Eclipses of the Sun and Moon have never ceased to provide us with a host of lessons about the nature of the universe around us.
The first of these lessons concerned the celestial bodies directly involved in eclipses: namely the Earth, Moon, and Sun. Indeed, back in antiquity, the proof that the Earth was round, and the first measurements of the respective sizes and distances of the Moon and Sun were deduced from the observation of eclipses. In the 19th century, it was the normally invisible atmosphere of the Sun that was revealed thanks to eclipses. Far from being the perfectly round, and sharply defined ball of hot gas that it appears to the eye — appropriately protected by suitable filters, of course — the Sun is found to be a sprawling giant, overflowing with energy, plasma, and particles, that extends its influence throughout the whole Solar System.
Eclipses also provoked the discovery of helium, the second most abundant element in the Sun, and in the universe as a whole. The first demonstration of an astrophysical nature resulting from eclipses is the one given by Aristotle concerning the fact that the Earth is round. The astronomical views of this Greek philosopher are well-known to us, thanks to his two works, known to us as Meteorology and On the Heavens, dating from the 4th century BC.
Like other thinkers of his day, Aristotle believed that all heavenly bodies were spherical, because to him heavenly bodies were a reflection of divine perfection, and the sphere is the most outstandingly perfect geometrical figure. But this argument was not a physical demonstration, because, naturally, Aristotle did not have any experimental means of confirming the spherical nature of the planets and stars. As far as the Moon was concerned, the philosopher adopted an explanation attributed to the Pythagoreans, namely that the observed appearance of the Moon throughout its various phases corresponded to a spherical body, half of which is illuminated by the Sun.
The golden age of Greek astronomy flourished at Alexandria. Since its foundation under the reign of Ptolemy Soter 3rd century BC , the Alexandrian school brought together brilliant mathematicians and geometers, such as Euclid, Archimedes, and Apollonius. Similarly, the greatest ancient astronomers Aristachus of Samos, Eratosthenes, and Hipparchus, as well as Ptolemy 2nd century BC , all worked there. Aristarchus BC is nowadays known for having been the first to voice the heliocentric theory, i.
His statement does not appear in any known work, but it was reported by Archimedes and by Plutarch. The only work of Aristarchus that has come down to us relates to the sizes and distances of the Sun and the Moon. The Alexandrian astronomer completely reopened this question, which had been discussed since the 4th century BC.
The Pythagoreans had positioned the heights of the celestial bodies according to musical intervals. Eudoxus, the brilliant disciple of Plato, had estimated the diameter of the Sun as nine times that of the Moon. As for Aristarchus, he devised an ingenious geometrical method of calculating the distance ratios of the Sun and Moon. Aristarchus of Samos tried to calculate the relative diameter of the moon and sun, as deduced from the line subtending the arc that divides the light and dark portions of the moon during an eclipse.
He found that the Sun lay at a distance between 18 and 20 times that of the Moon. In fact, it is times as far. By an argument based on the observation of eclipses, he determined the diameter of the Moon as one third of that of the Earth, which is very close to the actual value. He also announced that the diameter of the Sun is seven times that of the Earth. Even though Aristarchus considerably underestimated the size of the Sun, because it is actually times as large as the Earth, he had grasped the essential fact that the daytime star was much larger than the Earth.
It was precisely this result that led him to the heliocentric hypothesis. He did, in fact, argue that under these circumstances, it was logical to believe that the Earth and the other celestial bodies revolved around the Sun, rather than the reverse.
Aristachus was before his time. The world had to wait until and the work by Copernicus, before the heliocentric theory was again put forward, this time with success. A century after Aristachus, and again at Alexandria, Hipparchus developed a complete theory of the Moon.
Whence the average of The total solar eclipse mentioned is that of 20 November BC.
Francis Bacon (Stanford Encyclopedia of Philosophy)
The actual value of the Earth-Moon distance is 60,4 terrestrial radii. If the Universe is finite, it seems necessary for it to have a center and a frontier. The center poses hardly any conceptual difficulty: it suffices to place the Earth there, like the geocentric systems of Antiquity appearances lead one in this direction , or the Sun, as Copernicus did in his heliocentric system. In the fifth century BCE, the Pythagorean Archytas of Tarentum described a paradox that aimed to demonstrate the absurdity of having a material edge to the Universe.
His argument would have a considerable career in all future debates on space: if I were at the extremity of the sky, could I extend my hand or stick out a staff? It is absurd to think that I could not; and if I could, that which is found beyond is either a material body, or space. I could therefore move beyond this once again, and so on. If there is always a new space towards which I can extend my hand, this clearly implies an expanse without limits.
There is therefore a paradox: if the Universe is finite, it has an edge, but this edge can be passed through indefinitely. This line of reasoning was taken up by the atomists, such as Lucretius, who gave the image of a spear thrown to the edge of the Universe, and afterwards by all the partisans of an infinite Universe, such as Nicholas of Cusa and Giordano Bruno. Mostly renowned as a cartographer, he also made terrestrial and celestial globes, various instruments such as quadrants, a planetarium and a tellurium.
He invented mechanical devices for improving the technics of printing. As an astronomer, a former student of Tycho Brahe, Willem Blaeu made careful observations of a moon eclipse, he discovered a variable star now known as P Cygni, and carried out a measurement of a degree on the surface of the earth as his countryman Snell did in The Blaeu family has its origin in the island of Wieringen, where about , Willem Jacobszoon Blauwe — the grandfather of Willem — was born.
From his marriage with Anna Jansdochter sprang six children. The second son, Jan Willemsz.
From his second marriage with Stijntge, Willem Jansz. Blaeu was born at Alkmaar or Uitgeest. At an early age, Willem Blaeu went to Amsterdam in order to learn the herring trade, in which he was destined to succeed his father.
But Willem did not like this work very much, being more inclined to Mathematics and Astronomy. He did not attend a university and worked first as a carpenter and a clerk in the Amsterdam mercantile office of his cousin Hooft. However, in he became a student of Tycho Brahe The celebrated Danish astronomer demanded a high standard of his pupils. Some were invited by him, others were undoubtedly taken on special recommendation.
We may therefore presume that young Blaeu had reached a good standard of education and technical skill, since he was considered worthy to become a student of the great astronomer. As it is well-known, Tycho Brahe had his own cosmic system, a sort of compromise between the Ptolemaic and Copernican. Willem Blaeu, although a supporter of the Copernican system, remained cautious during the rest of his career. In his books he mentioned the Copernican model as one of the existing theories, besides the Ptolemaic and Tychonic. It will not only save him for confrontations with religious people, but this attitude was also beneficial for his sales.
After his return from Hven in , Blaeu settled in Alkmaar. Very little is known of his stay here. He married, probably in , Marretie or Maertgen, daughter of Cornelis from Uitgeest. Here too, his eldest son Joan was born.
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Cosmology developed rapidly after the completion of general relativity by Albert Einstein, in In this theory, the Universe does not reduce to a space and a time which are absolute and separate; it is made up of the union of space and time into a four dimensional geometry, which is curved by the presence of matter. It is in fact the curvature of space-time as a whole which allows one to correctly model gravity, and not only the curvature of space, such as Clifford had hoped.
The non-Euclidean character of the Universe appeared from then on not as a strangeness, but on the contrary as a physical necessity for taking account of gravitational effects. The curvature is connected to the density of matter. In , Einstein presented the first relativistic model for the universe. Like Riemann, he wanted a closed universe one whose volume and circumference were perfectly finite and measurable without a boundary; he also chose the hypersphere to model the spatial part of the Universe.
In truth, the cosmological solutions of relativity allow complete freedom for one to imagine a space which expands or contracts over the course of time: this was demonstrated by the Russian theorist Alexander Friedmann, between and At the same time, the installment of the large telescope at Mount Wilson, in the United States, allowed for a radical change in the cosmic landscape.
In , the observations of Edwin Hubble proved that the nebula NGC was situated far beyond our galaxy. Very rapidly, Hubble and his collaborators showed that this was the case for all of the spiral nebulae, including our famous neighbor, the Andromeda nebula: these are galaxies in their own right, and the Universe is made up of the ensemble of these galaxies.
Alfred North Whitehead
Beyond this spatial enlargement, the second major discovery concerned the time evolution of the Universe. In , indications accumulated which tended to lead one to believe that other galaxies were systematically moving away from ours, with speeds which were proportional to their distance. At the beginning of XVIIth century, the way was open for new cosmologies, constructed on the basis of infinite space. In other terms, physical space was not mathematicized.
The introduction of a universal system of coordinates which entirely criss-crossed space and allowed for the measurement of distances was a reflection of the fact that, for Descartes, the unification and uniformization of the universe in its physical content and its geometric laws was a given. Space is a substance in the same class as material bodies, an infinite ether agitated by vortices without number, at the centers of which were held the stars and their planetary systems.
The tendency toward the radical geometrization of an infinite space, initiated by Descartes, was consummated by the Englishman Isaac Newton At the heart of this immobile framework, Newton explained celestial mechanics in terms of the law of universal attraction, from now on considered responsible for gravitation and the large scale structure of the Universe.