Harold Corwin

IC 117 = NGC 560. Every now and then, I can do really silly things. When I started on this project, I did not have a good feel for the original data that went into the NGC and ICs. So, I tended to overlook obvious points that now leap off the page at me. Javelle's descriptions, for example.

Dreyer called the 58th nebula in Javelle's first list "pF, S, dif, III 441 [NGC 560] sf." Well, the only object north-preceding NGC 560 is a pretty bright star. So, I took that star to be IC 117 without checking Javelle's original paper.

Are pretty bright stars "pF," "S," and -- especially -- "dif" [diffuse]?" Not usually. So, 29 years later, when I ran across my note that this was a star, I looked at Javelle's original monograph to find out what's wrong. Dreyer got the data copied into the IC correctly, but he apparently interpreted Javelle's footnote "On a vu les nebuleuses NGC 558, 560, 564" simply as "III 441 sf".

NGC 560 would indeed be "sf" if IC 117 were at the position that Javelle says it is, but there is nothing there. The star is about 20 arcsec northeast of Javelle's position, so it is definitely not the object he measured. Re-reducing his observation does us no good as that lands within the round-off error of the correctly-copied NGC position.

Did Javelle perhaps make a sign error? Nope -- there is nothing at any of the offsets implied by such an error. How about his comparison star? Well, on the night in question, he observed another nearby nebula (IC 116) using BD -2 221 as his comparison star. He claims to have used BD -2 220 for IC 117; is it possible that he used 221 instead?

Actually, yes. Reducing his observation with the modern position for BD -2 221 drops his position within 2 arcsec of the nucleus of NGC 560. There is no doubt that IC 117 is identical with NGC 560.

By the way, Carlson claims in her 1940 collection of Mt. Wilson identifications that IC 117 = NGC 558. I do not know what this is based on -- the nominal position of IC 117 is closer to NGC 560 than it is to NGC 558.

― IC Notes by Harold Corwin

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