Probably discovered by Charles Messier in March or April, 1781.
Independently rediscovered by William Herschel on April 12, 1789.
[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, see below - hf]
(manuscript:) Nebula near Gamma UMa, same right ascension a bit near this star and 1 deg .. more south. Discovered by M. Méchain on March 12, 1781.
[As found by Henk Bril in 2006, this position coincides well with NGC 3953, not NGC 3992. Therefore, we acknowledge this here by assigning that object the designation M109B. We prefer to keep also the name "M109," or if you prefer "M109A," for NGC 3992, first because this is widely used, and second to acknowledge Messier's probable or possible original discovery of this object. - hf]
[This position is not apparent in the author's xerox copy. If Messier actually observed NGC 3992, it would have been his personal, original discovery - hf]
Although Flammarion found Messier's notation of the position of the nebula near Gamma Ursae Majoris [..] he made no attempt to number it, and because Méchain did not give precise positions, Dr. Hogg omitted identifications of this and the other nebula near Beta Ursae Majoris. From my study of this region, the nebula near Beta is unambiguously NGC 3556, while an examination of the critical limiting magnitude of the catalogue indicates that the one near Gamma must be NGC 3992, a fact confirmed by the position Messier added to his personal copy. Thus, if the objects from M104 to M107 are included, it seems logical to me to number NGC 3556 and NGC 3992 as M108 and M109 respectively, especially as they are mentioned in the original catalogue.
[While exactly so for M108, the limiting magnitude of the Messier objects would allow for another identification: NGC 3953 of mag 10.1. A closer look at the position mentioned in Messier's manuscript notes suggests this latter identification more than that with NGC 3992.]
Last Modification: January 6, 2021