NGC/IC Project Restoration Effort

===== NGC 4548 is almost surely M 91, though M 58 = NGC 4579 (which see) has also been suggested as M 91. Curiously, M 58 plays a role either way. The story -- as far as I've traced it -- was first presented by William C. Williams of Fort Worth, Texas in a letter to Sky and Telescope (December 1969 issue, page 12). Briefly, he suggests that Messier used M 89 = NGC 4552 as a reference object to find offsets to M 91 -- but then mistakenly applied those offsets to M 58 to come up with the position for M 91 that he lists. Going through this exercise, Williams shows that Messier's position for M 91 can be retrieved in just this way. Further, he suggests that applying the offset to the correct comparison object (M 89) leads us to the correct object. In other words, Messier misidentified his comparison object (if you've read more of these notes, you'll know that he was not the last to do so!) Let's repeat Williams's calculation with positions at the epoch of Messier's observation. I'm using the facsimile of Messier's paper from the Connaissance de temps for 1784 reproduced in "The Messier Album" by Mallas and Kreimer (Sky Publishing, 1978); this gives the date of observation as 18 March 1781. With the positions of the three relevant galaxies in hand for that equinox (precessed from my selected positions for the objects), here is what we have: Galaxy RA (B1781.21) Dec Notes M 91 12 26 28 +14 57.1 From Messier's table NGC 4548 12 24 24.6 +15 42 15 Precessed from J2000 NGC 4552 12 24 36.6 +13 45 51 Ditto; M 89 NGC 4579 12 26 40.4 +13 01 28 Ditto; M 58 The offsets of M 91 from NGC 4579 -- -12.4 seconds and +1d 55m 38s -- applied to NGC 4552 yeilds 12 24 24.2, +15 41 29 which is very close to the position for NGC 4548. The total V magnitude of NGC 4548, 10.2, is within range of whatever telescope Messier was using that night (a similar galaxy M 98 = NGC 4192 has V_T = 10.1; Messier measured its position less than a month later on 13 April 1781). So, this is at least a plausible scenario, and is probably the correct explanation for M 91 even though it requires several assumptions about Messier's observing techniques (see above, and more at NGC 4579). Owen Gingerich (in his introduction to "The Messier Album") suggests M 58 itself for M 91. That is almost certainly not correct; see NGC 4579 for that story. Finally, see NGC 4571 for more on M 91.

===== NGC 4548 17.5" (5/23/87): bright, moderately large, elongated 3:2 SW-NE, 3'x2', gradually increases to a bright core and a very small nucleus. M88 is 50' WSW.