Journal of Astronomical History and Heritage, 15(1), 19-29 (2012). THE M51 MYSTERY: LORD ROSSE, ROBINSON, SOUTH AND THE DISCOVERY OF SPIRAL STRUCTURE IN 1845 Wolfgang Steinicke Gottenheimerstr. 18, D-79224, Umkirch, Germany. E-mail:
[email protected] Abstract: In April 1845 Lord Rosse discovered the spiral structure of M51 with his 72-inch reflector at Birr Castle. Already in March the new telescope had been pointed at the object in Canes Venatici, later nicknamed the ‘Whirlpool Nebula’. Two experienced astronomers were present: Sir James South and the Reverend Thomas Romney Robin- son. The problem is that there is no record that they noticed the spiral structure, even though it was immediately seen by Lord Rosse the next month. The solution presented here is based on evidentiary facts, highlighting the nine- teenth century astronomical praxis. Focal points are bias, fantasy and a sometimes fatal conspiracy of eye and brain. Keywords: Spiral structure, nebulae, star clusters, Lord Rosse, Birr Castle, Leviathan of Parsonstown, Whirlpool Nebula, nebular hypothesis, visual observation, drawings. 1 DISCOVERY OF M51 AND JOHN the core, surrounded by a divided ring (Figure 2); it HERSCHEL’S ‘RING NEBULA’ appears as Figure 25 in the Slough catalogue. Herschel M51 (NGC 5194) is a nearby Sbc-galaxy with a visual magnitude 8.4 and a size of 11.2 × 6.9. It was dis- covered on 13 October 1773 by Charles Messier (1730–1817) with a 3.5-inch refractor at Paris. The description, published in his famous catalogue of 1781, reads: “… very faint nebula without stars …” 1 (Messier, 1781: 247).