Messier 91

Discovered most probably by Charles Messier on March 18, 1781.
Independently rediscovered by William Herschel in the on April 8, 1784.

Messier: M91.

March 18, 1781. 91. 12h 26m 28s (186d 37m 00s) +14d 57' 06"
Nebula without star, in Virgo, above the preceding No. 90: its light is still fainter than that of the above.

(At the position Messier has given, no object is present which he could have seen, thus M91 was missing until 1969, when William C. Williams discovered that Messier had probably measured its position from M89, while he thought he used M58, and plotted it wrong.)

(Following the entry for M91 in the Connoissance des Temps for 1784, Messier added the note below:)
Note. The constellation of Virgo, & especially the northern Wing is one of the constellations which encloses the most Nebulae: this Catalog contains thirteen which have been determined: viz. Nos. 49, 58, 59, 60, 61, 84, 85, 86, 87, 88, 89, 90, & 91. All these nebulae appear to be without stars: one can see them only in a very good sky, & near their meridian passage. Most of these nebulae have been pointed to me by M. Méchain.

(This notion is apparently the first impact of the Virgo cluster of galaxies into the scientific literature, i.e. the discovery of that galaxy cluster).

William Herschel: H II.120.

II.120. April 8, 1784.
L. r.
Large. Round.

JH's M91 candidate, H III.602 (NGC 4571)
H III.602. January 14, 1787.
vF. cL. vgbM. s. cBst.
Very faint. Considerably large. Very gradually brighter toward the middle. South of it is a considerably bright star.

[1811: PT Vol. 1811, p. 226-336; here p. 290-291]
12. Of the remarkable Situation of Nebulae.
The number of compound nebulae that have been noticed in the foregoing three articles [on multiple nebulae] being so considerable, it will follow, that it they owe their origin to the breaking up of some former extensive nebulosities of the same nature with those which have been shewn to exist at present, we might expect that the number of separate nebulae should far exceed the former, and that moreover these scattered nebulae should be found not only in great abundance, but also in proximity or continuity of each other, according to the different extents and situations of the former diffusions of such nebulous matter. Now this is exactly what by observation, we find to be the state of the heavens.
In the following seven assortments we have not less than 424 nebulae; [..]
(*) [7th assortment] Ten large nebulae. [including H II.120 = M91]

John Herschel (1833): h 1345.

h 1345 = H II.120.
Sweep 419 (April 23, 1832)
RA 12h 26m 51.8s, NPD 74d 33' 45" (1830.0). [Right Ascension and North Polar Distance]
B; L; pmE; gbM.
Bright; large; pretty much extended; gradually brighter toward the middle.

Sweep 24 (April 3, 1826)
RA 12h 26m 51.9s, NPD 74d 34' 2" (1830.0).
pB; R; bM; 60".
Pretty bright; round; brighter toward the middle; 60" [diameter].

Sweep 422 (April 28, 1832)
RA 12h 26m 52.0s, NPD 74d 33' 40" (1830.0).
B; R; pslbM; 60".
Bright; round; pretty suddenly little brighter toward the middle; 60" [diameter].

JH's M91 candidate, h 1367:
h 1367 = M.91 ??
Sweep 243 (March 24, 1830)
RA 12h 29m 0s +/-, NPD 75d 17' +/- (1830.0).
A bright * 9m, and 2 or 3 smaller; close by the B star and sp it, is a small well defined body which may be a close double star, and np is also a F neb. The place set down is that of Messier's 91st neb, but I do not think this can be that object, whose existance even seems questionable.
A bright star of 9th magnitude, and 2 or 3 smaller; close by the bright star and south preceding [SW] of it, is a small well defined body which may be a close double star, and north preceding [NW] is also a faint nebula. The place set down is that of Messier's 91st nebula, but I do not think this can be that object, whose existance even seems questionable.

Dreyer states that h 1367 doesn't exist, and identified it with h 1362 = H III.602 (NGC 4571):
h 1362 = H III.602
Sweep 24 (April 3, 1826)
RA 12h 28m 28.4s, NPD 74d 48' 12" (1830.0).
vF; pL; E; vgbM; attached like a tail to a star (place that of the *). (See fig. 66)
Very faint; pretty large; extended; very gradually brighter toward the middle; attached like a tail to a star (place that of the star). (See fig. 66)

Smyth

[from the description of M88]
[has its erroneous position from Messier in a chart]
.. "in the immediate nebulous neighborhood" of 88 Messier ..

John Herschel, General Catalogue: GC 3093.

GC 3093 = h 1345 = H II.120.
RA 12h 28m 22.7s, NPD 74d 43' 45.7" (1860.0). [Right Ascension and North Polar Distance]
B; L; lE; lbM. 5 observations by W. & J. Herschel.
Bright; large; little extended; little brighter toward the middle.

JH's M91 candidate, GC 3120 (h 1367):
GC 3120 = h 1367; M91??.
RA 12h 30m 30.8s, NPD 75d 26' 55.8" (1860.0).
np this place is a F neb; not M.91, whose existence ?. 1 observation by W. & J. Herschel.
North preceding this place is a faint nebula; this is not M.91, whose existence is doubtful.

Dreyer states that GC 3120 = h1367 doesn't exist, and identified it with GC 3113 = h 1362 = H III.602 (NGC 4571):
GC 3113 = h 1362 = H III.602.
RA 12h 29m 59.1s, NPD 74d 58' 8.1" (1860.0).
vF; L; E; vgbM; cB * att. 2 observations by W. & J. Herschel.
Very faint; large; extended; very gradually brighter toward the middle; considerably bright star attached.
Remark: Figure in P.T. 33 [JH 1833], plate vi, fig. 66.

Dreyer: NGC 4548.

NGC 4548 = GC 3093 = h 1345 = H II.120.
RA 12h 28m 23s, NPD 74d 43.8' (1860.0). [Right Ascension and North Polar Distance]
B, L, lE, lbM; = M91
Bright, large, little extended, little brighter toward the middle.

JH's M91 candidate, NGC 4571:
NGC 4571 = GC 3113 = h 1367 = H III.602; M 91??, d'A.
RA 12h 29m 50s, NPD 75d 1.7' (1860.0).
vF, L, E, vgbM, *9 nf nr; = IC 3588.
Very faint, large, extended, very gradually brighter toward the middle, a star of 9th magnitude is nearby north following [NE].
Remark: 4571. M91 (12h 30m 30s, 75d 30', 1781 Mar. 18, neb without stars) must have been a comet. h [J.H.] found np [north preceding, NW] the place a F [faint] neb[ula] (which he calls h 1367), the place of which he did not determine. It was doubtless [H] III.602 = h 1362, which he had not seen in the same sweep.
Figure in P.T. 33 [JH 1833], plate XIV, fig. 66.

William C. Williams: Identification of M91

[From a letter by William C. Williams to the Editor, Sky & Telescope, December 1969, p. 369]
"It can be simply demonstrated that the lost Messier object M91 is very probably the galaxy NGC 4548, at right ascension 12h 32m.9, declination +14d 46' (1950 coordinates).
As Owen Gingerich pointed out in his article "The Missing Messier Objects" (Sky and Telescope, October, 1960, page 196), M91 was the last of a group of eight nebulae observed by Charles Messier on the night of March 18, 1781. The French astronomer described it as a "nebula without stars" and fainter than M90. He gave its position as 12h 26m 28s, +14d 57' 06"; precessed to 1950, this is 12h 35m.0, +14d 02'. In what follows, 1950 coordinates are used.
My solution of the puzzle assumes that Messier determined the position of NGC 4548 by measuring its right ascension and declination relative to those of the nearby galaxy M89 (since there are no suitable reference stars in the vicinity):

      NGC 4548      12h 32m.9   +14d 46'
      M89           12h 33m.1   +12d 50'
      difference        -0m.2    +1d 56'
  

It is further assumed that, in calculating the coordinates of the new object, by mistake he applied the observed differences to M58, a 9th-magnitude galaxy Messier had recorded two years earlier:

      M58           12h 35m.1   +12d 05'
      difference        -0m.2    +1d 56'
      "M 91"        12h 34m.9   +14d 01'
  

It reproduces the Messier position to 0m.1 in right ascension and 1' in declination.

The Skalnate Pleso Atlas Catalogue gives the visual magnitudes of NGC 4548 and M90 as 10.8 and 10.0, respectively. This checks with Messier's statement that M91 was the fainter of the two. The same source gives the size of NGC 4548 as 3.7 by 3.2 minutes of arc."