1543

1543 - Unraveling -  Nicolaus Copernicus publishes De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres) 

Go outside, look overhead, and celestial objects will roll past. The sun will start its journey in the east and travel across the sky over the course of the day, disappearing to the west as the day ends.  The moon - whatever phase it is in - will do the same. At night the stars, if watched long enough, will seem to move as a unit, all traveling this same way.  Everything in the sky must be traveling in circles around the solid, unmoving earth. This had been so clear that it had been realized by the ancient thinkers like Ptolemy and incorporated in historical records, like the book of Joshua, where the sun stops overhead to allow the Israelites to overcome the Amorites. 

However, even those old records were from people who noted some discrepancies in what was perceived. The moon had phases, and did not move the same way as the sun. Sometimes one would be blocked out in an eclipse, but this did not occur every month or otherwise regularly. Even more significantly, a few of the stars moved on their own paths, not as part of the large group. These "wandering stars" - called planets from the Greek for "wanderer" -  would often even move backwards compared to the other stars and the sun - west to east, that is,  rather than east to west. Two of these planets were only visible early in the morning, as the sun rose, or in the early evening as the sun set, while the others moved through the night with the fixed stars. 

The solidness of the unmoving earth, though, meant those oddities must still be explainable by movement around it.  Astronomers proposed epicycles, circular movements of these traveling stars around points in space, which kept the perfection of circles and the solid earth in the explanation of what could be plainly seen. 

Nicolaus Copernicus was not the first to suggest that the problem was in the concept of an unmoving earth surrounded by celestial objects.  If the earth moved around the sun, the model became simpler overall, the epicycles unnecessary. Nonetheless, this proposed change was not immediately accepted. One problem was that the movement of the earth would cause the positions of the stars to change over the course of the year. This difference had never been noted, so how could it move? There were religious reasons to prefer the earth-centered view, often because the Bible, presumed infallible, described events that depended upon it. There would be additional refinements over the next century - requiring outright rejections of previously accepted concepts - before Copernicus' view would be recognized as the way our local universe really worked.