Harold Corwin

John Herschel has this in three sweeps with accordant positions. That position is also close to the cluster we usually put this number on, so the identity is not in question. He may also have picked up the cluster in a fourth sweep as well; see NGC 222 for that speculation.

What interests me is the relationship of this cluster to two others in the area which have traditionally carried the numbers NGC 222 and NGC 231, both of which see. John Herschel refers to these in his description in Sweep 625 on 20+- September 1835 (see my comment at the end of this note about the uncertain date): "The first of an irregular string of nebulae and stars which descends at an angle of about 45 degrees from the centre to the edge of the field (i.e. in a northeastern direction)."

The string of stars is certainly there (see my notes about NGC 222 and NGC 231), though there is no nebulosity involved, at least in the DSSR2 image of the area -- John Herschel was probably seeing the unresolved background light of the SMC.

Archinal and Hynes also have a discussion of this string of clusters, noting that it is probably Dunlop's number 2 in his 1826 list.

Finally, I see that in his table of Sweeps, John Herschel has "no such sweep" noted in place of a date for Sweep 625. Sweep 624 was made on 19 September 1835, and Sweep 626 on 21 September. The date I've adopted is an interpolation, of course, but it may have been on either of the adjoining dates. Another possibility is that the Sweep is indeed non-existent, and those objects marked with its number may be from other sweeps.

In May 2016, I checked John Herschel's sweeps in the Herschel Archive and find his comment "Sweep 625 No[.] omitted by mistake -- no such Sweep exists." On the right-hand page, he also writes, "625 = 626". He notes the date as 21 September 1835. So much for the mysterious "no such sweep"!

― NGC Notes by Harold Corwin

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