102. Nebula between the stars omicron Bootis and iota Draconis: it is very faint, near it is a star of 6th magnitude Messier in Connaissance des Temps for 1784, p. 267Charles Messier compiled his "Catalogue of Nebulae and Star Clusters" during the years 1758 to 1781 (or 1782 if one counts the last additions by his colleague Pierre Méchain, which are contained in most modern versions of the catalog). Contrary to prior and contemporary observers who had a large number of errors (nonexistent objects) in their lists, the entries of his catalog correspond to actual astronomical objects in all cases, perhaps with one exception, his entry number 102 (there are positional errors for 3 other objects, M47, M48, and M91, which could be figured out by the time).
For this object (M102), Messier gives above description together with M101 and M103 as communicated to him "through M. Méchain, which M. Messier has not yet observed." He gives no position for M102 (and M103) in the published version of the catalog (although he has added positions from hand in his personal copy, see below).
About two years after the entry was made and published, Pierre Méchain retracted his discovery and claimed that the observation was an error, a duplicate observation of M101, and a star chart error of Messier. In a letter to Bernoulli, dated May 6, 1783, he wrote:
On page 267 of the "Connaissance des Temps for 1784" M. Messier lists under No. 102 a nebula which I have discovered between omicron Bootis and iota Draconis: this is nothing but an error. This nebula is the same as the preceding No. 101. In the list of my nebulous stars communicated to him M. Messier was confused due to an error in the sky-chart.This reference got longly forgotten and was only recovered by Helen Sawyer in 1947/48; based on this letter she also added the objects M105 to M107 to the modern Messier Catalog . Kenneth Glyn Jones , and even more definite, Owen Gingerich in his contribution to Sky & Telescope  (reprinted in Mallas/Kreimer's Messier Album ) regard this issue as solved, in the sense that M102 is a duplication of M101. One could easily join this conclusion, as the discoverer himself admitted a fault and retracted.
However, there remain some doubts and arguments which still allow for other possibilities, as also Kenneth Glyn Jones admits. First of all, both Méchain and Messier were very careful observers, indicated by the fact that M102 is the only possible "non-object" left in the catalog, and only for 3 further objects there were positional errors. Also, as the descriptions for M101 reads:
it appears not necessarily obvious that this is the same object as M102, with the description given above. In addition, Méchain's "retraction" occured over 2 years after the "discovery", giving room for speculations anyway, and it was never published in the Connaissance des Temps, where Messier's catalog and supplements were first printed, although both astronomers became associate editors of this periodic publication: Messier in 1785 and Méchain in 1786.
March 27, 1781. 101. 13h 43m 28s, +55deg 24' 25". Diam. 7'. Nebula without stars, very obscure and pretty large, between 6' and 7' in diameter, between the left hand of Bootes and the tail of Ursa Major. Difficult to distinguish when graticule lit.
Moreover, Messier had added by hand a position for M102 to his personal copy of the catalog, which both Owen Gingerich and Kenneth Glyn Jones have claimed to be erroneous "because there is no obvious object". As Messier was certainly a careful observer, it is probable that he has seen 'something', but maybe he did a reduction error again. The question arises if he found another object, either one of the candidates discussed below, a comet, or even some completely other one ? We will come back to this question later, as it suggests a very interesting possibility.
At last, there are at least two candidates, the more probable being NGC 5866, near the position Méchain describes; wouldn't one know of his letter mentioned above, today's astronomers would most probably believe that this was the observed object !
To step the way down, note that omicron Bootis is about 40 degrees away and south of iota Draconis, thus (at least) one of them must be a mistype; Admiral Smyth in his 'Bedford' Catalogue suggests the obvious possibility that it must read 'theta' instead of 'omicron' Bootis. The other possibility would be, according to J.L.E. Dreyer in Notes and Corrections to the NGC, that iota Draconis was mistaken for iota Serpentis; then M102 would be situated near the position of the faint galaxy NGC 5928, at RA 15h23.9m, Dec +18d15' (1950).
However, Dreyer's proposition of NGC 5928 can be waived with great certainty for the obvious reason that it is only a 14th mag galaxy, according to the RNGC , so that both Méchain and Messier could hardly have seen it with their instrumentation, even under exceptionally good conditions. Thus we can also exclude Dreyer's assumption that iota Draconis was a mistake for iota Serpentis, since there's no sufficiently bright object in that region of the sky, and are left with the possibility proposed by Smyth, that omicron Bootis was mistaken for theta Bootis.
Between those stars, about 3 degrees SW of iota Draconis, is a small group of galaxies, the brighter of which could be viewed as candidates for M102:
Another fact makes NGC 5866 a good candidate for
Imagine you want to find NGC 5866 with a telescope, how do you procede ? I would look for the stars iota Draconis and theta Bootis and then locate the 5.25 mag star GC 20332 (= HD 134190, SAO 29407) which is little more than 1 degree south and almost exactly at the same right ascension. This star is one of 5 in the rectangular region between RA/Dec limits given by the two stars and listed in Becvar's catalog of stars brighter than 6.25:
RA (1950.0) Dec (1950) mag 23 theta Boo 14h23m48.8s +52d04'52" 4.06 GC 19627 30 56.9 55 37 03 5.99 GC 19666 32 45.2 57 17 12 6.25 GC 19742 36 40.0 54 14 19 5.52 * GC 20332 15 04 59.9 54 44 53 5.21 GC 20641 18 36.8 52 08 16 5.52 12 iota Dra 15 23 48.8 59 08 26 3.47(NGC 5866 is at RA 15h05.1m, Dec +55d57' for 1950.0). A misestimate of a 5.25 as 6th mag star would eventually be not too far off, so that the `6th mag star' in Messier's description might be GC 20332. Then the description matches well with that visually 10th mag lenticular galaxy, as it appears probable that Méchain perhaps wanted to describe a route to his newly `discovered' object. Another, though perhaps less probable, possibility is that the star mentioned is the 6.8 mag star HD 133666 (SAO 29393) lying only 0.4 degrees NW of NGC 5866. The good match of Méchain's description with this galaxy suggests that this may have been the object he had seen in his discovery observation.
However, as Méchain has disowned the discovery, one may keep the position that due to his claim, Méchain's discovery was spurious and eventually a duplicate observation of M101 as he claimed. As also Don Machholz admits, it may well be that he was correct with this statement, then there remains only the puzzle of Messier's handwritten position.
For me, the author of this article, some light came into this mystery when Dr. Don Greeley communicated to me the handwritten positions Messier had added to his personal copy of the catalog printed in the Connaissance des Temps for 1784. He points out:
The positions in Messier's catalog were very faded and difficult to interpret. It was necesary to make a copy of that page so dark that the printing on the page behind it showed through. I made a slide of the page and when projected on a flat white wall showed that M102 was "14.40" in RA and "56." in Dec. M103 was much harder to see but is probably "1.20" in RA and "61." in Dec. Now they must be corrected for precesion for modern charts.The acuracy of Messier's values is probably indicated by the rough decimals, but for the following considerations I suppose them more acurate as they really are. When precessed to modern times, there is little surprise that the position for M103 becomes RA = 1:34.6, Dec = +62.1 (2000.0), close (little more than 1 degree north and very little east) to the correct position of this cluster, which is RA = 1:33.1, Dec = +60.7. Messier's position of M102 becomes
In accordance to the claims of Owen Gingerich and Kenneth Glyn Jones, there is actually no striking object close to this position in the sky. It is however interesting that the position lies between the stars iota Draconis and theta Bootis (so that at least it is apparently validated that the "omicron" in the description is a typo). On a closer look, one also fails with a sign error in a positional difference, as it had occured for M47, or a parallel shift due to taking a wrong reference star or object as for M91. But, and that is apparently most interesting, the position is almost exactly at the correct declination for NGC 5866 and M101, and it is almost exactly 5 degrees (20 min) west (preceding) of NGC 5866 in right ascension (is is also roughly 10 degrees east of M101, but much less acurately; the 2000.0 position of NGC 5866 is RA 15:06.5, Dec +55.7, while that of M101 is RA 14:03.2, Dec +54.3). The particular interest connected with this arises from the fact that another missing object, M48, was also measured nearly exactly 5 degrees false (in that case in declination, though). A look in the sources suggests that Messier has normally used sky charts with grids of lines every 5 degree, as e.g. his chart showing the path of the comet of 1779. Then a deviation of exactly 5 degrees may have several simple reasons: A wrongly labelled chart, an erroneous look on the neighboring label, a wrong count to an un-numbered tick, etc. One should perhaps also keep in mind that Méchain, in his letter, speculated that "M. Messier was confused due to an error in the sky-chart"!
Therefore, in the opinion of the present author, it appears probable that Charles Messier has observed NGC 5866 when he measured the position of M102 (which he could probably locate without much difficulty because of Méchain's acurate description), but due to some reductional error, plotted it exactly 5 degrees west (preceding) of its correct position.
The object that really deserves the designation "Messier 102" should be identical to one of the two observed by Méchain and Messier, may they be identical or not. As nobody is still alive who has witnessed them during their observation and recording, we can currently not reconstruct what they actually observed. Méchain's description gives good evidence that the object M102 could be NGC 5866, which most probably everybody would believe if he had not retracted the discovery in the letter mentioned, or if this letter had stayed forgotten. It may now depend on taste to speculate which was erroneous: The observation or the letter. Moreover, Messier has probably observed NGC 5866 and taken it for M102, but again made an error in data reduction. Once more, it is a question of taste if these facts entitle the lenticular galaxy NGC 5866 to bear the designation "M102".At least, observers who want to go for sure that they observed all Messier objects should thus turn their telescopes to aim NGC 5866. They will be rewarded by quite an easy, beautiful object.
Messier's list comprises one hundred and three objects. For all of them, positions are given, except for 102 and 103. For these only the descriptions are given, as follows:Shapley and Davis remark that "The visual observations of these neighboring nebulae by the Herschels show N.G.C. 5866 much the brighter, and its identity with M 102 can be accepted as quite probable."
102. Nebuleuse entre les etoiles Omicron du Bouvier and [sic!] Jota du Dragon; elle est tres-faible; pres d'elle est une etoile de la sixieme grandeur [Nebula betrween the stars Omicron Bootis and Jota Draconis; it is very faint; near it is a star of sixth magnitude].
103. Amas d'etoiles entre Epsilon et Delta de la jambe de Cassiopeia [Cluster of stars between Epsilon and Delta Cassiopeiae].
By a star chart, or the sky, you will see that, taken as it stands, no object could be well selected for M102, since Omicron Bootis is too far from Jota Draconis. If, however, Omicron is a misprint for Theta, it becomes intelligible, and M102 is perhaps N.G.C. 5866, altho in Norton's Atlas it is apparently identified as N.G.C. 5979. On our photographs, however, N.G.C. 5866 appears to be the brightest object in this region.
M 102 = NGC 5866, Type S [galaxy], [No other data], Notes: `= M 101'Perhaps this reflects the situation :-)
``Messier's list contains as its object 102 a galaxy [it's the first time I hear that Messier did know that - hf] near the position of NGC 5866. However, his description points to a duplication of M101 [I cannot verify this, see both descriptions given above; my impression is more that NGC 5866 might match the description - hf]. Did Messier do a mistake of 1 hour in right ascension [How, as he gave no measured position :-) - hf] ? Therefore, the designation M102 is nonunique.''
`A small but brightish nebula on the belly of Draco with four small stars spreading across field N of it. Doubt as to wether this is the nebula discovered by Méchain in 1781 since Messier describes it as ``very faint'', situated between omicron Bootis and iota Draconis. If omicron Bootis should be theta Bootis, this is probably the object seen by Méchain and J.H.'s 1910, being the brightest nebula of five in that vicinity.'The problem is that John Herschel's number 1910 is NGC 5879, which is however not the brightest of the group; the brightest is NGC 5866.
 Kenneth Glyn Jones. Messier's Nebulae & Star Clusters. 2nd edition, Cambridge University Press, 1991. Practical Astronomy Handbooks Vol. 2. 1st edition 1968, Faber.  John H. Mallas and Evered Kreimer. The Messier Album. 1st edition. Sky Publishing Corporation, 1978 (Second revised printing 1979).  Owen Gingerich. The Missing Messier Objects. Sky & Telescope, Vol 20, October 1960.  Jack W. Sulentic and William G. Tifft. The Revised New General Catalogue of Nonstellar Astronomical Objects. The University of Arizona Press, 1973.  Don Machholz. Messier Marathon Observer's Guide -- Handbook and Atlas. MakeWood Products, P.O.Box 1716, Colfax, CA 95713, USA. 1994  Helen B. Sawyer, 1948. Méchain's additions to Messier's catalogue. Astronomical Journal, Vol. 53, p. 118  Camille Flammarion, 1917. Nébuleuses et Amas d'Étoiles de Messier. L'Astronomie. Revue de la Societé Astronomique de France, November 1917. P. 385-400, here p. 385-386. Also see Flammarion's table of Messier's catalogue online.
Last Modification: 20 Feb 2000, 18:45 MET