Galieo Galilei was an Italian astronomer, physicist and mathematician who is widely credited as being the founder and father of modern science. Perhaps Galileo's biggest influence on today's science is that he was willing to stick to his findings despite the fact that the Catholic church felt he was in direct confrontation with their teachings. Galileo also made several advances in scientific fields and inventions which are still relied upon in some form or another to this day.
Leading the Charge In Experimentation
During Galileo's time the main way in which science was practiced still leaned heavily on "authority," meaning that whoever was the leading authority of that region provided the answers, and the public at large were expected to agree based mainly on faith. Galileo did not take statements at face value and investigated the causal effects of different variables. In effect, Galileo designed how experimentation would be carried out in the future.
Galileo changed the way mathematics was perceived by stressing that it was, in fact, the key to understanding how the world actually worked. His pioneering in this field allowed scientists like Sir Isaac Newton to build upon his work. Newton specifically used Galileo's work to help formulate his own laws of motion and explain how gravity works and affects objects.
While Galileo did not invent the first telescope, he did refine it to the point that he was able to see farther than any telescope of its time. This allowed him to see into outer space as well as set the basis for the kinds of powerful telescopes that we use today.
While Galileo was not the first scientist to posit that the earth actually revolved around the sun — along with the other planets — he is credited as being the one man who proved Copernicus' theory beyond a reasonable doubt. Using his telescope he was also able to demonstrate that the sun and the other planets were in fact naturally occurring bodies and not some sort of supernatural entities to be feared or mistrusted.
Early Test for the Speed of Light
Since ancient Greece, scientists have attempted to measure the speed of light. With no way of measuring its speed, these ancient academics believed that the speed of light was practically limitless. However, in the early 17th Century, Galileo performed one of the earliest experiments to measure this by telling his assistant to cover and uncover lanterns at specific times while he reported on the light's appearance and disappearance from far away. While he concluded light was too fast to be measured, his experiment would pave the way to future experiments which would eventually result in discovering this incredibly fast velocity.