There have been many great scientists all around the world.  The first of these lived in ancient Greece and Arabia thousands of years ago.  They discovered the size of the Earth, named many of the stars and constellations, and were very smart.  Sadly, for a very long time after these scientists died people did not remember what they had discovered.  That is until around 500 years ago when a period of time known as the Renaissance began.

The first of these Renaissance scientists was a man named Copernicus.  In 1543, Copernicus published a book about a new idea he had.  Most people in his day thought that the Earth was at the center of the Universe, an idea known as a Geocentric Universe.  They thought that the stars were little holes in a glass ball which surrounded the Earth. 

Shortly after the death of Copernicus, the next great Astronomer was born.  His name was Galileo Galilei.  Galileo was the first Astronomer to use what we call science to find out what the Universe is made of.  Copernicus was right, but he was only guessing.  Galileo was not going to guess.