History Online - Astronomy

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Ancient and Medieval Chronology

Star coordinates

Two star coordinate systems were known in the Middle Ages, namely, ecliptical and equatorial systems. From now on, we refer the reader to [102] with respect to all problems of star astronomy. Beginning with the 17th century, only equatorial coordinates have been used in catalogues. Ecliptical coordinates are only seen in the Almagest and in 16th-century catalogues, the last of them being by Tycho Brahe. The reason for this is very simple: Equatorial coordinates are much easier to measure (to a greater accuracy at that) than ecliptical ones. On the other hand, at the time of Ptolemy and from the 15th to the 17th century, astronomers believed ecliptical coordinates to be "eternal", i.e., suitable at once for all epochs, with the latitudes being invariable in time, and the star longitudes varying at a constant rate (meaning the longitudinal precession). Therefore, the two principal questions discussed by astronomy in the 15-17th century were: Do stars possess their own movement, or is the configuration of the starry sky invariable, and what is the rate of precession? Both answers given by Ptolemy are wrong. He answered in the negative in the first case, though Arcturus' own movement was discovered in the mid-18th century. He determined the rate as 1 per 100 years, its modern value being 49.8s a year, i.e., 1.4 each 100 years. It is curious to see Tycho Brahe assert that Copernicus allegedly assumed that the precession was 1 a century, and Tycho Brahe ascribed the first refining of the figure to himself ([104]).

The division value for star coordinates in the Almagest is 10 minutes. More precisely, one-sixth of a degree, since the Almagest coordinates were given in degrees and their fractions with an extremely inconvenient (from the modern standpoint) method for the latter notation (e.g., see [102], [108]). The coordinate accuracy does not correspond to this value (see the analysis in [102] and in the following). Ptolemy declared that he had directly measured the ecliptical coordinates of all stars by means of an astrolabe, which is a rather complicated procedure [102]. Copernicus only refined the positions of several stars. Tycho Brahe rewrote the catalogue again, using the system of base stars and thus achieving the accuracy of one minute. His catalogue contains about 700 stars (against Ptolemy's 1022); however, after it had been published in the 16th century, the Almagest completely lost its scientific importance.

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