- Charles Messier was a French comet hunter in the 1750s. In Messier's day, astronomers did not have the benefit of good start charts like we have today, showing th epositions of galaxies, star clusters, and nebulae - what the great twentieth-century comet hunter Leslie Peltier termed "comet masqueraders." these objects were largely unknown and uncharted. Thus Messier began to build a catalog of what he called these "embarrassing objects". By 1765, Messier had compiled a list of 41 such objects. Of those, only 17 or 18 were his own discoveries; the rest had been seen previously by others (whom he acknowledged). Before submitting the list for publication, he decided to round it out with a few more objects so on 4 March of that year he determined the positions of M42, M43, M44, and M45. He presented his list of 45 nebulae and star clusters to the Academy of Sciences in Paris in February 1771, and it appeared in the Academy's Memoirs for that year, which were actually published in 1774.
Source - Deep Sky Companions - The Messier Objects by Stephen James O'Meara
NOTE: The calculations used in the lists above do not account for atmospheric refraction or the earth's precession among other things that can cause positional inaccuracies. The calculations are based upon the sidereal date at the observer's location, and the right ascension and declination of the listed object. The calculations are not accurate enough to point your telescope at an astronomical object, but can be used to determine if an object is above 30 degrees to within a reasonable accuracy for visual observations. The results are not guaranteed, you should check your list against other sources.