Mathematician, astronomer and physicist, b. 15 February 1564 (Pisa, Italy), d. 8 January 1642 (Arcetri near Florence).
Galileo was the son of the musician Vincenzo Galilei, who moved to Florence when Galileo was 10. He received his early education in the monastery of Vallombrosa near the city. In 1581 he enrolled at the University of Pisa to study medicine. A geometry lesson which he overheard created his interest in mathematics, and he changed to the study of science and mathematics, which at that time were considered separate objects of intellectual endevour.
In 1585 lack of funds forced Galilei to withdraw from university studies. He supported himself by lecturing at the Florentine academy and spent his time studying on his own. The publication of an essay on the hydrostatic balance in 1586 and a work on the centre of gravity of solid bodies in 1589 gave him the position of lecturer in mathematics at the University of Pisa. In 1592 he was appointed professor of mathematics at the University of Padua, a position he kept for 18 years.
For several years Galilei's work continued to concentrate on mechanics. Combining observation and mathematical theory, he showed that Aristotle's idea of bodies of different weights falling with different speeds was wrong and discovered the law of uniformly accelerated motion. (This discovery gave rise to the legend that he dropped objects from the leaning tower of Pisa, which is not based on fact.) He formulated the law of parabolic fall that describes the path of objects thrown into the air. He described the equilibrium of floating bodies.
As early as 1597 Galilei expressed his conviction in a letter to Kepler that Copernicus was correct when he said that the Earth rotates around the sun. (Giordano Bruno had publicly stated this position already in 1584.) In 1609 he learned about the recent invention of the telescope, immediately improved its magnification from 3 to 32 and used his new telescope for astronomical observations. Among many other discoveries he showed that the milky way was composed of stars, found the craters on the moon and discovered the satellites of Jupiter. His observations of the movement of sun spots gave him undisputable evidence that Copernicus was correct, and he began to publish his findings.
The catholic church, which adhered to Ptolemy's view that the Earth is the centre of the universe and on 5 March 1616 had decreed that the Copernican theory was "false and erroneous", initially took a sympathetic attitidude and granted Galilei permission to write about both the Copernican and Ptolemaic system as long as the final conclusion was that man cannot presume to know how the world really is because the ways of God cannot be known to man.
Galilei's book Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano ("Dialogue on the Two Main World Systems, Ptolemaic and Copernican"), which was published in 1632, ended with this conclusion and had the full imprimatur of the censors; but it was such a clear support for the Copernican idea that the church considered it a grave danger to its authority. It "discovered" a document, dated 26 February 1616, that allegedly banned Galilei from discussing the ideas of Copernicus "in any way". (The document was not publicly accessible until 1877, when historians agreed that it had been planted on Galilei.)
Galilei stood trial in 1633 and was forced to recant. His sentence of imprisonment was commuted to house arrest, which was enforced until his death in 1642. Three and a half centuries later, in 1992, the Catholic Church formally acknowledged its error in condemning Galilei.
Galilei was the first scientist of modern Europe to combine mathematics and physics for the study of nature and develop the scientific method of theory developed from observations and tested against additional observations. He maintained that the "Book of Nature is ... written in mathematical characters." By writing in Italian (rather than Latin) he made new scientific findings accessible to a wide audience. His lucid and engaging style served as a model for scientific writing in all of Europe.
van Helden, A. (1995) Galileo. Encyclopaedia Britannica 15th ed.