Caldwell catalogue

Lore & Background

Patrick Moore noted that the Messier catalogue, while widely used by amateur astronomers as a list of deep-sky objects for observation, was not originally compiled for that purpose. Messier's list was actually a compilation of known objects that might be confused with comets, and it excluded many of the sky's brightest deep-sky objects, such as the Hyades, the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 253). Additionally, because Messier compiled his list from observations in Paris, it did not include bright deep-sky objects visible in the Southern Hemisphere, such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae. Moore compiled a list of 109 objects to match the commonly accepted number of Messier objects (he excluded M110). Unlike the Messier catalogue, which is ordered roughly by discovery, the Caldwell catalogue is ordered by declination, with C1 being the most northerly and C109 the most southerly, although two objects (NGC 4244 and the Hyades) are listed out of sequence. Other errors in the original list have since been corrected: it incorrectly identified the S Norma Cluster (NGC 6087) as NGC 6067 and incorrectly labelled the Lambda Centauri Cluster (IC 2944) as the Gamma Centauri Cluster. The catalogue has generated controversy in the amateur astronomy community. Critics note that Moore did not discover any of the objects, which are often very well known and not 'neglected' as claimed. The presentation as a catalogue with distinct designations may cause confusion, as the 'C' designation is not commonly used. Some argue the list is arbitrary, omitting many easily viewable objects while including some not readily available to visual observers. Advocates, however, see it as a useful list of some of the brightest and best known non-Messier deep-sky objects, dismissing the controversy as fabricated by older amateurs unwilling to memorize new designations, despite every telescope database using the Caldwell IDs as the primary designation since the late 1990s. NASA/Hubble also lists the 109 objects by their Caldwell number.

Reader's Guide

The Caldwell catalogue's significance lies in its role as a practical extension of the Messier catalogue for amateur astronomers. By including bright deep-sky objects omitted from Messier's list—particularly those in the Southern Hemisphere—it provides a more comprehensive observing guide. Despite controversy over its arbitrary selection and the use of new designations, the catalogue has gained widespread adoption. Telescope databases have used Caldwell IDs as the primary designation since the late 1990s, and NASA/Hubble lists the 109 objects by their Caldwell number. The catalogue's legacy is thus one of broadening the accessible deep-sky observing list, though debates about its necessity and accuracy persist within the amateur community.

Did You Know?

• The Caldwell catalogue was compiled by Patrick Moore using his other surname, Caldwell, because 'M' was already used for the Messier catalogue.
• It contains 109 objects, matching the commonly accepted number of Messier objects, but excludes M110.
• Two objects in the catalogue—NGC 4244 and the Hyades—are listed out of the intended declination order.

Origins and the Gap in Messier's Legacy

Patrick Moore conceived the Caldwell catalogue to fill a gap he saw in the Messier catalogue, which amateur astronomers widely used as a guide to deep-sky observation. Moore pointed out that Messier's original list was never intended as an observing checklist; it was compiled to catalog objects that might be mistaken for comets. As a result, some of the sky's most brilliant deep-sky targets—the Hyades, the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 253)—were absent. Furthermore, because Messier worked from Paris, the catalogue overlooked luminous southern-sky objects such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae. To address these omissions, Moore assembled a list of exactly 109 star clusters, nebulae, and galaxies, matching the commonly accepted count of Messier objects (with M110 excluded). The finished catalogue was published in Sky & Telescope in December 1995, giving amateur observers a curated selection of bright, accessible targets that Messier's original work had left out.

Naming, Numbering, and Ordering

Because the letter M was already claimed by the Messier catalogue, Moore chose his other surname—Caldwell—to brand the new list. Each entry carries a C prefix followed by a number from 1 to 109, creating a compact designation system. Rather than following discovery order as Messier's entries do, the Caldwell catalogue is arranged by declination: C1 sits at the northernmost position in the sky, while C109 marks the southernmost. Two objects, NGC 4244 and the Hyades, break this strict sequence. The original publication also contained a couple of identification errors that were later corrected: the S Norma Cluster (NGC 6087) was mistakenly listed as NGC 6067, and the Lambda Centauri Cluster (IC 2944) was mislabelled as the Gamma Centauri Cluster. Despite these early slips, the declination-based ordering gave the catalogue a practical, sky-position logic that made it straightforward for observers to work through targets in a logical geographic progression across the celestial sphere.

Controversy Within the Amateur Community

The Caldwell catalogue has never been free of debate among amateur astronomers. Critics note that Moore did not discover any of the 109 objects; many are already widely recognized and hardly neglected, as Moore's framing implied. Others argue that presenting the list as a formal catalogue with its own C designations risks sowing confusion, since those identifiers were not part of the traditional observing vocabulary. A further complaint is that the catalogue was marketed as a natural extension of Messier, yet the selection feels arbitrary—some easily visible objects are left out while targets that are difficult for visual observation are included. Advocates, however, counter that the catalogue simply offers a handy compilation of the brightest and best-known non-Messier deep-sky objects. They dismiss the controversy as the product of older observers who were simply unwilling to learn a new set of designations, especially given how quickly the broader community embraced the system.

Adoption and Enduring Legacy

Despite the initial pushback, the Caldwell catalogue has become deeply embedded in the tools of modern amateur astronomy. Since the late 1990s, virtually every major telescope database has adopted the Caldwell identifiers as primary designations, meaning that a C number is now as instantly recognizable to a backyard observer as an M number. Even NASA's Hubble Space Telescope resources list all 109 objects by their Caldwell number, lending the catalogue an institutional weight that goes far beyond a single magazine publication. The catalogue's objects span a diverse range of deep-sky types—open and globular clusters, dark, diffuse, and planetary nebulae, supernova remnants, and galaxies—making it a versatile reference. Community resources such as the Caldwell Club, searchable online lists, and dedicated star charts further cement the catalogue's place in the observing culture, transforming what began as one astronomer's curated list into a shared standard for pointing a telescope at the night sky.