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Copernicus On the Revolutions, Book I, Chapter 11.

The question as to whether Copernicus believed in the physical reality of the celestial spheres has been a source of some controversy. For more details, see Westman (1980), Aiton (1981), Jardine (1982), Grant (1987).

The argument of Kuhn (1957) that Copernicus was influenced by the Neo-Platonic tradition of Sun worship is refuted by Rosen (1983).

Copernicus On the Revolutions, I, 11. Further details are given in Rosen's notes in Copernicus (1992) (see also Africa (1961)).

Copernicus made a number of his own observations, but they are not notable for their accuracy. In On the Revolutions, he described the instruments that he used for his observations, all of which were known in ancient times; indeed, Copernicus' descriptions of the instruments are based on those in Ptolemy's Almagest. He relied heavily also on the observations of others, particularly those of Ptolemy (in whom Copernicus put rather too much faith) and also Islamic astronomers such as al-Battani and al-Zarqali In Copernicus' time, a theory was considered accurate if it produced results in agreement with observations within the limit of observational accuracy, which was about 10' of arc, and this was the sort of accuracy that Copernicus strove for. He did not succeed, but the tables constructed from Copernicus' mathematical astronomy by Erasmus Reinhold-the Prutenic or Prussian Tables (1551) - were an improvement over the preceding Alfonsine Tables from the thirteenth century, and remained a standard source of information until superseded by Kepler's Rudolphine Tables in 1627. This improvement was due, not to his geometrical models being intrinsically more accurate than Ptolemy's (they are not) but simply because he recomputed many of the parameters that must be entered into the models in order to construct the tables and, since many of these had changed since the time of Ptolemy, the final results were more accurate.

The Commentariolus

Copernicus developed his heliocentric theory while in Italy. The first exposition of his theory was a short sketch (commentariolus) which was written in about 1507, circulated in handwritten form to a few scholars, but never printed during Copernicus' life. The theory presented in this brief document differs in several essential features from the one that appeared eventually in On the Revolutions, and thus represents a preliminary stage in Copernicus' development of a heliocentric theory. No mention of the Commentariolus is made in On the Revolutions, and no manuscripts have been found among his own books and papers, so it is possible that, by the time he wrote On the Revolutions, he did not wish to be associated with his earlier work. All the existing manuscripts of the Commentariolus are thought to be descended from a copy received in

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