The Caldwell Objects

of Herschel's telescope. W then e employed an
eyepiece that gave us a 15' field of view (again, to
match Herschel's) and swept the region. Both of
us saw and studied this asterism, but it seemed
unlikely that Herschel would describe such a
distinct asterism as a "cluster of coarsely scattered
stars." Archinal's thinking on the subject has
evolved since, however. John Herschel never
observed NGC 6882, so his records are of no help.
NGC/IC sleuth Harold G. Corwin (Infrared
Processing and Analysis Center) has buttressed
the notion of redundancy in the case of these two
clusters. He posits that William Herschel made a
15' error (one telescopic field diameter) in his
September 10,1784, observation. "Aside from the
declination difference," he says, "the observations
are consistent with
37
the idea that he saw the same cluster both nights.
This would make NGC 6882 [equal to] NGC
6885." Corwin further notes that William
Herschel found seven objects on September 10,
1784. When Corwin analyzed Herschel's offsets
for all of them — using the Smithsonian
Astrophysical Observatory's star catalog for star
positions and, when necessary, proper motions
— he found that Herschel made significant errors
in his positions for four of these objects. "I was
looking for a pattern in the discrepancies
between William Herschel's positions and the
modern positions," he writes, "but do not see
one. What I do see, though, is a possibility that
his data on that night were taken with somewhat
less care than normal. Distractions? Sleepiness?
Who knows? In any event, there are significant
errors in more than half of his posi-
tions for deep sky objects
that night."
Corwin also notes
that Dreyer documented a
case in which William
Herschel twice recorded the
same nebula (NGC 4208 =
NGC 4212) during the same
sweep — probably within
the same five-minute period
— and entered the object
twice in his lists. "So, it is
possible for [Herschel] to
have observed the same
object twice within a short
period of time without
recognizing it as one he had
seen before," Corwin says.
"This reinforces my feeling
that the 15 arc minute offset
in the position of NGC 6882
is a simple mistake on
[Herschel's] part.
The Caldwell Objects 147