Galileo’s career was marked by many discoveries; while successful discoveries remain in the mind, there were considerable flops as well. And some facts people think they know about his work are pure fiction.
Galileo Galilei made several discoveries and contributed to the way scientists should work; he was actively developing existing scientific theories and put forward several of his own devising (the list is probably not complete):
1. Gravitational centre of solid objects. His first published paper in 1586 Theoremata circa centrum gravitatis solidorum dealt with the gravitational centre of solid objects and was conventionally written in Latin as opposed to his later publications in Italian.
2. Pendulum. He was the first to recognise that the movement and the acceleration of a pendulum is relative to its length and independent of its weight or its deflection. He was convinced that a pendulum clock could be devised but the construction of one eluded him.
3. Acceleration. With his trials on an inclined plane, Galileo discovered that acceleration is a force distinct from speed. His biographer Vincenzo Viviani claimed that Galileo had done trials with objects falling from the Leaning Tower of Pisa. This is a myth, as watches with the necessary exactitude to make measurements meaningful were a thing of the far future.
4. Replicable laboratory conditions. Galileo’s detailed description of the inclined plane and its use made it possible for other scientists to replicate his findings one on one. Years later, Francis Bacon would demand that these replicable conditions should be part of any scientific publication; modern scientists would do good to remember that when they publish their claims to fame.
5. Mathematical formula. In Il Saggiatore in 1623, Galileo quite plainly stated that the only way to describe laws in natural sciences was by mathematical formulas (unlike Francis Bacon). The formulas would not be recognisable as such by modern algebraic conventions, though.
6. Surface of the moon. Using a telescope, Galileo was able to discern geological features on the moon and to draw a crude map of the surface. With this he contradicted Aristotle’s claim that the moon’s surface was flat.
7. Planets and stars. The same means showed him that unlike stars planets showed up as discs in his viewer. Later, he found that Venus showed a libration like the moon.
8. Moons of Jupiter. Galileo is credited with the discovery of the four largest moons of Jupiter. Frank Keim in his book Die Entdeckung der Jupitermonde 105 Jahre vor Galileo Galilei (The Discovery of Jupiter’s Moons 105 Years Prior to Galileo Galilei) proposes a theory of an earlier discovery, though.
9. Milky Way. The telescope also revealed to him that our galaxy was not a nebula but an agglomeration of individual stars.
10. Relative weight of ice and air. Galileo proved Aristotle wrong again in a simple but efficient experiment showing that ice swims on water because it is lighter and not because it is flat. In 1614, Galileo was able to determine the weight of air as a fraction of water; until then, scientists had believed air to be weightless.
11. Sunspots. The discovery of sunspots was disputed between Galileo and Christoph Scheiner. They also disagreed on their nature, Scheiner seeing them as satellites and Galileo as clouds.
12. Tides. Galileo proposed a theory that the tides were caused by the movement of the earth (and then used it as a proof for the heliocentric system of Copernicus). It took Isaac Newton to get out of that muddle.
13. Comets. In Il Saggiatore, Galileo gives a detailed description of comets as optical effects like the rainbow or polar lights. The book is in fact mainly a diatribe against Tycho Brahe’s ‘ape stars’ (as Galileo called them).
14. Elasticity. Galileo got near to a workable theory on elasticity but included a wrong assumption in his formula. At least, he got nearer than Leonardo Da Vinci had in his studies of the problem. He would probably have been amused by the fact that it took 300 years for scientists to believe that he was wrong; after all, many of Aristotle’s theories had lasted for 2000 years.
15. Neptune. Researchers at the University of Melbourne are of the opinion that Galileo was aware of the existence of Neptune. Officially, the existence of Neptune was first mathematically assumed and later confirmed by observation in 1846 by Johann Galle.