Six stages in the history of the astronomical unit

Giovanni Antonio Rocca wrote "The problem of solar distance and parallax was one of the most important in astronomy, well worth a lifetimes work by any astronomer." (see Ricciolo, 1651:732). This paper briefly reviews the values obtained for the Earth-Sun distance throughout the history of astronomy, and divides the investigation of the astronomical unit into six stages. It is suggested that a similar six stages can be recognized in the history of many other fundamental parameters in our subject.