Charles Messier mentions, in his catalog, with the description of his object M97, two objects which were later added to his catalog, by Owen Gingerich, as M108 and M109. Both these objects had been reported to him by Pierre Méchain. In the printed version, Messier did not give a position for any of them. Gingerich identified M108 with NGC 3556, which is since commonly accepted, and M109 with NGC 3992. The latter identification was also widely accepted, until Henk Bril investigated an old atlas from 1795, by Jean Fortin, who had included nebulae positions from Méchain. Henk found, in 2006, that Méchain's nebula was probably not NGC 3992, but the very similar looking galaxy NGC 3953.
According to Owen Gingerich, Messier added a position by hand to his personal copy of the catalog which was identified by Owen Gingerich in 1953 as that of H IV.61 = NGC 3992, which is now called M109. This position is given in Sky and Telescope for September 1953, p. 289, as "11'43=54d.5," or RA=11h 43m, Dec=+54.5 deg (c. 1781.3). In the xerox copy of this catalog owned by the author, this position is almost missing: only suggestions of the two first digits are just readable. This position is strange: RA coincides almost exactly with NGC 3953 (and with Gamma UMa). Dec with NGC 3992.
If Messier actually should have seen NGC 3992, it would be his original discovery, as Méchain almost certainly saw NGC 3953.
To acknowledge this situation, we have assigned the designation "M109B" to Méchain's object, NGC 3953, and keep "M109" for NGC 3992, first because t is widely in use, and second to acknowledge Messier's possible or probable discovery.
Shown above are two images of M109 (left) and M109B (right), taken by by Martin Germano with the same optics (8-inch f/5 Newtonian and ST-8XME CCD camera). M109 was exposed 780 minutes March 9-11, 2011, and M109B for 720 minutes March 12/17/18/31 and April 1, 2011 from his observatory in Thousand Oaks, California. The similarity of the appearance of these two barred spiral galaxies is evident.
.. more to come soon ..
For now, please consult Henk Bril's discussion at: http://www.astrobril.nl/FortinOther.html#M109.
Last Modification: January 7, 2021