Sky and Telescope - September 2015 - 35

pean observers, however, identify M102 with the smaller
galaxy NGC 5866, and here I shall argue that they are
probably right. My arguments parallel those of amateur
astronomer Hartmut Frommert, who has analyzed the
situation in detail, but I can oﬀer some simpliﬁcation.

PICK ONE Is the nonexistent "M102" actually NGC
5866 (below) and not M101 (left) as long thought? In
deep images at the same scale they appear utterly diﬀerent, but their bright cores look similar in small scopes.

M101: MASIL IMAGING TEAM; NGC 5866: MICHAEL COVINGTON

Mistaken Identity

tale grows complex. Since Messier's time the Messier
catalog has developed and matured, in several stages. It
originally gave incorrect positions for some objects, and
it failed to number the last few objects that Messier had
noted. The catalog took its more-or-less accepted present
form in the mid-20th century, when astro-historians
added M104 through M110 and addressed the questionable identities of M47, M48, M91, and M102.
Most American reference books treat M102 as a
duplicate observation of the galaxy M101, because M102's
"discoverer," Messier's contemporary Pierre Méchain,
later wrote that he had made this mistake. Many Euro-

NGC 5866, as the Cambridge Atlas of the Messier Objects
notes, is fainter than M101 (magnitude 9.9 vs. 7.7) but has
a much greater average surface brightness (20.8 versus
23.7 magnitudes per square arcsecond). Both are visible
in 10×50 binoculars under excellent conditions, and both
are easier to see than some other Messier objects, such
as M78 and M91. Thus, both should have been within
reach of Méchain's and Messier's modest instruments.
In large telescopes or long-exposure photographs they
look quite diﬀerent: M101 is a huge, nearby spiral seen
face-on, and NGC 5866 is a more distant galaxy appearing edge-on. However, their bright innermost areas are
all that show in a small scope, and these can look similar
as barely visible smudges. As amateur astronomer
Richard Jakiel told me, NGC 5866 resembles the central
part of M101 when a hazy sky or inadequate telescope
hides the latter's big disk of spiral arms. My ﬁrst casual
attempt to see NGC 5866 was successful, with an 8-inch
telescope, even though I was in town and the full Moon
was in the sky. So I don't think it's too faint for Méchain
to have seen and recorded.

Facing the Facts
One must account for four facts: (1) Messier's description of M102; (2) an obvious typographical error in his
description (the Greek letter omicron in place of theta);
(3) Méchain's retraction, asserting that M102 was just
M101; and (4) Messier's handwritten position for M102 in
his personal copy of his catalog.
By themselves, (1) and (2) point clearly to NGC 5866;
(3) does not, but I shall argue that it should not be taken
at face value; and (4) is a puzzle for either theory, to
which I shall propose a simple solution.
Start with Fact (1). In the ﬁ nal installment of his
catalog, published in the French astronomical almanac
Connoissance des Temps for 1784 (issued in 1781), Messier
adds, at the end of his list, three objects that his colleague Méchain had found. Messier gives coordinates
for only one of them. Here are his descriptions, in the
original French; my translations follow:
Par M. Méchain, que M. Messier n'a pas encore vue.
By Mr. Méchain, [objects] that Mr. Messier has not yet seen:

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