Source: http://solar-center.stanford.edu/galileo/ https://sciencing.com/list-discoveries-galileo-galilei-8249749.html 400th Anniversary of Galileo's Astronomical Discoveries Who was Galileo? Galileo Galilei (1564-1642) was a Tuscan (Italian) astronomer, physicist, mathematician, inventor, and philosopher. He was born in Pisa, and was the oldest of six children in his family. When he was a young man, his father sent him to study medicine at the University of Pisa, but Galileo studied mathematics instead. He later became professor and chair of mathematics at the University. Until about 1609 he taught mathematics, and made several discoveries in physics. He helped to mathematically describe ballistics, and the force of friction as it relates to motion. After experimenting with moving objects, he established his "Principle of Inertia", which was similar to Newton's First Law. Galileo then became interested in optics and astronomy, and in 1609 he built his first telescope and began making observations. The following year he published his first results, where he described the highlands and "seas" of the Moon, four of Jupiter's largest moons, and many newly discovered stars. He also discovered the phases of Venus and sunspots, thereby confirming that the Sun rotates, and that the planets orbit around the Sun, not around the Earth. But Galileo thought that most planetary orbits are circular in shape, when in fact they are elliptical, as shown by Johannes Kepler. Still, Galileo's observations have confirmed Copernicus' model of a heliocentric Solar System. They refuted the basic principles of Ptolemean cosmology, and put to rest Aristotle's theory that the heavens were "perfect and unchanging", which was supported by the Catholic Church. But the Church still allowed Galileo to conduct his research, as long as he did not openly advocate his findings. In 1632 Galileo angered the Pope when he published a book in which he openly stated that the Earth was moving around the Sun. He was put on trial by the Inquisition in Rome, where he was found suspect of heresy, and forced to say that all of his findings were wrong. He was first imprisoned, and later confined to his house near Florence. During the last ten years of Galileo's life, the Church monitored his travel and communications with others, and his writings were censored and placed in the Index of Prohibited Books. Galileo continued to write about physics, and in 1632 he put forward his concept of Basic Relativity in physics, which may be stated as follows: "the laws of mechanics will be the same for all observers moving at the same speed and direction with respect to one another." This fundamental concept later formed the basis for Einstein's Special Theory of Relativity. Until the time of Galileo, European scientists relied largely on Aristotle's approach of philosophical analysis to explain physical phenomena. Galileo demonstrated the advantages of experimentation, and argued that physics should be a mathematics-based science. Galileo was among the scientists, including Kepler, Newton and others, who began the Scientific Revolution in Europe. Galileo's work was instrumental in advancing the scientific method. His experimentational and mathematical approach to physics was revolutionary and ahead of his time. Did you know? Galileo became completely blind by the age of 74, though he probably never looked at the Sun directly through his telescope. Remember, you should NEVER look directly at the Sun! Galileo's telescopes had a magnification of only about 30x. He observed Neptune in 1612, but thought that it was a distant star. Galileo also observed Saturn's rings, but to him they appeared as two separate bodies attached to the planet. It is commonly believed that Galileo dropped balls of different mass from the leaning tower of Pisa, to demonstrate that they fell at the same speed. There is no historical evidence that Galileo actually did this. But it is possible that he suggested this experiment as a way to disprove Aristotle's erroneous belief that heavier objects always fell faster than lighter ones. Galileo attempted (unsuccessfully) to measure the speed of light by placing observers with lanterns about a mile apart. Galileo is credited with making the first working thermometer, though it was not very accurate. Galileo's father, Vincenzo Galilei, was a musician and music theorist. He helped to invent a new style of early Baroque music. Earth's Orbit Shortly after the telescope was invented in the Netherlands, Galileo fashioned his own from makeshift spectacle lenses. He learned how to make increasingly powerful telescopes, which he eventually used to to monitor the solar phases of the planet Venus. After noticing Venus went through similar phases to the moon, he concluded the sun must be the central point of the solar system, not the Earth as was previously assumed. The Principle of the Pendulum At just 20 years of age, Galileo was in a grand cathedral and noticed that a lamp swinging overhead took exactly the same period of time for each swing, even as the distance of a swing got progressively shorter. This principle of the pendulum made Galileo famous, and was eventually used to regulate clocks. The law states that a pendulum will always take the same amount of time to finish a swing because there is always the same amount of kinetic energy in the pendulum -- it is merely transferred from one direction to the other. The Law of Falling Bodies This law states that all objects will fall at an equal rate, when accounting for relatively minor differences in aerodynamics and weather conditions. Galileo demonstrated this theory by climbing to the top of the Leaning Tower of Pisa and dropping items of various weight off the side. All items hit the ground at the same time. Contrary to the conventional wisdom established by Aristotle, the speed of a heavy object's fall was found to not be proportional to its weight. Astrological Discoveries Galileo made several astronomical discoveries that people today simply accept as common sense. He discovered that the surface of the moon is rough and uneven as opposed to smooth as people had thought, and in 1610 he discovered four moons revolving around Jupiter. More important than either of these was his finding that many more stars exist than are visible to the eye, an assertion that came as a shocking surprise to the scientific community at the time. Mathematical Paradigm of Natural Law For centuries, natural philosophy, which at that time encapsulated such fields as physics and astronomy, was discussed and theorized from a qualitative standpoint. Galileo didn't just discover specific laws of the universe, he reformed the qualitative standpoint and established mathematics as the language of scientific discovery. He pioneered the scientific method and ushered in the modern practice of experimentation and calculated laws of nature. His doing so led to the revelations that many of the laws of Greek philosophers such as Plato and Aristotle were incorrect. https://www.quora.com/What-astronomical-observation-of-Galileo-has-helpedweaken-the-Ptolemaic-model The heliocentricity of Copernicus, Kepler and Galileo, explained the strange orbit of the inner planets, Mercury and Venus. As observed from earth, they describe an 8-shaped orbit. It is easy to understand why when plotting the orbits of all planets being around the sun and not the earth. Kepler became famous for finding that the earth's orbit was elliptical and not a perfect circle, something that irritated the Vatican: How could a perfect God put the earth on an orbit that was not a perfect circle? Galileo Galilei was the first one to really observe the planets with a telescope and find out, for example, that Jupiter had several moons. Those were, incidentally, later used as a time table to compute the latitude when at sea. Galileo Galileo Galilei was the first astronomer to use a telescope to study the heavens. Galileo made a number of observations that finally helped convince people that the Sun-centered solar system model (the heliocentric model), as proposed by Copernicus, was correct. These arguments can be divided into two kinds: Those that proved that the Ptolemaic model was incorrect; and those that undermined the broader philosophy of Aristotelianism that included the Ptolemaic model. We'll first consider some philosophically important observations and then the ones that proved Venus, at least, goes around the Sun and not around Earth. Sun and Moon One of the ideas that made Aristotelianism popular with the church during the middle ages was that the heavens are perfect. This also meant that they were unchanging, because if they change then either they weren't perfect before or they won't be perfect after the change. Galileo discovered spots on the Sun and also saw that the surface of the Moon was rough. People really tried hard to account for these observations without making the heavens imperfect; one suggestion was that over the mountains of the Moon there was a layer of clear crystal so the final surface would be smooth and perfect! Jupiter’s Moons Galileo saw near Jupiter what he first thought to be stars. When he realized that the stars were actually going around Jupiter, it negated a major argument of the Ptolemaic model. Not only did this mean that the Earth could not be the only center of motion, but also it knocked a hole in another argument. The supporters of the Ptolemaic model argued that if the Earth were moving through space, the Moon would be left behind. Galileo’s observations showed that the moons of Jupiter were not being left behind as Jupiter moved. Phases of Venus One observation definitely disproved the Ptolemaic model, although it didn't prove that Copernicus was right (as Tycho Brahe pointed out). This was the observation that Venus has phases, much like our Moon does. To the naked eye, Venus always appears as a bright dot in the sky. With a telescope, however, it is fairly easy to see the phases of Venus. Just as the Moon has phases, Venus too has phases based on the planet’s position relative to us and the Sun. There was no way for the Ptolemaic model (Earth centered solar system) to account for these phases. They can only occur as Galileo saw them if Venus is circling the Sun, not the Earth.