Tycho Brahe, the imperial mathematician to Emperor Rudolf II and his two successors Matthias and Ferdinand II, a mathematics teacher in Linz, Austria, and an adviser to General Wallenstein. He also did fundamental work in the field of optics, invented an improved version of the refracting telescope (the Keplerian Telescope), and helped to legitimize the telescopic discoveries of his contemporary Galileo Galilei.
Kepler lived in an era when there was no clear distinction between astronomy and astrology, but there was a strong division between astronomy (a branch of mathematics within the liberal arts) and physics (a branch of natural philosophy). Kepler also incorporated religious arguments and reasoning into his work, motivated by the religious conviction that God had created the world according to an intelligible plan that is accessible through the natural light of reason.
Johannes Kepler’s first major astronomical work, Mysterium Cosmographicum (The Cosmographic Mystery, 1596), was the first published defense of the Copernican system. As he indicated in the title, Kepler thought he had revealed God’s geometrical plan for the universe. Much of Kepler’s enthusiasm for the Copernican system stemmed from his theological convictions about the connection between the physical and the spiritual; the universe itself was an image of God, with the Sun corresponding to the Father, the stellar sphere to the Son, and the intervening space between to the Holy Spirit. His first manuscript of Mysterium contained an extensive chapter reconciling heliocentrism with biblical passages that seemed to support geocentrism.
The extended line of research that culminated in Astronomia nova (A New Astronomy)—including the first two laws of planetary motion—began with the analysis, under Tycho’s direction, of Mars‘ orbit. Based on measurements of the aphelion and perihelion of the Earth and Mars, he created a formula in which a planet’s rate of motion is inversely proportional to its distance from the Sun; by late 1602 Kepler reformulated the proportion in terms of geometry: planets sweep out equal areas in equal times—Kepler’s second law of planetary motion. He then set about calculating the entire orbit of Mars, using the geometrical rate law and assuming an egg-shaped ovoid orbit. After approximately 40 failed attempts, in early 1605 he at last hit upon the idea of an ellipse, which he had previously assumed to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fit the Mars data, he immediately concluded that all planets move in ellipses, with the sun at one focus—Kepler’s first law of planetary motion.
Kepler was convinced “that the geometrical things have provided the Creator with the model for decorating the whole world.” In Harmonices Mundi (Harmony of the Worlds), he attempted to explain the proportions of the natural world—particularly the astronomical and astrological aspects—in terms of music. Among many other harmonies, Kepler articulated what came to be known as the third law of planetary motion. He then tried many combinations until he discovered that (approximately) “The square of the periodic times are to each other as the cubes of the mean distances.” When conjoined with Christian Huygens’ newly discovered law of centrifugal force it enabled Isaac Newton and othersto demonstrate independently that the presumed gravitational attraction between the Sun and its planets decreased with the square of the distance between them.
In 1615, he completed the first of three volumes of Epitome astronomia Copernicanae (Epitome of Copernican Astronomy). Despite the title, which referred simply to heliocentrism, Kepler’s textbook culminated in his own ellipse-based system. The Epitome became Kepler’s most influential work. It contained all three laws of planetary motion and attempted to explain heavenly motions through physical causes. It was read by astronomers throughout Europe, and following Kepler’s death it was the main vehicle for spreading Kepler’s ideas. In the late 17th century, a number of physical astronomy theories drawing from Kepler’s work—notably those of Giovanni Alfonso Borelli and Robert Hooke—began to incorporate attractive forces (though not the quasi-spiritual motive species postulated by Kepler) and the Cartesian concept of inertia. This culminated in Isaac Newton’s Principia Mathematica (1687), in which Newton derived Kepler’s laws of planetary motion from a force-based theory of universal gravitation.
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