On the December 1995 cover of Sky & Telescope magazine, a small notice announced that Patrick Moore, the British amateur astronomer and broadcaster, had compiled a new list of one hundred and nine deep-sky objects intended as a complement to the Messier catalogue. He called it the Caldwell list, using the second part of his full name, Patrick Caldwell-Moore, to distinguish it from existing M-numbered objects.
The list was an attempt to address what Moore considered a peculiarity of Messier's catalogue. Messier, observing from Paris at a latitude of forty-eight degrees north, had naturally excluded objects too far south for him to see. He had also, for reasons of his own working method, omitted a number of bright northern objects that any modern amateur would consider obvious candidates for a survey.
The Caldwell list filled both gaps. It contains thirty-three objects south of the celestial equator, including some, like the Eta Carinae Nebula (C92) and Omega Centauri (C80), that southern-hemisphere observers had long known to be among the finest sights in the sky.
It also contains seventy-six northern objects that Messier had omitted, including the Double Cluster in Perseus (C14), the Veil Nebula complex (C33 and C34), and the Cocoon Nebula in Cygnus (C19).
The list was ordered, somewhat unusually, by declination rather than by right ascension. C1 is the most northerly object, the bright open cluster NGC 188 in Cepheus. C109 is the most southerly, the planetary nebula NGC 3195 in Chamaeleon.
Moore's rationale for the declination ordering was practical. An observer at, say, thirty-five degrees north could easily identify which objects were accessible from their latitude by simply reading down the list until the declination column dropped below their southern horizon.
The convenience was real. The list was adopted quickly, by amateur observers especially in the British Isles, where Moore's Sky at Night television program had been running since 1957 and where his name carried considerable weight.
It was adopted less quickly, and somewhat reluctantly, in the United States. American amateur publications spent the late 1990s debating whether the list was needed at all, given the availability of the Astronomical League's various observing programs, the Herschel 400, and the popular Deep-Sky Companions series by Stephen O'Meara.
By 2005, however, the Caldwell list had become a standard observing project. The Astronomical League introduced a Caldwell observing program. Sky & Telescope produced a Caldwell observing chart. Software packages including SkySafari and Stellarium added Caldwell labels.
Thirty years on, the list is still in use, and it is still slightly controversial. The controversy, such as it is, has three threads.
The first is the question of whether a list compiled by a popular broadcaster on his own initiative, without peer review or institutional sanction, ought to have the same standing as a list compiled by a working astronomer in the eighteenth century.
The objection is mostly aesthetic. Both Messier and Moore were, in their respective centuries, amateurs by training. Both compiled their lists for their own reasons. Both lists have proved useful.
The second thread is the question of duplication. Several Caldwell objects had been included in other widely used amateur lists before 1995, including the Herschel 400 and Burnham's Celestial Handbook. Critics argued that the Caldwell list added little.
The criticism overlooks the practical function of a numbered list. The Caldwell numbers, like the Messier numbers, are a shorthand. Saying "C14" is faster than saying "the Double Cluster in Perseus, also catalogued as NGC 869 and NGC 884." Speed of reference is, in observing programs, valuable.
The third thread is the question of inclusions and exclusions. Some observers have argued that the list is missing several bright northern objects that should have been included, including IC 405, the Flaming Star Nebula, and NGC 7000, the North America Nebula. Others have noted that several of the list's fainter southern objects are very difficult from any latitude north of about twenty degrees.
Moore himself, in interviews before his death in 2012, said that the list was a personal compilation and that he had made the inclusions and exclusions he thought useful. He invited other observers to make their own lists, and several have. None of the alternatives has displaced the Caldwell.
The list's persistence is, in part, a tribute to Moore's reputation as a communicator. It is also a function of the fact that the list works. It directs the moderately experienced amateur observer to a hundred and nine objects, most of which are worth seeing, in an order that is easy to follow from any latitude.
The Astronomical League's Caldwell observing program, by current count, has been completed by approximately twelve hundred observers since its introduction in 2001. The program requires the observation of at least seventy of the one hundred and nine objects, with sketches or photographs as documentation.
Completion certificates are signed by the program coordinator, an amateur astronomer named Frank Salatka, who has held the position since 2008. Salatka, when asked, said the rate of completion has held steady at roughly seventy certificates per year, with a slight increase in the past three years as more observers have used digital imaging to satisfy the documentation requirement.
The program is, in this respect, modest. Twelve hundred completions over a quarter of a century is not a mass movement. But it is a stable practice, and it represents the kind of slow amateur engagement with the catalogue that Moore presumably had in mind.
On the night of June 4, 2026, a member of the Salt Lake Astronomical Society named Hiroshi Tanaka completed his Caldwell program by observing C53, the Spindle Galaxy in Sextans, from a site at Stansbury Island in the Great Salt Lake. He had been working on the program for seven years. He sent in his observation forms, with sketches, two weeks later.
He received his certificate, signed by Salatka, in early August. He hung it on the wall of his observatory shed, next to his Messier certificate from 2019 and his Herschel 400 certificate from 2023. He has not yet decided what list to work through next.
There are a great many lists. The Astronomical League maintains over fifty observing programs. The amateur observer who wants to keep busy will not, in any plausible lifetime, run out of catalogues.
What the Caldwell list adds, to anyone willing to work through it, is a tour of the brightest deep-sky objects in the sky, ordered for convenience, with a single number for each. It is what Messier's list was, in 1781, with the addition of two centuries of accumulated observation and a southerly extension that Messier could not have made.
Moore would, one suspects, be content with this. He did not, in his lifetime, claim that the list was scientifically important. He claimed only that it was useful. Thirty years on, the claim still holds.
