The year 1986 marked the bicentenary of the birth of Arago (Estagel 1786 - Paris 1853) and, on that occasion, the Bibliothèque de l’Observatoire de Paris installed a showcase in his honour for the general public visiting the Perrault building. The showcase contained an extract of Victor Hugo’s poem Promontorium somnii [1] in which he described a visit to François Arago in 1834. This latter, who had been living at the Observatoire since the beginning of the century, had just been nominated "Directeur des observations" ("Director of observations") by the Bureau des longitudes which had been responsible for the governance of the Observatoire since its creation in 1795 and had so far only nominated "directeurs délégués" ("acting directors").
In the long prose poem devoted to this visit, Victor Hugo recounted how he had climbed up to the Observatory platform and from there observed the Moon. Research carried out in 1986 led to the conclusion that the observation had been done with the Gambey equatorial, installed in one of the domes on the terrace, since the large dome and its refractor – called today the Arago Dome - did not yet exist.
The bicentenary of the birth of Victor Hugo once again led many to interrogate the Observatoire de Paris about which instrument Arago and Hugo had used on that evening of 1834. The Bibliothèque thus asked Mme. Suzanne Débarbat, astronomer and historian at the Observatoire de Paris, as well as Mr. Michel Combes, astronomer at the Observatoire de Paris and specialist in optical instrumentation, to look once again into the history of this event ; this they did, on the basis of two critical editions of the works of Victor Hugo, that of René Journet and Guy Robert ( Victor Hugo. ″Promontorium Somnii″ , Paris : les Belles lettres, 1961 (Annales littéraires de l’Université de Besançon, 42) and that directed by Jean Massin ( Victor Hugo, Œuvres complètes, t. 12 , Paris : Le Club Français du Livre, 1969) which leans heavily on the former.
The terrace in question is certainly the upper one - Hugo wrote that Arago led him up to the "platform", a term used also by Arago to designate this terrace ; two small domes had been installed on the North side : they are no longer used but are still there. The small one, to the West of the meridian, had iron arches ; it was covered by a white sheet in order to reflect as much as possible the light and heat of the Sun. The other dome housed the azimuth circle made by Reichenbach, the famous Munich instrument maker, which Laplace had donated to the Observatoire de Paris around 1810, for lunar observations. This instrument is now in the Grande Galerie of the Observatoire de Paris ; it led to improved orbital parameters for the terrestrial satellite, a difficult problem for astronomers.
At about this time, the Bureau des longitudes had embarked on the installation on the terrace of an equatorially mounted refractor made by the French instrument maker Gambey. In 1834 this refractor was being tested, and it would not have been available for observations before 1835. During the extended trial period, the refractor was first installed in the smaller of the two domes, after which the emplacements of the Gambey equatorial and the Reichenbach circle were switched.
During this time, Arago and the Bureau des longitudes projected the installation at the Observatoire de Paris of a long focal length refractor, similar to the one at the Dorpat Observatory (Tartu in Finland). It would have to be powerful enough to enable the observation of binary stars, which had been discovered only a few decades earlier. Its realization was entrusted to the French instrument makers Cauchoix and Lerebours. This latter did have, in effect, objective lenses of various diameters ; they were tested one by one at the Observatoire de Paris during the 1820s and 1830s. The objective lenses were mounted in a suitable tube, firstly in one of the buildings which, on the East side, was next to the tower of the main building, then probably, although the texts are not too clear on the subject, on the upper terrace at an unknown date.
Although three instruments were definitely on the terrace in 1834, they were not capable of the same observations. Victor Hugo claimed in effect to have observed through "une lunette qui grossissait quatre cents fois" ("a refractor which magnified four hundred times") and recounted that Arago claimed that it brought the Moon from a distance of 90000 to 225 leagues, the former figure corresponding to the distance between the Earth and its satellite [2]. The Gambey equatorial, which as we have seen was not yet quite operational, can be eliminated since the diameter and focal length of its objective lens are insufficient to provide a magnification of 400. The Reichenbach circle, with a quite similar refractor, can also be excluded for the same reasons.
The remaining possibilities are Lerebours’ various objective lenses, which had been tested at the Observatoire de Paris, and whose diameters were on the order of twenty centimeters, with focal lengths spanning 5 to 6 meters. Their magnification, using one and a half centimeter focal length eyepieces, could reach the value of 400 quoted by Victor Hugo.
Moreover, only a large magnification could have enabled the poet to describe certain details. Initially, he couldn’t see anything in this refractor [3], which is understandable since the refractor was pointing towards that part of the Moon which was not illuminated by the Sun, which he called « lueur cendrée » ("ashen glow") and which Lalande, like Arago, referred to as « ashen light ». After that, he was able to distinguish "qu’un segment obscur" ("only a dim segment") because of the large magnification.
Seeing " un lever de Soleil dans la Lune " ("a sunrise on the Moon", Hugo had a revelation, and saw a lunar landscape, listing successively the Messala volcano, the Promotorium somnii, mount Proclus, mount Cléomèdes and mount Petavius. As emphasized by the scientific editors of the Promontorium somnii, these names were quoted in the order of the nomenclature of Cassini’s map (1625-1712). This map, reproduced in the Magasin pittoresque of the 23rd of March 1833, was stuck to one of the folios of the manuscript of Hugo’s Promontorium somnii. An analysis of the text has moreover shown that before the version published in 1864, Hugo had put Selene in place of Messala and Aristarcus in place of Promontorium somnii.
The map in the Magasin pittoresque shows the lunar features in the order that they would appear in the field of view of a refractor, on condition that the whole Moon be visible, or, as was noted in the XVIIth century, in the order corresponding to the motion of the shadow of the Earth on the Moon during a total eclipse. Clearly Victor Hugo had checked his own observations or his memory thereof, against this nomenclature. In effect, in this latter, Aristarcus is on the East of the visible face of the Moon, and the other features are on the West : Victor Hugo could not possibly have seen at the same time mount Proclus or mount Petavius since he had observed the Moon in its crescent phase. Having realized how impossible this was, Hugo changed his text using the poetic Promontorium somnii, which became the title of his poem...
Furthermore, Hugo, describing the progressive appearance of these lunar features, repeats the explanation that Arago had presumably given him, and which, he added, is simple : they appear gradually because of the "mouvement propre de la Lune" ("the Moon’s own motion"). Now, the only lunar motion which can be discerned on a short time scale is that of the Earth, due to which the various features of the celestial body enter the field of view of the refractor one by one. This suggests that one of the experimental refractors of the Observatoire had been used, most probably mounted on the large Cauchoix mounting which had just been acquired for trials and whose orientation had to be adjusted manually. In effect, the Gambey equatorial would have enabled the refractor to compensate automatically for the motion of the Earth.
In view of the magnification indicated by Victor Hugo, the field of view of the refractor could hardly have exceeded two minutes of arc, which corresponds to 1/15th of the lunar diameter (roughly 1/2 degree). It is thus hard to believe that he could have observed successively all the features he claimed to have seen since they are situated on an extensive North-South arc. However, the motion of the Earth can only reveal features situated in an East-West arc. It is therefore clear, as thought the critic of the text in Massin’s edition, that to finish his text in December 1863, the poet had leaned heavily on the map published in the Magasin pittoresque . The same critic also noted that Hugo’s library had the 1863 edition of Fontenelle’s Discours sur la pluralité des mondes, which evokes the "Promontoire des songes". It is also possible, as maintained by Journet and Robert, that Victor Hugo had been struck by the powerful vision conjured up by the name Promontorium somnii that was on Cassini’s map.
Whatever the reason, be it Fontenelle’s influence or that of Cassini’s map, be it the author’s logic, or the memory of something seen 30 years earlier, the observation of the Moon had marked the poet deeply, and was for him one of his "lasting memories".
The Bibliothèque wishes to thank S. Débarbat and M. Combes for the research they have carried out, thanks to which the readers of Victor Hugo can now satisfy their curiosity.
Laurence Bobis, Conservateur
Directeur de la Bibliothèque
[1] See the text on Wikisource,
[2] Since a league corresponds to 3,898 km, 90 000 leagues represents about 360 000 km. The average distance between the Earth and the Moon is 380 000 km.
[3] This is not, of course, a telescope in the strict sense of the word, but an astronomical refractor : in effect, a refractor has an objective lens, while a telescope has a mirror to concentrate the light. Before Foucault and the invention of silvere