Newton vs Galileo

Who were Galileo and Newton?

Galileo Galilei was born in Pisa on 15 February 1564. He taught mathematics at Pisa and then at Padua from 1592 to 1610, studying motion with inclined planes and pendulums. In 1609 he built an improved telescope and in March 1610 published The Starry Messenger, announcing mountains on the Moon, countless new stars and four moons of Jupiter. He became court philosopher and mathematician to the Grand Duke of Tuscany. His Dialogue Concerning the Two Chief World Systems (1632) led to his trial by the Roman Inquisition in 1633 and house arrest for life near Florence, where he wrote Two New Sciences (1638). He died at Arcetri on 8 January 1642.

Isaac Newton was born at Woolsthorpe in Lincolnshire on 25 December 1642 by the calendar then used in England, which is 4 January 1643 by the modern calendar. At Cambridge, during the plague years of 1665 to 1667, he worked out the foundations of calculus, his theory of colours and the first ideas of universal gravitation. He became Lucasian Professor of Mathematics in 1669, built the first working reflecting telescope, and published the Principia in 1687 and Opticks in 1704. He ran the Royal Mint from 1696, was President of the Royal Society from 1703, and died in London in March 1727.

Galileo vs Newton: comparison table

Was Newton born the year Galileo died?

Only on the English calendar. Italy used the new Gregorian calendar, so Galileo died on 8 January 1642. England still used the older Julian calendar, which was ten days behind, so Newton's birth was recorded as 25 December 1642. Convert Newton's birthday to the Gregorian calendar and it becomes 4 January 1643. The neat story that genius passed from one to the other in the same year depends on mixing two calendars.

They never met, and Newton never travelled to Italy. He knew Galileo through his books, which circulated widely in Latin translations.

How Newton built on Galileo

Newton said so himself. In the scholium after the laws of motion in the Principia, he writes that by using the first two laws, Galileo had found that falling bodies descend in proportion to the square of the time and that projectiles move in a parabola. Newton's first law, that a body keeps moving in a straight line unless a force acts on it, grew from Galileo's thought experiments with balls rolling on smooth planes, and from René Descartes, who stated the straight-line version clearly.

Newton's great step was to join Galileo's earth physics with Kepler's sky physics. The force that makes an apple fall is the same force that holds the Moon in orbit, weakening with the square of the distance. From that single law he derived Kepler's three laws of planetary motion, the tides and the precession of the equinoxes. Galileo had suspected the heavens and the earth follow the same rules; Newton gave the rule.

Newton was writing to Hooke, a rival, and the line was partly politeness. But Galileo and Kepler were among the giants he meant in practice. The step-by-step story from Copernicus through Kepler and Galileo is in how heliocentrism was proved.

Where they agreed and where they differed

Agreed: both believed nature is written in mathematics. Galileo said in The Assayer (1623) that the book of the universe is written in the language of mathematics, and without it one wanders in a dark labyrinth. Both tested ideas by experiment and measurement, both built their own instruments, and both were devout Christians who saw no conflict between studying nature and believing in God.

Differed in method: Galileo looked for exact descriptions (how fast, how far) and avoided asking about hidden causes. Newton gave a universal cause, gravity, though he refused to guess what gravity is: in the General Scholium of 1713 he wrote that he did not frame hypotheses about its cause.

Differed in temperament and fortune: Galileo was a brilliant, combative writer who mocked opponents in print and paid for it when Pope Urban VIII felt mocked by the Dialogue. Newton hated public controversy, delayed publishing for years, and fought his battles (with Hooke over light and gravity, with Leibniz over calculus) through letters and committees. His religious doubts about the Trinity stayed in private manuscripts, so he never faced a trial.

Myths about both men

• Galileo dropping balls from the Leaning Tower of Pisa: the story comes from his young disciple Vincenzo Viviani, writing years later, and may not have happened as told. His real evidence came from inclined planes.
• Galileo saying “and yet it moves” after his trial: the words first appear in print in 1757, more than a century later. See did Galileo say and yet it moves.
• An apple falling on Newton's head: Newton told friends in old age that seeing an apple fall in his garden set him thinking about gravity. Nothing hit his head. See did an apple fall on Newton's head.
• Galileo inventing the telescope: he improved a Dutch invention of 1608. Newton invented the first practical reflecting telescope.

Questions to ask Galileo and Newton

1. Galileo: what did you see the first night you turned your telescope to Jupiter?
2. Galileo: how did you measure falling bodies without a clock accurate enough?
3. Galileo: why did you reject Kepler's elliptical orbits?
4. Galileo: were you afraid at your trial in 1633?
5. Newton: what exactly did you take from Galileo's Two New Sciences?
6. Newton: how did you link the apple and the Moon?
7. Newton: why would you not say what causes gravity?
8. Both: can a scientist be a believer, and did your faith shape your science?

On Yuga you can ask these by voice to Galileo and Newton. They are AI recreations built from their books, letters and the historical record, and they can make mistakes. Check every fact against the Principia, the Dialogue and the standard biographies, and cite those.

Which one for your project?

Choose Galileo for a project on the telescope, the trial of 1633, the relationship between science and the Church, or experiments on motion you can repeat in class with a ramp and a ball. See questions to ask Galileo.

Choose Newton for a project on gravity, light and colour, calculus, or the strange private side of a scientist who also studied alchemy and prophecy. 2027 marks 300 years since his death: see Isaac Newton 300 years and questions to ask Isaac Newton.

Choose both for a timeline of the scientific revolution.