Nicolas-Louis de La Caille, James Dunlop and John Herschel: an analysis of the first three catalogues of southern star clusters and nebulae

Cozens, Glendyn John (2008) Nicolas-Louis de La Caille, James Dunlop and John Herschel: an analysis of the first three catalogues of southern star clusters and nebulae. PhD thesis, James Cook University.

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"If men like [John] Herschel are to spend the best years of their lives in recording for the benefit of a remote posterity the actual state of the heavens…what a galling discovery to find amongst their own contemporaries men [James Dunlop] who … from carelessness and culpable apathy hand down to posterity a mass of errors …[so] that four hundred objects out of six hundred could not be identified in any manner … with a telescope seven times more powerful than that stated to have been used!"

The denigration of James Dunlop and his catalogue of 629 southern nebulae and clusters produced in 1826 originated with John Herschel and was continued by others of his day. Was this criticism justified? Was James Dunlop guilty of "carelessness and culpable apathy"? Were there "four hundred objects out of six hundred" which could not be identified, and if so, was there an explanation for this large shortfall?

This question led to a search within Dunlop's 1826 catalogue to rediscover, if possible, some of the missing objects and to reinstate Dunlop, if justified, as a bona fide astronomer. In doing this, Dunlop's personal background, education and experience became relevant, as did a comparison with the catalogue of 42 southern nebulae and clusters produced by Nicolas-Louis de La Caille in 1751-2, and the 1834-8 catalogue of 1708 southern nebulae and clusters by John Herschel, who found the Dunlop catalogue so galling.

To place the three southern catalogues in their historical context, a brief overview of these and the first three northern catalogues was made. Biographical information, descriptions of their equipment and comments on their observing techniques were included, where obtainable, for each of the authors of the three southern catalogues.

However the main objective of this thesis was to determine which of the 629 objects in the Dunlop catalogue exist and then using these objects in a revised Dunlop catalogue, to statistically analyse and compare it with the content of the Lacaille and Herschel catalogues. In order to identify and compare the catalogues, positions given for an object by each astronomer were precessed to J2000.0 coordinates. These modern positions for an object could then be plotted onto modern photographic star atlases and digital images of the sky, to determine the accuracy of the original positions.

Analysis of the three non-stellar catalogues included the determination of the radial distance of each object from its "correct" position and diagrams of both difference in Right Ascension and difference in Declination against Right Ascension and Declination, in order to identify any trends. Each catalogue contained some copy or printing errors, but these were omitted from the statistical calculations performed. The results for the three catalogues, from the astrometric perspective, showed that the Herschel catalogue contained the most accurate positions, followed closely by the Lacaille catalogue with no obvious or systematic trends in their inaccuracies. In contrast, the Dunlop catalogue showed some clear trends in the positional inaccuracies which, regardless of mitigating circumstances, to some extent warranted John Herschel's criticism.

Finally an examination of the completeness of each catalogue was undertaken to determine the thoroughness of each astronomer. Firstly the effective aperture and theoretical magnitude limit for each telescope was calculated. Next the non-stellar objects were grouped into five types, open clusters, globular clusters, diffuse nebulae, planetary nebulae and galaxies, and a single working magnitude limit was found for each catalogue. A number of indicators were used to determine the working magnitude limit.

The number of faint objects of each type which were seen, and the number of bright objects which were missed by the three astronomers, was assessed. In both the Dunlop and Herschel catalogues galaxies gave the best indicator of the working magnitude limit. Globular clusters provided the best working magnitude limit for Lacaille.

In answer to the question, 'Was the Dunlop catalogue as bad as John Herschel claimed?' the reply must surely be that although there are definite problems within the catalogue, chiefly missing objects and positional inaccuracies, generally this catalogue achieved much of what Dunlop intended, that is, a comprehensive list of bright nebulae and clusters in the southern sky. Although partially justified, John Herschel and others have not granted to James Dunlop the recognition he deserves.

Item ID: 24051
Item Type: Thesis (PhD)
Keywords: astronomical catalogs; astronomical catalogues; Coelum Australe Stelliferum; deep-sky objects; diffuse nebulae; galaxies; globular clusters; history of astronomy; James Dunlop; John Herschel; Nicolas Louis de Lacaille; non-stellar objects; observational astronomy; open clusters; optical telescopes; Parramatta Observatory; planetary nebulae; refracting telescopes; Sir Thomas Brisbane; southern nebulae; Southern sky; southern star catalogue; southern star clusters; Sydney Observatory; telescopic double stars
Date Deposited: 03 Dec 2012 06:29
FoR Codes: 02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020104 Galactic Astronomy @ 33%
21 HISTORY AND ARCHAEOLOGY > 2103 Historical Studies > 210303 Australian History (excl Aboriginal and Torres Strait Islander History) @ 33%
02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020103 Cosmology and Extragalactic Astronomy @ 34%
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