A quantitative analysis of David Fabricius' astronomical observations (Q6633324)

Of the more than 200 observations used in his \textit{Astronomia Nova}, Kepler took most from Tycho Brahe's observation notebook, but also used two other sources: his own observations (eight) and those of David Fabricius (two).\N\NDavid Fabricius (1564--1617), a Reformed pastor who carried out his ministry in Ostfriesland, although not a professional astronomer, made important astronomical observations and was known by Tycho Brahe and Kepler, the latter considering that Fabricius was the best observer after Brahe. Fabricius and Kepler carried a ``correspondence of around 50 letters that acquired a central role in understanding the path that led Kepler to his first two laws because in them, Kepler communicated to Fabricius the progress in his model of Mars.'' (p.\ 618) Fabricius sent the record of his observations to Brahe and, after Brahe's death, to Kepler.\N\NThe authors looked at 526 observations that Fabricius sent to Brahe and Kepler between 1596 and 1609 and compared them with those calculated from modern theory. The observations were made from three cities, Resterhafe, Essens, and Osteel. By mentioning only the hour or the half-hour, but not the minutes, Fabricius did not provide an exact observation time, which provided a challenge to the authors' project of checking the exactness of Fabricius' observations. The errors found are ``orders of magnitude smaller than what is expected from Fabricius' observations.'' (p.\ 620) The ``findings preliminarily indicate that Fabricius' astronomical observational abilities were comparable to, and occasionally superior to, those of Brahe himself. So, Kepler was probably right when he says that, after Brahe's death, Fabricius was the best observer.'' (p.\ 629)