Aristarch of Samos

Aristarchus of Samos (Ancient Greek: Ἀρίσταρχος ὁ Σάμιος, Arístarkhos ho Sámios; Latin: Aristarchus; c. 310 BC. C.-c. 230 BC) was a Greek astronomer and mathematician, born in Samos, Greece. He was the first person known to propose the heliocentric model of the Solar System, placing the Sun, and not the Earth, at the center of the known universe. This proposal was made after studying the distance and size of the Sun (he determined that the Sun was much larger than Earth).

Aristarchus was one of the many wise men who made use of the emblematic Library of Alexandria, where the most privileged minds of the classical world met.

Their original works were probably lost in one of the several fires that the Library suffered. Of the heliocentric model of Aristarchus, only the quotations of Plutarch and Archimedes remain.

Heliocentrism

In the time of Aristarchus, the obvious belief was to think of a geocentric system. Astronomers at the time saw the planets and the Sun revolve around our sky on a daily basis. The Earth, for many, should therefore be at the center of everything. The approaches of the renowned Aristotle, made a few years earlier, left no room for doubt and came to reinforce this hypothesis. The Earth was the center of the universe and the planets, the Sun, the Moon and the stars were in fixed spheres that revolved around the Earth. But there were certain problems with such claims.

Some planets, such as Venus and, above all, Mars, describe wandering trajectories in the sky, that is, they sometimes move forwards and others backwards, which is in flagrant contradiction with the Aristotelian tradition, which said that all the movements and shapes of the sky were perfect circles. Before Aristarchus, Heraclides Ponticus found a possible solution to the problem by proposing that the planets could revolve around the Sun and the Sun in turn around the Earth. This was already a big conceptual leap, but it was still a partially geocentric model.

The dominant paradigm was Aristotle's geocentric theory developed in depth centuries later by Ptolemy. It was not until the work of Copernicus, some 1,700 years later, that the heliocentric model began to be considered as a consistent alternative.

The only work by Aristarchus that has survived to the present, On the Sizes and Distances of the Sun and the Moon, is based on a geocentric worldview. We do know from citations, however, that Aristarchus wrote another book in which he advanced an alternative hypothesis of the heliocentric model. Archimedes wrote:

You, King Gelon, are aware that the universe is the name given by most astronomers to the sphere whose center is the center of the Earth, while its radio is equal to the straight line that unites the center of the Sun and the center of the Earth. This is the common description as you've heard of astronomers. But Aristarchus has drawn a book consisting of certain hypotheses, where it is affirmed, as a result of the assumptions made, that the universe is many times greater than the universe recently mentioned. Its hypothesis is that the fixed stars and the Sun remain immobile, that the Earth revolves around the Sun in the circumference of a circle, the Sun lies in the center of the orbit, and that the sphere of the fixed stars, located with almost the same center as the Sun, is so great that the circle in which he assumes that the Earth turns is such proportion to the distance of fixed stars as the center of the sphere keeps to its surface.

Aristarchus thus believed that the stars were infinitely far away, and saw this as the reason why there was no visible parallax, that is, an observed motion of some stars relative to one another as the Earth moves around the Sun The stars are, in fact, much further away than was supposed in antiquity, and stellar parallax is only perceptible with the best telescopes. But the geocentric model was chosen as a simpler and better explanation of the lack of parallax. The rejection of the heliocentric vision was apparently very strong, as the following passage from Plutarch suggests (De facie in orbe Lunae, On the face of the Moon, c. 6):

Cleantes, a contemporary of Aristarchus, thought that it was the duty of the Greeks to process Aristarchus of Samos with the charge of impiety for moving the House of the Universe (i.e., the Earth) [...] assuming that the sky remains at rest and the Earth rotates in an oblique circle, while rotating at the same time on its own axis.

However, Professor Lucio Russo states in his book The forgotten Revolution (Springer Verlag) that the French philologist of the century XVII Gilles Ménage, probably influenced by the persecution of heliocentricists such as Giordano Bruno or Galileo, mistranslated this quote from Plutarch (changing an accusative for a nominative and vice versa), as evidenced by the fact that all the Versions prior to the translation by Ménage, which is the one that has been published since then, present the terms clearly inverted: it is Aristarchus who suggests that Cleanthes should be tried for impiety and not the other way around. This fact, already mentioned by Giacomo Leopardi in his History of Astronomy (1813), suggests the need for a reinterpretation of the reception of Aristarchus's ideas.

Distance from Sun

As shown in the attached diagram, Aristarchus calculated the angle between the Sun and the Moon when it was in the first or last quarter, that is, when the angle α It's 90o. Then, measuring β could solve the triangle. He noted that the Earth-Sol distance was much greater than that of Earth-Luna and that therefore the Sun had to be much larger (because both the solar disk and the lunar have an apparent diameter of about 32 minutes of arc).

Aristarchus argued that the Sun, Moon, and Earth form a right angle at the time of the waxing or waning quarter of the Moon. He estimated the angle opposite the larger leg to be 87°. Although he used correct geometry, the observational data were inaccurate, so he erroneously concluded that the Sun was 20 times farther away than the Moon, when in fact it is 400 times. further away. He pointed out that since the Moon and the Sun have nearly equal apparent angular sizes, their diameters must be in proportion to their distances from Earth. He thus concluded that the diameter of the Sun was 20 times larger than the Moon.

Criticism of his contemporaries to the movement of the Earth

This new representation of the astronomical system was severely criticized in Antiquity. The idea that the Earth moved was unacceptable and seemed to contradict both common sense and everyday observations. In addition, the hypothesis was directly opposed to the accepted classical philosophical doctrines, according to which the Earth should have a special role with respect to other celestial bodies, and its place should be the center of the Universe. These philosophers affirmed, based on the Aristotelian theory, that heavy bodies move naturally towards the center of the Earth. Another implication of Aristotle's theory of natural motions was that the bass, once it reached its natural place, stopped or stopped. The consequences of this theory reached conclusions that were partly true and partly false. It was deduced, for example, that the Earth must have a spherical shape, but also that the Earth remained completely immobile in the center of the Universe.

Ancient scientists realized that if the Earth rotates on its axis every 24 hours, the speed of a given point on the Earth's surface must be very high. How, then, could clouds or projectiles moving through the air exceed the speed and motion of the Earth? No movement to the East could ever be made because the Earth would always move ahead.

The main argument of the astronomers was clearly based on the unsuccessful observation of the phenomenon of annual parallax of the stars: if the Earth revolves around the Sun, there should be some variations in the relative positions of the stars, observed from different points of view. Earth orbit. If things were as Aristarchus claimed, there should be a displacement of the fixed stars in the course of a year, but the Greek astronomers had not noticed anything like that in their observations. This fact could be explained in two ways:

1. Earth does not revolve around the Sun;
2. the Earth revolves around the Sun, but the stars are so far away that the displacement is so small that it cannot be appreciated at first sight.

We know today that the second explanation was the correct one. But using the best stargazing instruments, the annual stellar parallax could not be discovered until 1838 with Bessel's investigations.

It is true that Aristarchus could not have been the only one who believed in his hypothesis, but in the ancient texts the names of his sacrilegious followers have been erased. The only one who is remembered is Seleucus, a Babylonian astronomer, who lived a century after Aristarchus and who returned to the heliocentric theory with argued grounds.

Eponymy

• The lunar crater Aristarchus carries this name in his memory.
• The Aristarchus asteroid (3999) also commemorates its name.