People have been observing the sky for thousands of years. In the ancient Greece and Arabia they made great discoveries about the Earth, and named many of the stars and constellations. Unfortunately, much of their knowledge about a lot of things got forgotten until the Renaissance at the beginning of which most people believed that the Earth was in the centre of the Universe and that stars were small holes in the glass that surrounded the Earth.
Nicolaus Copernicus changed that belief by starting the Scientific Revolution. He was a highly educated man, and spent 15 years at different universities, studying maths, astronomy, medicine and law, and spent much of his time reading Aristotle and other ancient scholars. He asked himself: "What if the Earth is not the centre of the Universe? What if the Earth is actually a planet circling the Sun?" He wrote a manuscript explaining his new theory about the Earth revolving around the sun which was read by many astronomers, and spread all through Europe. His theory attracted many mathematicians and various astronomers who came to Copernicus to learn more about his new theory. He published his book in 1543, the same year he died. If he lived longer, he would probably have been imprisoned for his ideas.
Galileo Galilei, another brilliant astronomer, was born in 1564. Scientists before Galileo, including Copernicus, studied the works of the classical philosophers and thinkers such as Aristotle and believed them to be true. Galileo was the first who wanted to experiment and test his ideas to prove them right. In this way he and laid the foundation for the scientific method. Galileo made many great discoveries and inventions, such as a compass and a thermometer. With his telescopes, Galileo was able to observe the skies no one had ever done before. In 1610 he spotted 4 objects surrounding Jupiter which turned out to be Jupiter’s four largest satellite moons: Io, Calisto, Europa and Ganymede. They were later renamed the Galilean satellites. Galileo discovered that the Moon wasn't a perfectly round ball which was common belief in his time. When he published his findings in a book, the powerful Catholic Church considered Galileo's ideas as heresy. At first they sentenced him to life in prison, but later allowed him to live at his home in Tuscany under house arrest where he continued to write until he became completely blind. He died in 1642.