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Type: galaxy (in pair), I
Mag: B=14.7, V=?
Size: 1.778′ x 0.363′
NGC 5730 and NGC 5731. Here is another pair found by WH to which he gave only one position. John Herschel states explicitly that he estimated the position of the preceding of the two with respect to the following object. His position for N5731 = H III 658 = h 1868 is good, but that for N5730 = H III 657 = h 1867 is off in declination: his offset places it north-preceding N5731, not south-preceding as it really is.
This has led to confusion only in CGCG which has the identifications reversed. All other major catalogues have this pair named correctly, though UGC placed colons on the names, indicating some uncertainty on Nilson's part about the identifications.
There is a small mystery, though: where did the position angle notation in GC and NGC come from? It is correct (90 deg, which helps pin down the identification), but neither of the Herschel's published catalogues give a measurement. It is probably buried in Sir John's unpublished papers, as are the details for other of his observations (see e.g. NGC 980 and NGC 982).
Synonyms: H III-657
Discovered in 1787 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "Two [NGC 5730 & NGC 5731] Both vF, vS, E in different directions, 2 or 3' distant in parallel, each south of a small star."
The RNGC (Sulentic and Tifft 1973) notes that this is a 14.0 mag galaxy. Their coded description reads EON,HISB,DIFFAN SEND.
The Messier objects
The Bennett objects
The Caldwell list
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