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Galileo Galilei
February 15, 1564 – January 8, 1642

Galileo Galilei was a Tuscan astronomer, philosopher, and physicist who is closely associated with the scientific revolution.

His achievements include improving the telescope, a variety of astronomical observations, the first law of motion, and supporting Copernicanism effectively.

He has been referred to as the "father of modern astronomy," as the "father of modern physics," and as "father of science."

His experimental work is widely considered complementary to the writings of Francis Bacon in establishing the modern scientific method.

Galileo's career coincided with that of Johannes Kepler.

The work of Galileo is considered to be a significant break from that of Aristotle. In addition, his conflict with the Roman Catholic Church is taken as a major early example of the conflict of authority and freedom of thought, particularly with science, in Western society.

Born in Pisa, Italy, Galileo was a mathematician and musician.

He attended the University of Pisa, but was forced to cease his study there for financial reasons.

He was offered a position on its faculty in 1589 and taught mathematics.

Soon after, he moved to the University of Padua, and served on its faculty teaching geometry, mechanics, and astronomy until 1610.

During this time he explored science and made many discoveries.

Galileo's father, 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.

Galileo contributed to the rejection of blind allegiance to authority (like the Church) or other thinkers (such as Aristotle) in matters of science and to theseparation of science from philosophy or religion.
Although the popular idea of Galileo inventing the telescope is inaccurate, he was one of the first people to use the telescope to observe the sky.
Based on sketchy descriptions of telescopes invented in the Netherlands in 1608, Galileo made

one with about 8x magnification, and then made improved models up to about 20x.

In 1609, he demonstrated his first telescope to Venetian lawmakers.

He published his initial telescopic astronomical observations in March 1610 in a short treatise entitled Sidereus Nuncius (Sidereal Messenger).

It was on this page that Galileo first noted an observation of the moons of Jupiter.

On January 7, 1610 Galileo discovered three of Jupiter's four largest moons: Io, Europa, and Callisto.

He discovered Ganymede four nights later.

He determined that these moons were orbiting the planet since they would occasionally disappear; something he attributed to their movement behind Jupiter.

He made additional observations of them in 1620.

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.

The heliocentric model of the solar system developed by Copernicus predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth side of the Sun.

Galileo was one of the first Europeans to observe sunspots, although there is evidence that Chinese astronomers had done so before.

Galileo's theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes, was a precursor of the Classical mechanics developed by Sir Isaac Newton.

One of the most famous stories about Galileo is that he dropped balls of different masses from the Leaning Tower of Pisa to demonstrate that their velocity of descent was independent of their mass (excluding the limited effect of air resistance).

Though the story is now generally rejected as false, however, Galileo did perform experiments involving rolling balls down inclined planes, which proved the same thing: falling or rolling objects are accelerated independently of their mass.

In his 1632 Dialogue Galileo presented a physical theory to account for tides, based on the motion of the Earth.

If correct, this would have been a strong argument for the reality of the Earth's motion.

The original title for the book, in fact, described it as a dialogue on the tides; the reference to tides was removed by order of the Inquisition.)
Galileo also put forward the basic principle of relativity.
It typically states that nobody is able to determine their speed without the use of an external point of reference.

This later provided the basic framework for Einstein's theory of relativity.






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