An ancient story

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The mythological world
In the ancient times, the cosmology was purely symbolic, and very often, the sky was seen as a lid over the Earth, the water of the oceans being the seal between the two parts.

The first real astronomers were the Mesopotamians, but they contented themselves with accumulating data, without looking for scientific explanations for the celestial phenomena.

During the VI th century B.C., the greek society becomes emancipated, and some people, in order to avoid the power of priesthood, begin to ask questions on the universe.

The greek philosophers

In Milet, a town nowadays in Turkey, these philosophers begin to think about the origin of the word, like Thales, or Anaximander who considers the universe as infinite and eternal, with the Earth isolated in the middle of the space.

 Pythagoras, the first one, tries to understant the cosmos with the help of mathematics, and that's why he is often considered as the ancestor of physical science.

Anaximene wonders how the stars can stay in the airs, and he imagines crystal spheres to support them. This idea will be promised to a great future ...

Aristarchus of Samos, in the III rd century B.C., is the first one to suggest the idea of an heliocentric system, and he imagines methods to calculate the distances of the Earth to the Moon and the Sun. But the lack of precise instruments will not allow him to obtain accurate values.

The collapse of the Greek civilization removes these ideas, and at the beginning of our era, Ptolemy imagines a geocentric system, where the celestial bodies are hung on crystal spheres.

As planets sometimes seem to move back in the sky, he conceives a set of small circles carried by big circles, to try to explain this fact : his calculations are right within 5°, which, for this time, is a true scientific wonder.

Apparent displacement of Mars
Apparent displacement of the planet Mars in the sky, from February to October 2001.
We clearly see appearing the backward movement in June and July.

(From the software Cartes du Ciel)

The Ptolemaic system
In order to explain the variations of the size of the Moon, and the noticeable motions of the planets, which sometimes seems to move back, Ptolemy conceives "épicycles", small circles, carried by big circles in rotation around the Earth.

 


The Renaissance in Occident

 

The Arab astronomers will try to improve the Ptolemaic system by complicating it, to obtain a better adjustment with observations, but they will never reach their aim.

 In 1543, Nicolaus Copernicus, a Polish astronomer, presents his heliocentric system : for him, planets are moving around the Sun, and the Earth has a precession motion, like a spinning top. The stars are hung on a fixed sphere, a little more distant than Saturn, the last known planet at this time.
This theory, although it is only presented as a simple help for calculation without any physical reality, is rejected by the Church, because it does not grant particular importance for the Earth.

In 1596, Johannes Kepler presents a scientific explanation for the Copernician theory : he suggests that the Sun exercises on the planets a force decreasing with the distance. This allows him to formulate the laws, relatives to the orbits of the planets. These laws are now known as Kepler's laws.
This work is really important, because it presents the first complete and valuable explanation of the geometric advantages of the Copernician theory.

At the beginning of the XVIIth century, Galileo invents the astronomical telescope, and allows the birth of the experimental cosmology : by measures of the parallax of stars, he discovers that these stars are much farther than previously imagined.
He presents the mechanical principle of inertia, and the idea of relativity : all the systems of reference are equivalent for the study of the universe, as long as their relative motions are uniform.

 


Newton and the gravitation

In 1666, Newton formulates the universal law of gravitation : every object in the universe attracts every other object with a force that is proportional to the product of their masses, and inversely proportional to the square of their distance.

This discovery allows him to find again the laws of Kepler, and hence to justify the motion of the planets.

For Newton, the space and the time are absolute, they are the same for every observer, and the space exists regardless of any material contents.
In order to describe this universe, we need three dimensions of space, and one for the time : to localize an event, we need three numbers (e.g. on the surface of Earth : longitude, latitude, altitude) and a date to precise when it takes place.

This is the first coherent mathematical model of the universe, but it is not exempt from problems, as Newton realizes himself :
Because the gravity attracts every object, why doesn't the universe collapse on itself ?

Three solutions are conceivable : the universe is infinite and eternal, or it is expanding, or then it is very young, and it had not enough time yet to collapse.
For philophical reasons, Newton decides upon an eternal and infinite universe.

In 1823, a German astronomer, Heinrich Olbers expresses an already suspected paradox, but which will remain known as the Olbers' paradox :
If the universe is infinite and homogeneous, with stars in all the directions, why is the sky black ?

Indeed, wherever you look, there should be a star. So the calculation shows that the brightness of the sky should be uniform and infinite.

As everybody can notice it, the sky is black. Thus, one of the hypothesis of the Newtonian universe is false : it cannot be static, infinite, eternal and uniform.

At this time, people thought that the universe was finite, or, as Olbers, that the light was absorbed by some invisible dust along its path.
This explanation is wrong, because, under the effect of light, the dust would eventually warm itself, and shine as the stars.
The modern hypothesis is built on the finite speed of light, and on the fact that the universe didn't always exist : so, we do no't still see the light of very remote stars, and in the same time, other stars have already died as bright objects.