Galileo Galilei
Galileo Galilei (1564 – 1642) was an Italian scientist who was a major figure in the Scientific Revolution. He was a pioneer in the modern combination of mathematical theory with systematic experiment in science.
His work in physics included experimentation to establish the behavior of falling bodies, as well as the first modern theoretical work on inertia (for which he was given credit by Newton) and relativity of motion (for which he was credited by Einstein).
He was one of the first astronomers to use a telescope, and the discoverer or co-discoverer of several phenomena that contradicted the accepted ideas of the heavens. His support of the Copernican idea that the Earth rotates around the Sun led to a trial before the Inquisition on a suspicion of heresy.
Experimental science
Among the figures of the scientific revolution Galileo occupies a high position because of his pioneering use of quantitative experiments with results analyzed mathematically. There was no tradition of such methods in European thought at that time; the great experimentalist who immediately preceded Galileo, William Gilbert, did not use a quantitative approach. (However, Galileo's father, the musician Vincenzo Galilei, had performed experiments in which he discovered what may be the oldest known non-linear relation in physics, between the tension and the pitch of a stretched string.)
In the 20th century the reality of Galileo's experiments was challenged by some authorities, in particular the distinguished French historian of science Alexandre Koyré. The experiments reported in Two New Sciences to determine the law of acceleration of falling bodies, for instance, required accurate measurements of time, which appeared to have been impossible with the technology of 1600. According to Koyré, the law was arrived at deductively, and Galileo's experiments, reported in some detail as if actually performed, were merely illustrative thought experiments.
Later research, however, has validated the experiments. The measurements on falling bodies (actually rolling balls) were replicated using the methods described by Galileo [1], and the precision of the results was consistent with Galileo's report. Later research into Galileo's unpublished working papers from as early as 1604 clearly showed the reality of the experiments and even indicated the particular results that led to the time-squared law [2] .
Astronomy
In July of 1609 Galileo heard a report that a traveler from Flanders had shown "a glass by means of which distant objects could be seen as distinctly as if they were nearby." This person or another traveler then negotiated with the government of Venice to sell his secret device at a high price. A telescope had in fact been the subject of a patent application the previous year by Hans Lipperhey, a Dutch spectacle maker, and had been known for months to Galileo's friend Paolo Sarpi. (Compare the story, found not only in Brecht's play "Galileo" but in some authors claiming to give a historical account, that Galileo attempted to claim full credit for the invention of the telescope.) Without seeing the telescope, Galileo soon figured out how to make a crude one, which was followed by series of improvements, including an 8-power telescope and one of 20 power, probably in November of 1609.
Before the start of 1610 Galileo was using his telescopes to observe the Moon and stars. He soon saw that the line of shadow separating the light and dark sides of the moon (the terminator) was highly irregular and moved in an irregular fashion, leading to the conclusion that the surface was rough, contrary to the unanimous belief at that time that all heavenly bodies were perfectly spherical and smooth. By measuring shadows he estimated the height of lunar mountains with reasonable accuracy, and noted that they seemed to be higher than terrestrial ones.
In January 1610 Galileo discovered Jupiter's four largest satellites (moons): Io, Europa, Ganymede, and Callisto. Over the succeeding months he was able to determine the orbital periods of the satellites, and he made additional observations to determine the periods more accurately in 1620. (Later astronomers overruled Galileo's naming of these objects, changing his Medicean stars to Galilean satellites.) The demonstration that a planet had smaller planets orbiting it was problematic for the orderly, comprehensive picture of the geocentric model of the universe, in which everything circled around the Earth.
Galileo noted that Venus exhibited a full set of phases like the Moon. This would be impossible if both Venus and the Sun were in orbit around the Earth, but follows logically if both planets orbit the Sun. (It is also consistent with the system of Tycho Brahe in which planets orbit the Sun, which orbits the Earth.)
Galileo made some of the earliest observations of sunspots, although not the very first. From their motions he deduced that the Sun rotates on its axis. Blemishes on the Sun were another observation that was repugnant to the philosophy of the time. And the annual variations in their motions, first pointed out to Galileo by Francesco Sizzi, presented great difficulties for either the geocentric system or that of Tycho Brahe.