Heliocentric theory
views from Earth. By Giovanni Cassini.
Heliocentrism (from the Greek: ἥλιος-helios «Sun» and κέντρον-kentron «center») is an astronomical model according to the which the Earth and the planets move around the relatively stationary Sun and which is at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The idea that the Earth revolves around the Sun was proposed from the s. III a. C. by Aristarchus of Samos. Although he did not receive support from other ancient astronomers, he was cited by Archimedes in The Sand Counter.
It was not until the 16th century, during the Renaissance, that a fully predictive mathematical model of a heliocentric system was introduced by the Polish mathematician, astronomer and Catholic cleric Nicholas Copernicus, with the posthumous publication in 1543 of the book De Revolutionibus Orbium Coelestium. This marked the beginning of what is known in the history of science as the "Copernican revolution." In the next century, Johannes Kepler extended this model to include elliptical orbits. His work was supported by observations made with a telescope that were presented by Galileo Galilei.
With the observations of William Herschel, Friedrich Bessel and others, astronomers came to accept that the Sun is not at the center of the universe; In the 1920s, Edwin Hubble showed that it was part of a much larger complex: the (Milky Way) galaxy, and that it was just one of billions of other galaxies.
First conceptions
To anyone who stops to look at the sky, it will seem that the Earth is static in one place while everything in the sky rises in the East and hides in the West once a day. With a little more scrutiny, however, more complicated movements will be observed. For example, that the rising points of the Sun and the Moon change throughout the year, or that some stars and planets disappear for many months, or that the planets sometimes appear to have moved in the opposite direction relative to the stars. in the background (this "apparent motion" is known as the retrogradation of the planets).
As these celestial motions became better observed and understood, better descriptions could be made; the best known was the "Ptolemaic System", which reached its most complete expression in the s. II d. The Ptolemaic system was a sophisticated astronomical system designed to calculate the positions of the planets to a high degree of accuracy. Ptolemy himself, in his Almagest, notes that any model describing planetary motions is merely a mathematical contraption, and since there is no way of knowing which is real, the simplest model and one that gives the correct numbers is the one that should be used. However, he rejected the idea of a rotation of the Earth as absurd, as he imagined that great winds would be created. His planetary hypotheses were convenient enough that the distances of the Moon, Sun, planets, and stars could be determined "creating spherical celestial orbits" as if they were "contiguous realities." This placed the stars within 20 astronomical units (a regression compared to Aristarchus of Samos's heliocentric scheme, which for centuries had necessarily placed stars at least two orders of magnitude further away). far).
Greek and Hellenistic World
- Pythagoreans
The non-geocentric model of the universe was proposed by the Pythagorean philosopher Philolaus (circa 390 BC). According to Philolaus, there is at the center of the universe a "central fire" around which the Earth, the Sun, the Moon and the planets revolve in a uniform circular motion. This system postulated the existence of an antiworld collinear with the Earth and the central fire, with the same period of revolution. The Sun revolves around the central fire once a year and the stars are fixed; the Earth always shows the same hidden face towards the central fire, for which reason it and the anti-Earth are invisible from the Earth. The Pythagorean concept of "uniform circular motion" to refer to celestial movements remained unchanged for the next 2,000 years or so, and it was to them that Copernicus referred when showing that the notion of a moving Earth was neither new nor revolutionary.
Pontic Heraclides (s. IV B.C.) explained the apparent daily movement of the celestial sphere by means of the rotation of the Earth. It is often said that he believed that Mercury and Venus orbited the Sun, which in turn (along with the other planets) orbited the Earth.
- Aristarchus of Samos
The first known person to propose a heliocentric system was—yet—Aristarchus of Samos (c. 270 BC). Like Eratosthenes, he calculated the size of the Earth and measured the size and distances of the Moon and the Sun in a treatise that has survived; in it, Aristarchus concludes that the Sun is six or seven times wider than the Earth and therefore hundreds of times more voluminous.
Copernican Revolution
Astronomical model
In the 16th century, Nicolaus Copernicus's De revolutionibus orbium coelestium presents a discussion complete description of a heliocentric model of the universe, in a very similar way to the way that Ptolemy, in his Almagest, had presented his geocentric model in the s. II d. C. Copernicus discusses the philosophical implications of the system he proposes, elaborates it geometrically in detail with selected astronomical observations to derive the model parameters from it, and writes numerous astronomical tables that allowed the past and future positions of stars and planets to be calculated. With this, Copernicus moved heliocentrism, from philosophical speculation, to predictive geometric astronomy; although in reality, he did not predict the position of the planets any better than the Ptolemaic system already did.
The point of view of modern science
Kepler's three laws (early 1600s) describe mathematically the motion of the planets in their orbits around the Sun. Three apparent proofs of the heliocentric hypothesis were given, in 1727 by Bradley, in 1838 by Friedrich Wilhelm Bessel, and in 1851 by Foucault. Bessel proved that the stellar parallax was greater than zero by measuring a parallax of 0.314 arcminutes from the star 61 Cygni. The same year, Friedrich Georg Wilhelm Struve and Thomas Henderson measured the parallaxes of two other stars, Vega and Alpha Centauri.
The idea that heliocentrism was not true in a strict sense either, was gradually acquired. That the Sun was not the center of the universe, but one among innumerable stars, was vehemently maintained by Giordano Bruno. Over the course of the 18th and XIX, the Sun's status as merely one star among many became increasingly obvious. By the 20th century, even before the discovery that there are many galaxies, it was no longer up for debate.
The concept of an absolute velocity, including "being at rest" as a particular case, is governed by the principle of relativity, also eliminating any obvious "center" of the universe as a natural origin of coordinates. Some formulations of Mach's principle consider that the frame at rest with respect to distant masses in the universe possesses special properties.
Even if the discussion is limited to the solar system, the Sun is not at the geometric center of any planet's orbit, but roughly at the focus of the elliptical orbit. Furthermore, given the fact that the mass of a planet cannot be neglected in relation to the mass of the Sun, the center of gravity of the solar system is slightly displaced from the center of the Sun (the masses of the planets, mainly Jupiter, represent the 0.14% of that of the Sun). This is why a hypothetical astronomer located on an extrasolar planet would observe a "wobble" in the movement of the Sun.
Modern usage of “geocentric” and “heliocentric”
In modern calculations, the terms "geocentric" and "heliocentric" are generally used to refer to coordinate systems that are chosen for practical reasons. In such systems, the origin may be selected as the center of mass of the Earth, the Earth-Moon system, the Sun, the Sun plus the major planets, or even the solar system as a whole. However, such selection of "geocentric" or "heliocentric" coordinates have only practical implications and not philosophical or physical ones.
Popular perception
Some people still believe in the geocentric model. About one in five Americans believe that the Sun goes around the Earth, according to 1999 and 2006 surveys. About a third of Russians believe in the geocentric model, according to 2011 surveys.
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