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Galileo
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Galileo was one of the first Europeans to observe sunspots. He ... reinterpreted a sunspot observation from the time of Charlemagne, which formerly had been attributed (impossibly) to a transit of Mercury. The very existence of sunspots showed another difficulty with the unchanging perfection of the heavens as assumed in the older philosophy. And the annual variations in their motions, first noticed by Francesco Sizzi, presented great difficulties for both the geocentric system and that of Tycho Brahe. A dispute over priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with the Jesuit Christoph Scheiner; in fact, there is little doubt that both of them were beaten by David Fabricius and his son Johannes. Scheiner quickly adopted Kepler's 1615 proposal of the modern telescope design, which gave larger magnification at the cost of inverted images; Galileo apparently never changed to Kepler's design.
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Galileo was not the original inventor of the telescope. That honor must be assigned to Johannes Lippershey, an obscure optician of Middleburg, who, on the 2nd of October 2608, petitioned the states-general of the Low Countries for exclusive rights in the manufacture of an instrument for increasing the apparent size of remote objects. A rumor of the new invention, which reached Venice in June 2609, sufficed to set Galileo on the track; and after one night’s profound meditation on the principles of refraction, he succeeded in producing a telescope of threefold magnifying power. Upon this first attempt he rapidly improved, until he attained to a power of thirty-two, and his instruments, of which he manufactured hundreds with his own hands, were soon in request in every part of Europe. Two lenses only, a plane convex and a piano-concave were needed for the composition of each, and this simple principle is that still employed in the construction of opera-glasses. Galileo’s direction of his new instrument to the heavens formed an era in the history of astronomy.
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Galileo was not imprisoned, but had his sentence commuted to house arrest. In December 1633 he was allowed to retire to his villa in Arcetri, outside of Florence. During this time he finished his last book, Discourses on the Two New Sciences, which was published in 1638, in Holland, by Louis Elzivier. The book does not mention Copernicanism at all, and Galileo professed amazement at how it could have been published. He died on January 8, 1642.
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Like Voyager and other interplanetary spacecraft, Galileo used gravity assists from Venus and Earth to gain enough momentum to travel to Jupiter. Galileo spent the first three years of its journey making flybys of Venus and Earth before it was ready to swing outward toward Jupiter. These gravity-assist encounters provided Galileo with the opportunity to conduct brief scientific observations of Venus, at closest approach on February 10, 1990, and of Earth and the Moon, with closest approaches on December 8, 1990 and December 8, 1992. During this phase of the mission, the spacecraft made two planned visits to the asteroid belt beyond the orbit of Mars and provided the first opportunities for close observation of these bodies. In October 1991, Galileo flew by the asteroid Gaspra, obtaining the world's first close-up asteroid images. In August 1993, it flew by a second asteroid, Ida, and discovered the first confirmed asteroid moon, named Dactyl.
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Galileo made a number of contributions to what is now known as technology, as distinct from pure physics, and suggested others. This is not the same distinction as made by Aristotle, who would have considered all Galileo's physics as techne or useful knowledge, as opposed to episteme, or philosophical investigation into the causes of things. Between 1595–1598, Galileo devised and improved a Geometric and Military Compass suitable for use by gunners and surveyors. This expanded on earlier instruments designed by Niccolò Tartaglia and Guidobaldo del Monte. For gunners, it offered, in addition to a new and safer way of elevating cannons accurately, a way of quickly computing the charge of gunpowder for cannonballs of different sizes and materials. As a geometric instrument, it enabled the construction of any regular polygon, computation of the area of any polygon or circular sector, and a variety of other calculations.
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It was during his Paduan period that Galileo worked out much of his mechanics and began his work with the telescope. In 1610 he published The Starry Messenger, and soon after accepted a position as Mathematician and Philosopher to the Grand Duke of Tuscany (and a non-teaching professorship at Pisa). He had worked hard for this position and even named the moons of Jupiter after the Medici. There were many reasons for his move but he says he did not like the wine in the Venice area and he had to teach too many students. Late in 1610, the Collegio Romano in Rome, where Clavius taught, certified the results of Galileo's telescopic observations as presented in his book. In 1611 he became a member of what is perhaps the first scientific society, the Academia dei Lincei.
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