Tycho Brahe’s observations of Præsepe Cancri

Tycho Brahe made several thousand observations of the celestial bodies, in general with an unprecedented accuracy and precision. A group of observations of a nebulous object known as Præsepe differs, however, significantly from the general picture. Over a period of 3 years, Tycho observed different astronomical coordinates for Præsepe; the values of meridian altitude differed by as much as 8 arcminutes for repeated observations, and the declination values by up to 5 arcminutes, systematic discrepancies such as these never seen before or since for Tycho’s stellar observations. In this article it will be demonstrated that these differences were due to the observation of three or four individual stars and not of a single arbitrarily defined centre in Præsepe, the open star cluster M44 as it is known today. Individual stars had not been resolved in Præsepe before, and consequently neither had their celestial coordinates been determined. Tycho selected a single of the observed stars to represent Præsepe and included its celestial coordinates in his star catalogues.