In the 16th century Europe, astronomers still favored Aristotelian explanation of the universe. The Greek philosopher Aristotle believed that the world was divided into two worlds. According to him, the sublunary world is composed of four elements: water, earth, air and fire. This world is constantly changing. The supralunar world is perfect and without change. It is composed of the moon, the planets, the sun and all the stars. The passage of comets was the only observable change in the sky. Aristotle argued that they were a sublunary phenomenon and therefore they were not a perfect astronomical phenomenon. An extraordinary event will come to question this two thousand year old explanation .