Is NGC 5866 M102 or not? There are probably as many opinions as there are observers concerning this. I would like to challenge you to do what I have done. That is, I compared M101 with NGC 5866 and the original descriptions in Charles Messier’s catalog. This was a fun project to do and one that helped me make up my own mind on this debate.
The original descriptions in Messier’s catalog are different, just as you would expect. These objects were discovered by Pierre Mechain and sent to Messier for his verification. After the publication of Messier’s final list, M. Mechain wrote a letter to Bernolli in Berlin stating that he had made a mistake and that M101 and M102 were in fact duplicate observation’s of the same object. You would have thought that this would have settled the debate, wouldn’t you? But why did Mechain only send a letter to Berlin and not to the French publishers too? Personally I think that Mechain was an honest man and made a duplicate observation as he stated in the letter.
The problem comes from the fact that M. Messier wrote the positions for M102 and M103 on his personal printed copy of the catalog that had been published. This would make you think that he had found and observed these objects, correct? Well, I think that this is just what M. Messier did, but here is the rub. He made an error in the position of both M102 and M103. His position for M103 was off by 1 degree and his position for M102 was off by 5 degrees. Sadly this is similar to the 5 degree error Messier made in calculating the position of M48, except in the other direction. That proves to me that he was human and made mistakes in math just like I do.
Here are the original descriptions of M101 and M102:
M101-Nebula without star, very obscure and pretty large, 6 to 7 minutes in diameter, between the left hand of the Herdsman and the tail of the Great Bear. It can be barely distinguished when the wires are lit.
M102-Nebula between the star omicron of the Herdsman and iota of the Dragon: it is very faint; near it is a star of sixth magnitude.
I think that you too will agree that these are not the same object based on the descriptions alone. But then there is the problem of misidentifying theta Bootes with omicron Bootes. Was the star mislabeled on his star chart or did he misread his chart in the dark while making his notes? We will never know the answer to this question. You see NGC 5866 (M102) falls between iota Draco and theta Bootes not omicron Bootes.
When I made my comparison of M101 and M102, I think the description of M101 is pretty much spot on, the closest 6th magnitude star is about one and a half degrees to the west southwest. So it is a pretty large, faint object without star. On the other hand, when I observed NGC 5866 (M102) with my 4 inch refractor, I was surprised by how faint it appeared , and that there was a bright magnitude 7.5 star 10 minutes to the southwest and a 6th magnitude star 30 minutes to the northeast. It seems that NGC 5866 matches the description of M102, and since this is the brightest galaxy with a 6th magnitude star near by in the area, and since Charles Messier, by recording the position of the galaxy in his personal copy of his catalog, appears to have followed up on Mechain and found and recorded this galaxy; I will except NGC 5866 as M102. Sadly we will never know for sure which galaxy Messier meant to be M102 but for me, it matches NGC 5866 well enough. There you have it, read Messiers original descriptions and compare them for yourself. See what you think.