From an early age, Galileo Galilei was curious about the world around him. He began conducting scientific experiments. No one had ever done that before.
He invented many useful devices, such as improved clocks and tools to help sailors find their way at sea. His greatest fame came after he invented a more powerful version of the telescope. This new telescope allowed him to observe the skies in more detail than anyone before him. His observations soon led him to believe that Earth revolved around the sun.
This belief landed him in trouble with the Catholic Church, which for many centuries had taught that the sun revolved around Earth. Church leaders put him on trial. They would determine whether Galileo lived or died.