Italian scientist and philosopher. Galileo was a true Renaissance man, excelling at many different endeavors, including lute playing and painting. He attended medical school in Padua. While in a cathedral, he noticed that a chandelier was swinging with the same period as timed by his pulse, regardless of its amplitude. He began to study the isochronism of the pendulum in 1581, as well as the motion of bodies. Using an inclined plane, he showed that all bodies fall at the same rate. He also investigated cohesion, and concluded that a waterfall breaks when the weight of the water becomes too great, the same reason that water pumps could only raise water by 34 feet.

Galileo described his views on dynamics and statics in Dialog on the Two New Sciences, which emphasized mathematics over rhetorical arguments. Galileo was one of the earliest to propose abstract dynamical theories which were ideal and would not be observed under less than ideal circumstances. Galileo observed the supernova of 1604 and tried unsuccessfully to measure its parallax. According to Copernicus's theory, the Earth's motion must produce a parallax, but no such parallax was found until Bessel. Galileo grew interested in the heavens, and built his own a telescope in 1609 after the discovery of lenses was reported from Holland. Galileo used his 30 power telescope to discover craters on the Moon, sunspots which rotated with the Sun, the four largest satellites of Jupiter, and phases of Venus. This last observation demonstrated that the Copernican theory was correct, since phases would only be observed if Venus were always closer to the sun than to the Earth. Galileo published his observations in Siderius Nuncius (The Starry Messenger) (1611). For some famous quotes and diagrams from Siderius Nuncius, see MacRobert (1990). A complete translation is contained in van Helden (1989).

Galileo also proposed Galilean relativity, which states that the same definitions of motion are valid everywhere. The resultant Galilean transformation is correct for low speeds, but must be replaced by the Lorentz transformation for relativistic speeds. Galileo also said that motion is continuous and can only be altered by the application of a force. Galileo enunciated the law of fall (which states that distance traveled is proportional to the square of time) and the time law (which states that velocity is proportional to time). There is an apocryphal story that Galileo dropped two balls of different masses simultaneously from the leaning tower of Pisa to demonstrate that bodies fall at the same rate.

Galileo lay down the chief elements of his mechanics in Dialog on the Two Chief Systems of the World (1632), which was supposed to be an objective debate between the Copernican and Ptolemaic system. Unfortunately, Galileo put the Pope's favorite argument in the mouth of one of the characters, then proceeded to ridicule it. Galileo suddenly lost favor with the church, and was forced to recant his Copernican views and put under house arrest. Misner et al. (1973 p. 38) give some quotes by Galileo. One of the most telling is "In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual" (1632). A very similar twentieth century quote is attributed to Einstein.

Additional biographies: MacTutor (St. Andrews), Firenze, Bonn