The Library of Original Sources, Vol 5



Galileo Galilei was born at Pisa, February 18, 1564. His father saw no prospects in his own specialty, mathematics, and started Galileo in medicine. In 1583 while watching a great lamp swinging in the cathedral at Pisa, he noticed that no matter what the length of the oscillation the time was the same. Fifty years later, he applied this principle to the invention of a clock. About this time he became interested in mathematics and in spite of all dissuasion from his father took up its study instead of medicine.

In 1588 on account of a treatise on the center of gravity in solids he was appointed lecturer at the university of Pisa. In the next two years he was busy studying the subject of motion. He disproved the Aristotelian theory that bodies fall with speed proportioned to their weight by letting bodies fall from the tower of Pisa and showing that things naturally fall with the same speed. He studied the laws of motion, and proved by experiments on long inclined planes that falling bodies have a uniformly accelerated motion. From this he deduced the principle of inertia that bodies would go on moving in the same direction forever if not interfered with by some other force, and that the motion of a body is the result of the independent forces acting upon it. This doctrine of inertia was an answer to the objection of the anti-Copernicans that if the earth went round the sun a body thrown into the air would be left behind.

In 1592 he took the chair of mathematics at Padua, where he remained till 1610. About 1600 he made a crude thermometer. Hans Lippershey, an optician at Middleburg had in 1608 discovered the telescope and the rumor of it having been carried to Galileo, he made oneof his own, and improved it until he obtained a power of magnifying 32 times. With this he discovered the mountainous surface of the moon, the fact that the Milky Way is made up of many small stars, and finally in 1610 the moons of Jupiter. This was an example of the theory of Copernicus put in practise and was a strong argument for the system. He discovered also that Venus shows different sides, the same as the moon and used this as another argument that planets revolve round the sun.

In 1610 he moved to Florence. In 1613 he began to show openly his adherence to the Copernican system. After his being heard in regard to the theory, he was ordered (1616) by the consulting theologians not to hold, teach, or defend, the doctrine. Galileo returned understanding that the doctrine could be held only as an hypothesis. His friend Maffeo took the papal chair as Urban VIII in 1623, and Galileo began his dialogues on the system of the world. They were finished in 1630 but printed only after much trouble to get permission and on the promise of its not being heretical. The book at once became popular and had great influence, but it became apparent that it was a thinly veiled argument for the prohibited Copernican theory. The next year he was called before the Inquisition and compelled to recant.

In 1637 he discovered the librations of the moon, and a few months afterward became permanently blind. He died in 1642 on the 8th of January, the day Sir Isaac Newton was born.


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Chicago: "Galileo," The Library of Original Sources, Vol 5 in The Library of Original Sources, ed. Oliver J. Thatcher (Milwaukee, Wisconsin: University Research Extension Co., 1907), 291–292. Original Sources, accessed December 2, 2022,

MLA: . "Galileo." The Library of Original Sources, Vol 5, in The Library of Original Sources, edited by Oliver J. Thatcher, Milwaukee, Wisconsin, University Research Extension Co., 1907, pp. 291–292. Original Sources. 2 Dec. 2022.

Harvard: , 'Galileo' in The Library of Original Sources, Vol 5. cited in 1907, The Library of Original Sources, ed. , University Research Extension Co., Milwaukee, Wisconsin, pp.291–292. Original Sources, retrieved 2 December 2022, from