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Thematic Essay: Physics, from Leonardo to Hertz | Article View | |
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Copernicus |
The first major scientific breakthrough in the mid-1500s was called the Copernican revolution, and it revolutionized science. Nicolaus Copernicus, a Polish mathematician and astronomer, was dissatisfied with the accepted picture of our solar system as developed by the ancient Egyptian astronomer Ptolemy, who did his work around 150 ad. Ptolemy had carefully studied the motion of the planets, but he did so under the assumption that Earth is at rest while the Sun and all the planets somehow move around it. Other early astronomers had noticed that the planets sometimes moved across the sky ahead of the stars, but that they also sometimes reversed themselves. Ptolemy explained this motion as the result of a set of small circles, called epicycles, on which the planets moved. He hypothesized that the epicycles moved on larger circles called deferents, which were centered on Earth, and that the combination of these motions caused the planets’ forward and reverse movements. Ptolemy did not have instruments to make precise observations of planetary motions, and his data were skewed by his failure to realize that Earth also moves.
Copernicus made the bold, courageous assumptions that Earth is just another planet and that it revolves around the Sun. Why bold? Because it seemed absurd to assume that our solid, stable Earth could actually be speeding through space. Why courageous? Because religious authorities had adopted the satisfying idea that the entire universe was centered on Earth, and to refute that idea was to go against the church.
Copernicus’ description of his theory of the universe, written near the time of his death in 1540, is typical of the way modern physicists think and how they strive for simplicity in the description of nature. Copernicus describes his theory of the universe thusly:
First and above all lies the sphere of the fixed stars, containing itself and all things, for that very reason immovable; in truth the frame of the Universe, to which the motion and position of all other stars are referred. Though some men think it to move in some way, we assign another reason why it appears to do so in our theory of the movement of the Earth. Of the moving bodies first comes Saturn, who completes his circuit in thirty years. After him, Jupiter, moving in a twelve year revolution. Then Mars, who revolves biennially. Fourth in order, an annual cycle takes place, in which we have said it contained the Earth, with the lunar orbit as an epicycle. In the fifth place Venus is carried round in nine months. Then Mercury holds the sixth place, circulating in the space of eighty days. In the middle of all dwells the Sun. Who indeed in this most beautiful temple would place the torch in any other or better place than one whence it can illuminate the whole at the same time? Not ineptly, some call it the lamp of the universe, others its mind, and others again its ruler. And thus rightly in as much as the Sun, sitting on a royal throne, governs the circumambient family of stars … We find, therefore, under this orderly arrangement, a wonderful symmetry in the universe, and a definite relation of harmony in the motion and magnitude of the orbs, of a kind it is not possible to obtain in any other way.
As the modern physicist-historian Thomas Kuhn points out, Copernicus’s finding was “an ‘epochal’ turning point in the intellectual development of Western man.” Unlike more modern scientific revolutions, Copernicus’s system of a central Sun orbited by the seven known planets did not immediately affect science, but its social, cultural, moral, and political influence was rapid and profound. Copernicus’s paper was published in 1543, but its influence on physics had to wait for German astronomer Johannes Kepler, Italian astronomer Galileo Galilei, and English physicist and mathematician Sir Isaac Newton. The conceptual challenge centered on the need to understand the new idea that Earth actually moved.
By centering motions around the Sun, the Copernican system made the orbits of planets simpler, whereas the Earth-centric Ptolemaic system created the need for complex epicycles. Still, the overall accuracy of the two systems turned out to be about the same given the relatively low accuracy of astronomical measurements at the time. Consequently, observational precision played a key role in the advance of physics and astronomy. The champion of accuracy in the late 16th century was the Danish astronomer Tycho Brahe, who had valuable help from Kepler.
