14 - The contribution of Eustachio Manfredi to studies on aberration of light.

A few years before the Marsili Observatory was set up in 1699, Jacques Cassini, son of Gian Domenico, had published a paper (119) examining observations regarding the variations in elevation of the Polar Star, recorded in a letter of 1698 sent by John Flamsteed to John Wallis (1616-1703) and included by this latter in the third volume of his works (120). On the basis of observations carried out from 1689 to 1697 and an analysis of Hooke’s observations of 1669, Flamsteed confirmed the seasonal variations in elevation of the Polar Star noted by Picard in 1671 (121). In discussing Flamsteed’s results and confirming their accuracy, Cassini did however show how Flamsteed had made a mistake in attributing the variations observed to a parallactic motion, not having taken account of the inclination of the terrestial equator on the ecliptic.

Since 1704 the Parisian astronomers had been conducting observations regarding possible variations in transit times along the meridian of two stars. In 1707 Giacomo Filippo Maraldi (1665-1729), nephew and assistant of Gian Domenico Cassini, communicated a series of observations to Manfredi concerning the differences in transit of Sirius and Arcturus on the meridian, carried out between 1704 and 1705. Manfredi immediately began similar observations in Bologna, from the Marsili Observatory (122).

Though of top quality, the Bolognese observations were carried out over too short a period of time - because of the interruption in work caused by closure of the Marsili Observatory - to be able to confirm the effect contained in the Parisian ones.

Manfredi had however begun a systematic study of the effects of parallactic motion, the results of which he collected in a pamphlet of 1722 that was to form the first part of the work entitled De annuis inerrantium stellarum aberrationibus, published in 1729.

Jacques Cassini had raised the question of parallaxes again in 1717 (123), having observed, from 1714 to 1715, the meridian elevation of Sirius with a telescope set along the meridian and noticing small variations in elevation during the different seasons of the year. His observations corresponded to a parallactic half amplitude movement of eight arcsecs and were in close agreement with the measurements contained in Huygens’s Cosmoteros.

A few years later, in 1725, Samuel Molyneux (1689-1728) began observations, in the Royal Observatory at Kew, of the star  Draconis with a vertical telescope, as Hooke had done more than fifty years before. At first, in 1726, Molyneux and James Bradley (1692-1762), who helped him with the observations, had convinced themselves that the declination of the star changed constantly, dropping continuously until in May it inverted its movement, reaching the highest point of its oscillation in September, with an excursion of 39 arcsecs.

The Bolognese astronomers received notice of these observations towards the end of 1726 from the copy of a letter William Derham (1657-1735) - English gentleman who was a lover of astronomy and author of two books entitled Physico-Theology and Astro-Theology - addressed to his brother Thomas, British Ambassador in Rome (124). In this letter Derham begged his brother to thank Marsili for having procured him a copy of the Italian translation of his Physico-Theology and then mentions the initiative of Molyneux.

As soon as the equipment was operative again in the Specola in May 1729, the question of parallaxes was the first issue the Bolognese astronomers addressed with their observations.

Selecting certain stars that were bright enough that they could be observed during day-time too, Manfredi set about the actual observing using Lusverg’s mural semicircle, the biggest instrument in the observatory [file 16]. At first the observations dealt almost exclusively with Sirius and Arcturus, the same pair of stars that had already been observed in Paris and Bologna twenty years before.

At the beginning of 1729 Manfredi collected his studies in a pamphlet and on April 2 the papal archiater Antonio Leprotti, a close friend of his, wrote him regarding the difficulties he ran with censorship because of the suspicion that the thesis of the Earth’s motion was given too much attention in the text (125). On May 30 of the same year, however, the work got the imprimatur of the Bolognese inquisitor.

The book, the above mentioned De Annuis Inerrantium Stellarum Aberrationibus, is dedicated to cardinal Giovanni Antonio Davia, archbishop of Rimini, lover of astronomical studies and probably a convinced Copernican, given that in 1726 he had donated to the Bolognese observatory a large armillary sphere - currently on exhibition in the "Globe Room" of the museum [file 52] - which represented that very system of the world (126).

The final chapters of the book which critically examine the observations available, first those of the parallactic motion in elevation, then those regarding the effects in right ascension, demonstrate how the experimental evidence appeared at that time contradictory. Manfredi in fact discovered that Robert Hooke’s observations and those of Jacques Cassini on Sirius could be accounted for by parallactic motion. Such an explanation could not however be applied to Flamsteed’s observations or those regarding the difference in transit times of Sirius and Arcturus. In relation to these latter observations Manfredi reported very close agreement between the data obtained in Paris from 1704 to 1705 and the data obtained in Bologna in recent years. It is not all that clear what problems ecclesiastic censoreship might have had with this publication but Manfredi was asked to openly express himself against the Copernican theory. An examination of the manuscript (127) would seem to show that the expression "Copernici sectatores" (the sectarians of Copernicus) was introduced as a correction, probably the only modification that was made.

Already in October 1728 Manfredi had widened his programme to include 14 stars, making it thus more systematic and leaving the impression that he had already, by that date, received news of Bradley’s discovery.

After a year’s observations of 12 stars with a new zenithal sector of George Graham (1673-1751), Bradley began in 1728 to notice a certain regularity in their motion, a sign this of a general phenomenon, not just limited to a few stars. The amplitude and type of periodicity of this motion excluded any parallactic motion or change in the Earth’s axis explanation. As soon as Bradley was sure in his mind that the cause of these variations was due to the combined effect of the Earth’s motion and the propagation of light, he explained it in a letter to Sir Edmund Halley, president of the Royal Society, that was then published in the December 1728 issue - an issue that came out in February 1729 - of the Philosophical Transactions (128).

Halley referred Bradley’s work to the Royal Society only summarily - perhaps to guarantee him priority for the discovery - but Leprotti sent Manfredi a copy of a letter by Jean-Théophile Desaguliers (1683-1744) to Brooke Taylor (1685-1731), published in the Journal Literaire of 1729, from which it was possible to extract some details.

With Leprotti and Thomas Derham, Manfredi was still working on translation of the letter and a complete understanding of Bradley’s work (129), preparing then a short study on the question. The study, finished in October 1730, got round the difficulties of the Inquisition (130) and came out in 1731 in the first volume of the Bolognese Institute’s Commentarii with the title De novissimis circa fixorum siderum errores observationibus, remaining for years the only study confirming Bradley’s work.

The study described in detail the series of Bolognese observations - carried out between October 1728 and October of the following year - and accurately analyzed the observations of Peter Horrebow (1679-1764, Römer’s successor and author in 1727 of Copernicus Triumphans) and those of Bradley. Manfredi illustrated how the Bolognese observations were in perfect keeping with the new law expounded by Bradley, underlining the fact that the law had been deduced from variations of motion in declination and that verification dealt with variations in right ascension, something therefore that was relatively independent from the observations Bradley had worked on and destined thus to strengthen faith in the correctness of the interpretation he had proposed. In the final part, however, Manfredi is careful not to express an opinion that would tie him too closely to Bradley’s explanation confirming the Earth’s motion.

We know for sure that the problem of the study’s actual structure was discussed with Leprotti who, in November 1730, wrote Manfredi:

"On the other hand it will not be possible to print the afore-mentioned dissertation, except as you thought, and an examination of Horrebow’s observations found inconclusive for the annual parallax will greatly help in lifting suspicions!" (131)

In October of the previous year, in fact, Manfredi had already written Leprotti, claiming he had spoken, in his study, of a moving Earth, as a hypothesis, only out of censorial necessity (132).

The doubt still remains however that Manfredi was content to consider the physics part of astronomy as relatively hypothetical, inclined to give greater weight, in terms of truth value, to the purely geometric part. In the Instituzioni Astronomiche, published in 1749, which collect his university lectures, he in fact affirmed that the ultimate truth to be discovered in astronomy were the mathematical rules that accounted for the movement of the stars in the sky; that the "physical" theories on the why of these movements were somewhat hypothetical; that such theories could at times do great damage, diverting attention away from the correct interpretation of events - and here he cites the cartesian vortices - even though he had to admit that Newton’s system accorded wonderfully with the facts and was the only one which, in his opinion, had any chance of being correct.(133)

  1. - J. Cassini: 1699, Mem. Ac. Roy. Sciences (ed. di Amsterdam), p. 247.
  2. - J. Wallis: 1699, Opera Mathematica, Tomo 3, p. 701, Oxoniae.
  3. - J. Picard: 1693, Voyage d'Oranibourg, in Anciennes Mem. Ac. Roy. Sciences (ed. di Parigi del 1729), Tomo VII, p. 193.
  4. - E. Manfredi: 1729, De annuis inerrantium stellarum aberrationibus, Bologna, p. 68 Cartella di Manfredi segnata "Parallasse annua delle stelle fisse / cose diverse" che contiene l'estratto delle osservazioni di Parigi comunicato da G. F. Maraldi e la sinossi delle osservazioni bolognesi del 1707-8. Arch. Dip. Astron. Bologna, busta VII.
  5. - J. Cassini: 1717, Mem. Ac. Roy. Sciences (ed. di Amsterdam), p. 330.
  6. - W. Derham a T. Derham, in data 19 ottobre 1726, copia. Arch. Dip. Astron. Bologna, busta VII.
  7. - Biblioteca Comunale Archiginnasio Bologna. Fondi speciali Leprotti, I.
  8. - G.G. Bolletti: 1751, Dell'origine e de' progressi dell'Instituto delle Scienze di Bologna..., L. della Volpe, Bologna, p. 112.
  9. - Arch. Dip. Astron. Bologna, busta VI.
  10. - H. Sloan a T. Derham, in data 28 gennaio 1729, copia in Arch. Dip. Astron. Bologna, busta VII.
  11. - Arch. Dip. Astron. Bologna. Busta VII, cartella di Manfredi intestata "Aberrazione Bradleyana". Leprotti a Manfredi, in data 30 agosto 1729, Biblioteca Comunale Archiginnasio Bologna. Fondi Speciali Leprotti, I. E. Manfredi: 1731, De novissimis circa fixorum siderum errores observationibus, in De Bononiensi scientiarum et artium Instituto atque Academia Commentarii, Tomo I, Bologna, p. 599. Leprotti a Manfredi, in data 1 ottobre 1729, fondo cit.
  12. - Leprotti a Manfredi, in data 29 novembre 1729 e 2 dicembre 1730, fondo cit. Manfredi a Leprotti in data 25 ottobre 1730, Biblioteca Comunale Archiginnasio Bologna, Fondo speciale Manfredi.
  13. - Leprotti a Manfredi, in data 1 novembre 1730, Fondi Speciali Leprotti, I.
  14. - Manfredi a Leprotti, in data 25 ottobre 1730. Biblioteca Comunale Archiginnasio Bologna, Fondo speciale Manfredi.
  15. - E. Manfredi: 1749, Instituzioni Astronomiche, Bologna, pp. V e IX.