The observers

Many astronomers have made important discoveries. Here are the main ones :
Thales of Milet (600 BC)
Described the Earth as being flat.
Pythagoras (530 BC)
Considered Earth to be a motionless sphere situated in the center of a series of eight concentric spheres : the Sun's, the Moon's, Mercury's, Venus's, Mars's, Jupiter's, Saturn's and finally the stars' (called the Pythagorean spheres).
Anaxagore (environ : 500-428 BC)
Realized that lunar eclipses occurred when the Moon was at the opposite end of the Sun and also discovered the correct explanation : The Moon enters in the cone of shadow produced by the Earth.
Eudoxe (406-355 BC)
Drew the first map of the sky and showed that the Sun realized a circular path on the star's sphere : the Ecliptic. He also invented the solar quadrant.
Aristarque of Samos (310-230 BC)
He was the first to suppose that Earth turns on its axes and also around the Sun. But his hypothesis of a heliocentric system was not accepted during that period.
Eratosthene (284-192 BC)
Measured precisely the Earth's circumference as well as the ecliptic's oblique angle.
Hipparchus (v. 190-120 BC)
Showed that the Moon was actually four times smaller then Earth and calculated the distance that separated it from Earth. He located the position of many stars and created the first catalogue. He established the seasons' inequality, this by discovering that the Sun does not turn at the same speed all year long. He discovered the precession of the equinox. By comparing his own results with his predecessor's, he certified that the position of the celestial North Pole is not regular.
Ptolemy (120 BC)
Besides the vision of a geocentric system, he established, by using the epicycle method, tables (almageste) allowing to foresee the position of planets many centuries previously.
Nicolas Copernicus (1473-1543)
Studied the celestial body movements. His theories, that were revised many times, attempted to prove that the stars' movement did not exactly agree with the rule of the absolute mouvement according to which everything should turn around the center of the universe, meaning Earth, at a variable speed. Knowing about preceding Greek works, he showed the inaccuracy of Ptolemy system and displayed his viewpoint on a heliocentric system in one of his works written in 1543. However, he was able to give no proof for his reasonings and was also unable to explain why objects fell towards the center of the Earth and not towards the Sun, being the center of the solar system. Unfortunately his works were interrupted by his death in 1543.
Tycho Brahe (1546-1601)
Made many observations using aiming instruments and established a very precise map of the sky In 1572 he observed a new star's birth which then died in 1574. In spite of his numerous observations, he kept the idea of a geocentric system. In 1599, he settled in Prague and took as his assistant the best astronomer in Bohemia (Johannes Kepler).
Galilee (1564-1642)
He found the proof that Copernicus was missing to be able to confirm his theories. He proved that whatever their weight, all bodies take the same time to " fall ". His main credit was that he used mathematics to analyze his results. He elaborated in one night, using his information in perspectives or optics, a telescope which enlarged by three. Only the patent was deposited by Hans Lippershey in 1608. In one of his works, he gave the astronomy results of his observations (thanks to his telescope) like the mountains on the Moon, Jupiter's satellites, Venus's periods, the Sun's dark spots,... In 1623, he published a work in which he expressed a large number of his scientific thoughts on the Earth's monuments. His ideas brought him two trials at the inquisition. He escaped condemnation to the in 1633 because he officially denied his works. He still carried out works on clock-making before dying in 1642, the year Isaac Newton was born.
Kepler (1571-1630)
Johannes Kepler was interested from childhood in astronomy and rejoined his works to the Copernican theories. He became Tycho Brahe's assistant. On his death bed, his master revealed observations on planets that he had kept secret, but Kepler was occupied only by his observations of Mars and was studying a Super Novae that had appeared in 1604. He found that his own observations did not coincide with the system that Tycho Brahe had revealed to him nor with the Copernican system. In his work that appeared in 1609, he demonstrated that the planets turned around the Sun while forming ellipses. He also published a formulation of three rules concerning these movements. He died three years after he wrote his last work.
Newton (1642-1727)
His works are very important as much as in mathematics (he established the bases of modern analysis) as in physics. In optics he published a new work on light (corpuscular theory) studied the decomposition of the white light by a prism and creates a spherical mirror telescope. In mechanics, he creates the basis of the rules of movement and establishes the rule of universal gravitation. This rule explains that the force created between two bodies is attractive and directly proportional to the product of their mass and indirectly proportional to their distance squared. This theory of gravitation allowed the astronomers that succeeded him to explain many phenomenons likethe precession of the equinox.
Still today man make many discoveries in astronomy. Let's quote Einstein with his works on the equivalence of mass and energy and most of all on general relativity.
All these men, and many others, have participated an will still participate in the discovery and understanding of the fascinating universe that surrounds us.

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Last modifications: 3 July'99