Aristarque de Samos

See also: Aristarque

Aristarque de Samos , in Greek old Ἀρίσταρχος (env. 310 - 230 av. J. - C.), born with Samos, in Greece, is a astronomer and a mathematician.

One knows few things about the life of Aristarque de Samos, if not that he was probably the pupil of Strato de Lampsacos, at time when this one taught in Alexandria.

Of its writings only the work reached us On dimensions and of the distances from the Sun and the Moon . It is extremely probable which it wrote of other works disappeared at the time of the destruction of the Bibliothèque of Alexandria. Its theory on the Héliocentrisme is known for us thanks to the comments of Archimedes:

“You are not without knowing that by the Universe, the majority of the Astronomers mean sphere having his center in the center of the Earth (...). However, Aristarque de Samos published certain writings on the astronomical assumptions. The presuppositions which one finds in his writings suggest a universe much larger than that mentioned above. It starts in fact with the assumption that the fixed stars and the Sun are motionless. As for the ground, it moves around the sun on the circumference of a circle having its center in the Sun. ”
(Archimedes, Foreword of the treaty the sand glass .)

It would also seem that he invented a Gnomon hemispherical more powerful than those of his time.

The Astéroïde (3999) Aristarque was named in its honor.

The mathematician and the astronomer

Its measurements of the diameter and distance of the the Moon and the Sun are remarkable more for their ingeniousness and the mathematical methods used that for their exactitude.

Aristarque de Samos had already observed that the Moon spends about an hour to traverse a distance equal to its diameter. It observes in addition that the eclipse S of the moon last two hours. He concludes from it that the moon entirely remains in the cylinder of shade of the ground during two hours and shows whereas the diameter of this cylinder is equal to 3 moon diameters. He concludes from it that the diameter of the ground is three times larger than that of the moon. In addition to inaccuracies in the evaluations, Aristarque makes an error of reasoning while speaking about cylinder of shade without taking account of the cones of half-light. The ground is actually 3,7 times larger than the moon.

It measures then under which angle one sees the Moon of the Earth. It finds 2°. Thanks to complicated geometrical considerations now made simpler by the Trigonometry, it can deduce from it the distance the Ground-Moon according to the diameter from the moon (28,5 Moon diameters). However its measurement is seriously vague: the angle is actually much smaller (0,5°) and thus places the Moon much more far from the Earth. A more precise calculation was completely realizable at its time and was led by Hipparque (162-126).

For the distance Ground-Sun (T-S), it observes the moon at the time of one of its exact districts. The angle Ground-Moon-Sun is then right. Ground, the Moon and Sun draw a right-angled triangle TLS, rectangle in L. It is enough for him to measure the angle Sun, Ground, the Moon. It from of then deduced a framing from the report/ratio of the distances Moon-Sun and Ground-Sun. It finds for the angle Sun, Ground, the Moon an almost right angle (90° - 3°). It shows whereas the distance Ground-Sun is approximately 19 times larger than the distance the Ground-Moon. Unfortunately, its measurement is seriously false. Only precise instruments which will not appear that more than thousand years later will allow to evaluate this angle with 90° - 0,15°. What further places the Sun 20 times.

Sun having roughly the same apparent diameter that the Moon, that means that its real diameter is 19 times larger (actually 400 times larger).

It is with the lighting of this result that Aristarque starts to doubt the theory of the geocentrism: it seems more logical to him than the smaller planets turn around larger planets. It thus places the Sun in the center of the universe and described the earthmoving like a rotation on itself combined with a circular motion around the sun.

However, if the ground moves, it should see fixed stars according to an angle different according to the period from the year. Aristarque puts forth the assumption that this difference in angle (parallax) exists well but is not detectable because the fixed stars are located very far from the Earth. Its assumption is exact. This parallax is now measurable.

Opponents

All these inaccuracies and the force of the prejudices of its time explain the fact that this assumption quickly fell into the lapse of memory. Its detractors (Archimedes, Cleanthes) reproach him for putting at evil physics Aristote. Their arguments are mainly:

the Earth as a seat of the heaviest element has its natural place in the center of the world (and yet Aristarque proved that the Sun was much bulkier than the Earth).
the circular motion is not natural. The natural movement is rectilinear.
If the Earth moved, one would see stars following of the different angles in summer and winter (and yet Aristarque answered this objection).
If the Earth turned around itself of west in is, the objects not fixed on the ground would fly away towards the west. (It is necessary to await Newton to refute this argument).
It is sacrilege to have moved the hearth of the world and to be itself opposite with the dogma of the Ground-divinity and the fire of Hestias.

Its theory presents a contrast seizing with future the Cosmologie of Ptolémée.

See too

Nicolas Copernic

External bond

a demonstration of Aristarque

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