In the year 1515, Nicolaus Copernicus theorized that the Earth was a planet and discovered the idea of a heliocentric solar system, a system in which all planets revolved around the sun. He built his own observatory in 1513 to help him watch the actions of planets.
In 1514, Copernicus published his first manuscript with his theories called Commentariolus. The manuscript summarized Copernicus's heliocentric solar system and featured a sketch describing each of the seven aspects in the system. Both Commentariolus, and his later manuscript De Revolutionibus Orbium, were both considered heretical and banned. Two centuries later, when the ban was lifted, Kepler expanded on Copernicus's work and fixed some of the errors.