In the late 1700s serious study of the comet
s was undertaken, mostly lead by Charles Messier
(who found 13 comets on his own, and co-discovered 6 more - an historical LINEAR
Ironically, Messier is not known for his comets as much as he is known for the list of 'fuzzy things that look like comets but aren't". This list was composed to keep those fuzzy blobs in the sky that certainly weren't stars themselves, but weren't moving from and thus distracting him from his studies (though some objects don't look fuzzy at all, such as M45, also known as the Pleiades). One-upsmanship may have entered adding M42, M43, M44, and M45.
The additions of M42, M43, M44 and M45 are something of an oddity in the Messier list which consists mostly of faint objects that are easily mistaken for comets rather than the magnitude 5 Orion Nebula which covers 85 x 60 minutes in the sky (most other Messier objects cover about 20 minutes and are magnitude 7 or higher) and the very bright Pleiades at magnitude 1.4 and covering 110 minutes. These well known objects were added on March 4, 1969 and brought the list to 45 items. Why would Messier do this? The easiest and most obvious explanation was that Messier wanted a longer list than the one published by Lacaille in 1755 with southern sky objects and had 42 objects.
The first iteration of the list had 45 items and was published in 1774. These are also the brightest of the Messier List that gets dimmer as it goes down (harder to identify and so found later). It is not an intentional ordering.
There are two 'mistakes' in the Messier list at M40 and M73 which are not 'deep sky objects'. M40 is a faint double star system and M73 is an asterism of four close stars. M102 is also a mistake that may have been a duplicate of another entry but at the wrong coordinates. There are also a striking number of omissions in the list - such as the spectacular galaxies NGC 253 and NGC 2403.
The telescopes that Messier used ranged in power from 44X to 138X, and are comparable with those telescopes used today by amateurs looking for deep sky objects. The Astronomical League will award certificates to anyone who is able to detect 50 Messier Objects with binoculars or 70 objects with a telescope. Information on the Messier Club can be found at http://www.astroleague.org/al/obsclubs/messier/mess.html.
To date, there are 1864 people who have received this certificate.