Tycho Brahe's observations of Præsepe Cancri.

This article focuses on Tycho Brahe's observations of Præsepe, the open star cluster M44 in the constellation Cancer, demonstrating that his seemingly inconsistent measurements were actually precise observations of three or four individual faint stars rather than a nebulous object. Through detailed analysis of Tycho's recorded meridian altitudes, declinations, right ascensions, and angular distances relative to fundamental stars α Leonis and γ Geminorum, the study identifies specific stars (notably HIP 42549) as the targets of his observations. It further explains Tycho's use of the term "nebulosa" to denote very faint stars rather than truly nebulous objects, which he did not include in his star catalogues. The article concludes that Tycho and his assistants resolved individual stars in M44 with remarkable accuracy more than two decades before Galileo's telescopic observations, highlighting Tycho's exceptional observational skills in pre-telescopic astronomy.