Unwinding the discovery of spiral nebulae

Evidence for spiral structure in distant galaxies was first noticed byWilliam Parsons, the Third Earl of Rosse, in April 1845 within a fewmonths of the first trial of his great six-foot reflector the "Leviathanof Parsonstown" on 11 February 1845. Despite the significance ofthis discovery there are puzzling inconsistencies in the story, andthe discovery date – sometime in April – is curiously vague. Herewe review the chronology of observations of the two principalplayers in the story: Messier 51 and Messier 99. The former wasidentified by Lord Rosse as having a spiral arrangement in thespring of 1845, and the latter "the following spring". The RevdThomas Romney Robinson, the third Director of the ArmaghObservatory, was observing with Lord Rosse during February andMarch 1845, and again in 1848, but he apparently only confirmedRosse's detection of spirality in both galaxies around 11 March1848. No-one doubted Lord Rosse's discovery of spirality in M51(and the following year also in M99), but it was almost three yearsbefore the observation was confirmed by another astronomer