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.
Since NGC 4548 is probably the long-lost M91, as a Messier Club member I would be pleased to see this fine spiral galaxy returned to the Messier catalogue.
WILLIAM C. WILLIAMS
Fort Worth, Tex. 76133