Sky and Telescope - September 2015 - 36

Identified at Last?
+60°

ι
DRACONIS
URSA MAJOR

5866

5866 oﬀ by
20m in RA

Missing M102
Epoch 1781

5866 oﬀ by
1h in RA

Messier's
handwritten
M102

ζ

81
80

83

Mizar
+55°

Star magnitudes

3
4
5
6
7
8

BOÖTES

θ

84

M101

M101 wrong way
from θ Boö

86

ι

82

CANES
VENATICI

κ

21
24
15h 00m 47

39

14h 30m

101.
R.A. 13h 43m 28s (208°° 52′ 4″) ["208" should read "205"].
Declination 55° 24′ 25″.
Nébuleuse sans étoile, très-obscure & fort large, de 6 à 7 minutes de
diamètre, entre la main gauche du Bouvier & la queue de la grande
Ourse. On a peine à la distinguer en éclairant les ﬁls.
Nebulosity without a star, very obscure and rather large, from
6 to 7 arcminutes in diameter, between the left hand of Boötes
and the tail of Ursa Major. Diﬃcult to distinguish when the
micrometer wires are illuminated.
102.
[No R.A. or declination given.]
Nébuleuse entre les étoiles ο du Bouvier & ι du Dragon: elle est trèsfoible; près d'elle est une étoile de la sixième grandeur.
Nebulosity between the stars ο Boötis and ι Draconis: it is very
faint; close to it is a sixth-magnitude star.
103.
[No R.A. or declination given.]
Amas d'étoiles entre ε & δ de la jambe de Cassiopée.
Cluster of stars between ε and δ of the leg of Cassiopeia.

Here M101 and M103 are easy enough to identify. But
the description of M102 is problematic.
Fact (2) is that in Messier's description of M102, "ο"
Boötis must be a misprint for "θ" Boötis. Historians agree
on this, because nothing else makes sense. In order for
M101 and M102 to be confusable they must be fairly close
36

September 2015 sky & telescope

13
14h 00m

+50°

η

24
13h 30m

together. Omicron (ο) Boötis is far away from M101 and
from the other reference star, Iota (ι) Draconis. Theta (θ)
Boötis, on the other hand, is a near neighbor of Iota Draconis, and there is no Greek-lettered star between them. It
would be natural to describe an object as "between" Iota
and Theta. It is not clear whether the misprint was on
Messier's star chart or arose from a mistake by Messier or
by the printers of the Connoissance des Temps.
Fact (3) is that, shortly after the catalog was published, Méchain retracted his observation of M102, stating that it was an error: a duplicate observation of M101.
The Berliner Astronomisches Jahrbuch for 1786, issued
in 1783, contains a letter from Méchain, printed in German, announcing additional discoveries and asserting
that M102 was a mistake. About M102 Méchain says the
following (as quoted by Helen Sawyer Hogg in a 1947
article in the Journal of the Royal Astronomical Society of
Canada; my translation follows):
Seite 267 der Connoissance des tems f. 1784 zeigt Herr Messier
unter No. 102 einen Nebelﬂeck an, den ich zwischen ο Bootes und
ι Drachen entdeckt habe; dies ist aber ein Fehler. Dieser Nebelﬂeck
ist mit dem vorhergehenden No. 101 ein und derselbe. Herr Messier
hat durch einen Fehler in den Himmelscharten veranlasst, denselben nach dem ihm mitgetheilten Verzeichnisse meiner Nebelsterne
verwechselt.
On page 267 of the Connoissance des tems [an older spelling]
for 1784, Mr. Messier shows under No. 102 a nebula that I am
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