Galileo Galilei (1564–1642) was born in Pisa and taught mathematics at the universities of Pisa and Padua. He was a lively writer and a stubborn arguer. Two of his inventions changed science: a telescope good enough to study the sky, and a way of slowing falling motion down so it could be measured.
Key idea
The telescope: from a rumor to the moons of Jupiter
In 1608 spectacle makers in the Netherlands applied for patents on a tube with two lenses that made distant things look near. Galileo heard about it in 1609. He had never seen one.
About ten months ago a report reached my ears that a Dutchman had constructed a telescope, by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near … I prepared a tube, at first of lead, in the ends of which I fitted two glass lenses, both plane on one side, but on the other side one spherically convex, and the other concave.
His first tube magnified about three times. He ground his own lenses and soon reached about eight, then about twenty times. In January 1610 he turned one on Jupiter and saw four small "stars" that moved from night to night, always staying near the planet. They were moons going around Jupiter. He also saw mountains on the Moon, the many stars of the Milky Way, and later the phases of Venus.
Why it works
Worked example: Magnification
- Magnification $\approx \dfrac{98}{4.9}$.
- $98 \div 4.9 = 20$.
Answer: About 20 times, close to Galileo's best telescopes of 1610.
Moons circling Jupiter meant not everything goes around the Earth. Galileo argued for Copernicus's idea that the Earth goes around the Sun. That led to his trial by the Roman Inquisition in 1633. He was ordered to abandon the idea and spent his last years confined to his house near Florence.
The rolling ball: making falling slow enough to measure
Aristotle had taught that heavy things fall faster than light ones. Falling is too quick to time with the clocks of Galileo's day. So Galileo rolled a bronze ball down a long, smooth groove tilted gently, which slows the motion while keeping its pattern. He timed it with a water clock: water flowing out of a tank during the roll was weighed.
… in such experiments, repeated a full hundred times, we always found that the spaces traversed were to each other as the squares of the times, and this was true for all inclinations of the plane …
Key idea
Worked example: Squares of the times
- Distance grows as the square of the time: $3^2 = 9$ times as far.
- $9 \times 10 = 90$ cm.
Answer: 90 cm.
Story or history?
The story that Galileo dropped two balls of different weight from the Leaning Tower of Pisa comes from his student Vincenzo Viviani, who wrote it down years later. Galileo's own books do not describe doing it, and many historians doubt it happened as told. What is certain is the inclined plane work he describes, and his clever argument with a heavy and a light stone tied together.
| When | What happened |
|---|---|
| 1564 | Born in Pisa |
| 1589 | Teaches mathematics at the University of Pisa |
| 1592 | Moves to the University of Padua, where he stays eighteen years |
| 1609 | Hears of the Dutch spyglass and builds his own |
| 1610 | Sees four moons of Jupiter; publishes The Sidereal Messenger |
| 1633 | Tried by the Roman Inquisition; confined to his house |
| 1638 | Two New Sciences printed in Leiden, with the inclined plane experiments |
| 1642 | Dies at Arcetri, near Florence |
What it led to
The telescope opened astronomy, and Galileo's law of falling led straight to Newton. His method, a measured experiment described so others can repeat it, with the results written as mathematics, became the pattern of modern physics.
Common mistake
Read the source on the Great Books Ladder
- Galileo: Two New Sciences: the inclined plane and the tied stones, in Galileo's own words
Read more: Galileo Galilei.