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It has become popular to say that for thousands of years there has been a “consensus” to assert that the Earth is the center of the universe, that it is stationary, or that it is flat.
different assumptions
Of course, Earth’s place in the universe has long troubled humans. But their responses were not as consistent as one might think. during antiquity, Astronomy scientists The Greeks proposed various models, some of them very creative.
Croton philolaus, in the fifth century BC, it is believed that the Earth revolves around a central fire different from the Sun. The latter, like other stars and a planet called Anti-Earth, also revolves around this fire. Philolaus is at the same time the first to write that the earth is moving. Copernicus will also refer to it in the introduction to his book. The revolutions of the heavenly spheresyear 1543.
However, it is the archetype of the Greek philosopher Aristotle, a century later, that he is better known. According to him, the Earth is a perfect sphere placed at the center of the universe. All other stars describe perfect circular motions around them.
This form says geocentrism It was popular in antiquity. It must be said that the arguments are convincing. From our point of view as earthlings, the stars seem to move in the sky and the earth is stable under our feet. In addition, if the Earth moved, we think we should see the position of the stars change from hour to hour, which is not the case. Today we know that the reason for this is that the stars are far beyond what the Greeks could have imagined.
for the historian of science Kostas GavrogluGreece at the time remained very open to “counter-intuitive” discussions about the Earth’s place in the universe. Aristark SamosA century after Aristotle suggested, moreover the earth is moving around the sun. He came to this conclusion based on his observations of the phases and eclipses of the Moon: the diameter of the Earth would be three times that of the Moon, and the Sun would be 19 times longer than the Moon. According to him, the sun should be in the center because it is much larger.
This model seems to have been a reasonable choice for several hundred years: for example we find in a textbook dating from the year 420 a variant of the heliocentric theory – the sun at the center – proposed by Heraclides du Pont (c. 340 BC), for whom Venus and Mercury orbit the Sun, but not the Earth (however, he argues that the latter orbits within 24 hours).
Critics of the geocentric model
However, these old, heliocentric models are not without flaws. For example, they do not explain the retrograde motion of certain planets: when we observe their trajectories over a period of several months, they seem to stop, retrace their steps, and then resume their “forward” trajectory.
The Greek philosopher and mathematician Ptolemy (100-168) proposed a mathematical model to reconcile the whole. According to him, the stars have a spiral path caused by a system of immaterial and moving spheres. He also imagines a point near the center of the orbits of these balls called the equant. Thus it is able to predict the movement of the planets.
Although Ptolemy’s model was adopted by the majority until the time of Copernicus, some astronomers criticize it, especially in Arab world. Members of the Maragha Observatory and astronomer Ibn al-Haithan, among others, question the importance of the equant. They correct this error using mathematical tools similar to those used by Copernicus. However, their models remain geocentric. On the other hand, the Arab astronomer Ibn al-Shatir published the heliocentric theory a century and a half before Copernicus. It is not known whether Copernicus read the work of this precursor.
Finally, some medieval European philosophers questioned the Earth’s place in the universe. for example, Nicholas de Coz (1401-1464) based on metaphysical arguments To say that it would be logical that the Earth is not at the center of the universe: if the latter is infinite, then there is no center for that, and the Earth can move without its motion being apparent to us. Historians consider him to be another ancestor of Copernicus.
What is Copernicus’ contribution?
Copernican So the time comes when the problems of Ptolemy’s model are criticized for some time. After years of research and reflection, Copernicus came up with a simple mathematical solution: placing the Sun at the center of the universe.
His model has the advantage of resolving celestial phenomena incompatible with Ptolemy’s model such as the phases of Venus or Mercury. In addition, Copernicus suggests that the distance between the Sun and the Earth is small compared to the distance between us and the stars. Thus, he explains why the movement of the Earth in space does not change the position of the stars in the sky. His model lays the foundations for modern astronomy.
There’s still one final detail: imagine the planets following perfectly circular orbits around the sun. It was the German Johannes Kepler who would write, after 66 years and supporting mathematical calculations, that the orbits actually have the shape of an ellipse. And it was Isaac Newton who, after another 80 years, would explain by gravity the laws laid down by Kepler.
Photo: a printed copy of the first edition of Copernicus (1543).