The Caldwell Objects

the most visually pleasing objects in the heavens.
His was a catalog of "comet masquer-aders," as
the late comet discoverer Leslie Peltier called
them. Messier started his list in 1758, after he
encountered a stationary fuzzy patch (Ml) near
Zeta (ζ) Tauri while looking for the Comet of
1758. As comet hunter David Levy writes in the
first chapter of Deep-Sky Companions: The Messier
Objects, "Realizing he had been fooled by the
sky's version of a practical joke, Messier began to
build a catalogue of what he called these
'embarrassing objects.'"
Messier's first catalog, published in 1774,
contained not 110 but 45 "embarrassing" objects.
Actually, his original list, compiled in 1765, had
only 41 objects. "Before submitting the list for
publication," Levy explains, "he decided to round
it out with a few more objects." In order of their
inclusion, these are the two components of the
Orion Nebula (M42 and M43), the Beehive
Cluster (M44) in Cancer, and the Pleiades star
cluster (M45) in Taurus. But why not the Double
Cluster?
To understand Messier's intent, we have to
forget what we now know about the nature of the
objects in his list, forget how spectacular these
objects look in photographs and CCD images.
Instead, we have to journey back in time some
two and a half centuries and think like Messier,
who was a comet hunter, not a celestial tourist.
Like Messier, I have spent several years
hunting for comets. Unlike Messier, I have never
discovered one that was not already known. But
I have had the privilege to meet many
"embarrassing objects." Identifying them has
been relatively easy for me because most are
plotted on the star charts I use in the field.
Messier and his contemporaries did not have the
luxury of star charts with reams of nebulae
plotted on them. And nothing is more unproductive
to a comet hunter than sitting behind
450
an eyepiece, waiting to see if an uncharted object
will move; the agony increases as twilight
advances, or as the object nears the western
horizon. Understandably, then, Messier wanted
to "cage" these celestial pests, once and for all, by
cataloging their positions.
When Messier systematically began searching
for comets in 1758 competition was scarce.
"For about 15 years nearly all comet discoveries
were made by Messier," Gingerich writes in The
Messier Album, "so that he almost considered
them his own property." These comets were not
the dim and distant objects that (mostly
professional) astronomers are accustomed to
discovering today (mostly on films or CCD
images, well before amateurs have a chance to
spy the comets in their eyepieces). In Messier's
day all comets were swept up visually, and all
were bright and relatively near the Sun. Of the 15
comets discovered from 1758 to 1774 (when his
first catalog was published), 7 were discovered
by Messier (and he independently found one
other first discovered in 1759 by observers in
Lisbon); seven were discovered with the unaided
eye; and more than half were sighted within two
weeks of perihelion (each comet's closest
approach to the Sun). No comets were found
more than about 2 months before or after
perihelion, and the faintest ones shone near the
limit of naked-eye visibility at magnitude 6.5.
These statistics provide us with some valuable
insight into where Messier and other comet
hunters concentrated their searches.
It has long been known that to maximize
their chances of discovery, comet hunters search
a "haystack" of sky centered on the ecliptic, near
the position of sunset or sunrise. In the Guide to
Observing Comets (edited by Daniel W E. . Green
and published as a special issue of the
International Comet Quarterly), veteran comet
observer John Bortle explains how these searches
are conducted:
Deep-Sky Companions: The Caldwell Objects