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The Caldwell Objects

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37<br />

Collinder reversed the two dimensions again in<br />

his catalog; his Collinder 416 (NGC 6882)<br />

measures 20' and his Collinder 417 (NGC 6885)<br />

measures roughly 8'. <strong>The</strong> positions Collinder<br />

gives for the two objects are precisely the same<br />

positions that Trumpler gives for NGC 6882 and<br />

NGC 6885. In other words Collinder's position<br />

for NGC 6885 is the same as Trumpler's position<br />

for NGC 6885, but Collinder's diameter for NGC<br />

6885 equals Trumpler's diameter for NGC 6882.<br />

And Collinder's position for NGC 6882 is the<br />

same as Trumpler's position for NGC 6882 but<br />

Collinder's diameter for NGC 6882 is roughly the<br />

same as Trumpler's diameter for NGC 6885. "My<br />

guess," Archinal says, "is that Collinder simply<br />

copied Trumpler's positions (since they match<br />

exactly) for NGC 6882 and NGC 6885, not<br />

realizing that his identifications were the reverse<br />

of Trumpler's." Wilson, who has also been trying<br />

to solve this puzzle for years, simply tossed her<br />

hands into the air after she searched numerous<br />

catalogs for answers, but found only more<br />

questions. "No one really agrees with each other<br />

on what NGC 6882 or 6885 is!" she says. "Every<br />

reference in [the] amateur literature gives<br />

conflicting information. It is obvious that the two<br />

clusters have been confused with each other since<br />

[the time of William] Herschel."<br />

What a mess! But forget all the confusion.<br />

Let's wipe the slate clean and look at William<br />

Herschel's original discovery notes. (Such a step<br />

is fundamental in solving any identification<br />

problem; one must always refer to the original<br />

sources so far as is possible.) Just where did<br />

William Herschel position NGC 6885 and NGC<br />

6882?<br />

William Herschel made only one observation<br />

of each cluster on different (though subsequent)<br />

nights. First he discovered the object now known<br />

as NGC 6885. On September 9, 1784, he found<br />

NGC 6885 1' east and 27' south<br />

146<br />

of 18 Vulpeculae. Interestingly, Herschel's discovery<br />

position of NGC 6885 is not centered on<br />

20 Vulpeculae but on a point about 6.5' westsouthwest<br />

of that star. In fact, Herschel's<br />

discovery position for NGC 6885 matches the<br />

center of NGC 6882 as plotted on the Millennium<br />

Star Atlas. Furthermore, when I took an unbiased<br />

look through my telescope I saw a faint but<br />

dense patch of stars northwest of 20 Vulpeculae<br />

and a brighter but looser scattering of suns to the<br />

southwest of that star; Herschel's position for<br />

NGC 6885 is roughly midway between these two<br />

concentrations. Now at least one piece of the<br />

puzzle appears to make sense. Without question<br />

there is a cluster of stars (or the semblance of<br />

one) at Herschel's position for NGC 6885, and it<br />

is my recommendation that star charts place a<br />

20'-wide NGC 6885 there. Seen another way,<br />

NGC 6882 in the Millennium Star Atlas should<br />

read NGC 6885.<br />

Now let's look at NGC 6882.<br />

On September 10, 1784, William Herschel<br />

discovered NGC 6882 just 1m 12s east and 12'<br />

south of 18 Vulpeculae. Of it he penned " clus- A<br />

ter of coarsely scattered stars" — which is<br />

uncannily similar to his description of NGC<br />

6885. Indeed, some astronomers have argued<br />

that Herschel's observation of NGC 6882 is simply<br />

a repeat viewing of NGC 6885. Supporting<br />

this allegation is the position of NGC 6882 when<br />

plotted on a star chart. William Herschel positioned<br />

NGC 6882 just south of the star 19<br />

Vulpeculae, where no cluster is evident; this may<br />

explain why many of the early star charts do not<br />

show NGC 6882. Archinal found a small oval<br />

asterism at Herschel's position for NGC 6882 on<br />

the National Geographic Society-Palomar<br />

Observatory Sky Survey and once suggested that<br />

this was the missing NGC 6882. In September<br />

1998, Archinal and I used the 31-inch reflector at<br />

the Warren Rupp Observatory, stopped down to<br />

give us the effective aperture<br />

Deep-Sky Companions: <strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong>

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