The Caldwell Objects

A typical pattern for evening hunting is to
begin in late twilight by choosing a point near
the horizon and about 45° north or south of the
sunset point and sweeping parallel to the
horizon until reaching an azimuthal point
around 45° on the other side of the sun…. After
completing the first sweep, the instrument is
raised about one-third to one-half of a field
diameter and the observer proceeds to sweep
backward to the original starting point. This
procedure should be continued until one
reaches at least 45° altitude above the horizon.
The search pattern is reversed when scanning
the morning sky.
Of the 41 deep-sky objects in Messier's original
list, 17 clearly were discovered first by
Messier; and of the 109 objects in the final catalog
only 37 were Messier discoveries. Just as we
might expect from a comet hunter, all of his
original finds were within 45° of the point where
the ecliptic met the horizon at the time of
discovery. In fact, all but one (M40) of the first 41
objects lies reasonably within that limit; 60
percent of them were within about 25° of that
point. Since it was Messier's intent to catalog as
many of these "embarrassing" objects as possible,
he thoroughly searched the literature on
previously reported nebulae and, after observing
and obtaining accurate positions for them, added
these objects to his list. Messier did not care
whether these objects were his discoveries,
because they were not comets. He meticulously
acknowledged each discoverer in his catalog. In
fact, he was probably quite happy to do so —
their discoveries only benefited him.
One question immediately surfaces, however:
why did Messier include in his catalog the bright
and obvious objects M31 in Andromeda, M6 and
M7 in Scorpius, and M8 in Sagittarius? All were
well known at the time, some since
antiquity. The answer would be obvious to an
18th-century comet hunter. All of these objects lie
in the comet "haystack" at various times of the
year, and all look nebulous to the naked eye.
Obviously, no intelligent observer would confuse,
say, M31, for a comet when it was in the
dark of the sky, but what about in a twilight sky?
In Messier's day, twilight comets were not
infrequent. The first and only comet discovered
in 1758 (when Messier started looking), was
sighted from Bourbon Island in the Indian Ocean
just 15 days before perihelion; the comet shone at
magnitude 2.5 and had a 1.5°-long tail. Messier's
fourth comet was sighted (on March 8, 1766) in
the twilight; the 6th-magnitude comet, which
was approaching conjunction with the Sun, never
escaped the twilight and was last seen by Messier
with difficulty low over the horizon. On October
13, 1773, one year before Messier's first list was
published, Messier discovered another twilight
comet; this one shone at magnitude 4.5, about the
same brightness as both M6 and M8. In 1780
Messier would find yet another twilight comet, a
telescopic 7th-magnitude one. So it was not only
reasonable but also prudent that Messier include
in his catalog the positions of all cometlike
objects, however bright, as long as they were in
the comet haystack.
It's easy to imagine how the compact glows
of M6, M7, or M8 could be confused as comets
when hiding in the twilight, many would agree,
but M31? Interestingly, in the last 20 years, I
know of two cases where respected comet
hunters have reported the discovery of a
binocular comet in the twilight sky only to be
"embarrassed" when they learned that they had
simply rediscovered M31. Of course, the small
cometlike form of M32 is almost sitting on top of
M31; how strange it would have been for Messier
to include M32 in a catalog of pseudocomets
while neglecting the conspicuous,
Appendix Β 451