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This is the first edition of Messier’s catalog of cloudy spots or “nebulae,” numbered from M1 to M45. The Great Orion Nebula is M42. In 1781, Messier published a final catalog of 103 nebulae, which are now called “Messier objects.”
Comet watching required an ever more complete mapping of the sky. In multiple articles, Messier traced the routes of comets through various constellations, mapping the nebulae along the way so as not to confuse them with the comets.
Earlier in this same volume, Messier published an article, with a star map, describing the second comet of 1770-1771, which passed near Orion.