Galileo Galilei (1564-1642)

Galileo:

• did not dispel misconception that he was an inventor of the telescope;
• was the first to use the telescope for astronomical observations;
• discovered that the Moon was not a smooth ball, but a world on its own, with craters, mountains, and lowlands;
• observed sunspots and calculated the rotational period of the Sun;
• resolved the Milky Way into myriads of very faint stars; this suggested that stars were at different distances and that these distances were humongous;

``The Copernican model and the lack of parallax required the stars to be at a great distance; the telescope made such a heresy believable.''

Galileo also:

• found that planets looked like disks in the telescope, but stars did not;
• observed all four phases of the Venus (gibbous phases could not be explained by the Ptolemaic model);
• discovered four larges satellites of Jupiter; they are still called Galilean moons; this was another blow to the dying Ptolemaic system.

His discoveried were written down in his book called Starry Messenger. Luckly to us all, he wrote it in his native language, Italian, rather in the scholar language of the day, Latin. Had he done otherwise, me may have been using a Latin textbook for this class!

However, his most important contribution was perhaps not in Astronomy, but in Mechanics. If the sky contradicted the Ptolemaic system, which was based on the Aristotelian physics, then perhaps the Aristotle's teaching about five elements was also wrong!

He invented a thought experiment. He made some real experiments by rolling balls over a slide. However, he never succeeded in obtaining permission from the mayor of the town of Pisa (who was concerned about the safety of pedestrians) to throw his balls down the Leaning Tower.¹

Galileo understood that if everything moves together uniformly, there is no perception of motion. In other words, moving uniformly is indistinguishable from being at rest. This understanding made the heliocentric model possible: the Earth moves, but we move together with it, thus not feeling this movement.

Galileo was a great scientist, but his life ended in misery. He was brought to trial for heresy in 1633, was forced to recant his scientific beliefs, and was confined to his home for the rest of his life. Only in 1980 he was finally exonerated.

Isaac Newton (1643-1727)

If anyone can be called ``the founder of modern science'', than it should be Newton.

At the age of 25, during 18 months when Cambridge University was closed during an epidemic of plague, Newton

• developed mathematical calculus;
• founded optics, the science of light propagation;
• initiated his work on (what is now called Newtonian) mechanics.

He was made the Lucasian Professor of Mathematics at Trinity College at the age of 26. Three years after that he was elected to the Royal Society of London, and he spent the rest of his career participating in various committees and meetings.

His main work is called Philosophiae Naturalis Principia Matematica (The Mathematical Principles of Natural Philosophy), or, in short, Principia. It was published in 1687.