Sky and Telescope - September 2015 - 38

made chart. We know Messier created his own charts of
comet paths, as did other astronomers of the time.
What I think happened is that Méchain or Messier
used a chart on which one of the circles of right ascension was mislabeled by one hour. NGC 5866 and M101
diﬀer in right ascension by almost exactly an hour, and
in declination only slightly. A one-hour mistake in the
right direction would make M102 seem to be a slightly
imprecise duplicate observation of M101.
The most likely scenario, in my view, is that Méchain
saw NGC 5866 but did not measure its position, only
noting that it was between Omicron and Theta and close
to a sixth-magnitude star. Later, he or Messier found this
star ﬁeld on a chart whose 15-hour circle was erroneously labeled 14, read oﬀ the rough coordinates, and
noticed that these were close to (just a little more than 1°°
from) the coordinates of M101. Because this was not at
all a precise measurement, Méchain would conclude that
it was the same object.
A less likely alternative is that M102 was plotted correctly and M101 was plotted on a separate chart whose
14-hour circle was labeled 15. This, likewise, would make
M101 and M102 come out with coordinates only a little
more than a degree apart.
There remains Fact (4), Messier's handwritten
position, which matches nothing at all. One possible
explanation is O'Meara's, already mentioned - that a
position measurement was "ﬂipped" in right ascension
relative to Theta Boötis. Another, oﬀered by Frommert,
is that NGC 5866's coordinates were misread due to a
5-degree (20-minute) error in labeling or reading right
ascension on a map.
I think what Messier really did was much simpler.
If Méchain never gave him a measurement for M102 -
only a description saying it was between Theta and Iota
- then all Messier could do was to look at his map and
select a point midway between the two stars, and write
down its position using very round numbers, choosing
a right ascension circle marked on his map (14h 40m)
and going up to what looked like the right declination
(about 56°). That would at least give him something to

PARIS OBSERVATORY LIBRARY (2)

Identified at Last?

PAIR OF STARS Left: Charles Messier started publishing his famous astronomical catalog about a year after this
portrait was painted circa 1770, when he was about 40 years
old. Right: Pierre Méchain, who was 14 years younger than
Messier, is depicted here in later life.

compare to other people's reports and to use in planning his own observations.

A Matter of Choice
So I think it's probable, though not certain, that
Méchain really saw NGC 5866, complete with the
adjacent 6th- or rather 7th-magnitude star, and Messier
may have seen it later when conﬁrming the observation.
Méchain's later impression that M102 was a duplicate of
M101 was a mistake.
We will never have complete certainty, but I would
rather add NGC 5866 to the Messier list as M102, with
some remaining doubt, than leave it out. We accept
other Messier objects that are at least as uncertain, such
as M48 and M91. You are free to make up your own
mind, but as for me - I ﬁnished observing the American Messier list almost 30 years ago - now I need to
observe one more object. ✦
Michael Covington is the author of Astrophotography for the
Amateur, Digital SLR Astrophotography, and other books.
By day he is a computational linguist retired from the Institute for Artiﬁcial Intelligence at the University of Georgia.
He welcomes mail at astro@covingtoninnovations.com.

F U R THE R RE AD IN G FO R THE C U R I O U S
"Messier 102"
Hartmut Frommert
messier.seds.org/m/
m102d. html
"The Missing
Messier Objects"
Owen Gingerich
Sky and Telescope,
October 1960, pp. 196-199.

"Catalogues of Nebulous
Objects in the Eighteenth
Century"
Helen Sawyer Hogg
Journal of the Royal
Astronomical Society
of Canada, vol. 41 (1947),
pp. 265-273.

September 2015 sky & telescope

The Messier Album
John H. Mallas &
Evered Kreimer
Sky Publishing Corp., 1978.
"M102: Mystery Solved"
Stephen J. O'Meara
Sky & Telescope,
March 2005, pp. 78-79.

Atlas of the
Messier Objects
Ronald Stoyan,
Stefan Binnewies,
Susanne Friedrich, and
Klaus-Peter Schroeder
Cambridge University Press,
2008.

http://messier.seds.org/m/m102d.html http://messier.seds.org/m/m102d.html

Sky and Telescope - September 2015

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