This is the first edition of Messier's catalog of cloudy spots or "nebulae," numbered from M1 to M45. In 1781, Messier published a final catalog of 103 nebulae, which are now called "Messier objects." The Great Orion Nebula is M42.

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.