The Caldwell Objects

Earth turns, the cluster would climb ever higher
in the sky, well away from the comet haystack.
Meanwhile the comet hunter's sweeps would
take him (or her; Caroline Herschel was a nearcontemporary
of Messier) lower and lower as he
(or she) searched for brighter and brighter comets
near the Sun. So only someone who did not know
the sky or was unskilled in the comet-hunting
trade would have been fooled by this bright
circumpolar glow; it would be folly to list the
Double Cluster as an
object that could be mistaken for a comet by a
skilled observer. And though Messier's skysweeping
technique is not known with certainty,
Peter Lancaster Brown, in his book Comets,
Meteorites, and Men, writes that "from sketchy
accounts [Messier] left, there is no reason to
suppose that it differed much from those in use
by present generation amateur-visual observers
dedicated to making a persistent search of the
skies night after night, month after month and
year after year."
Appendix Β 453