Two particular aspects of his scientific personality stand out. On the one hand is his committed professionalism, reflected in his geographical studies - Manfredi’s method for computing longitudes, by means of star occultations from the side of the moon, is reported in the cited reference (101) -, his compilation of the Ephemerides Bononienses (102), his work on reform of the calendar (103), his determining of the basic elements of the apparent motion of the Sun (104), and the work as geographer and geodesist he was commissioned to do by princes and cities regarding the ever recurrent "question of the waters". Manfredi dealt with the "principal question" of the running of the Reno into the Po, the rivers of Ravenna and the Tiber. At the behest of the Senate of Luni and the archduke of Tuscany he also studied the course of the Serchio river. (105)
On the other is his interest as "philosopher" in doctrines regarding the physics of the sky, the debate on different "world systems", the principles of Newtonian physics (106).
One idea, furthermore, was always present in Manfredi’s work, as it had been in Marsili’s original programme: the possibility of finding practical interests in "natural philosophy" research - what Manfredi himself had defined as "benefits not to be laughed at, that civil society reaps from astronomy." (107)
The scientific programme for the Bolognese Specola, as conceived by Manfredi and discussed with Marsili, mirrored that of the London and Paris Observatories, and - especially, as we have seen, for Marsili - was more a programme of astronomical geography than astronomy real and proper. Marsili had in fact also been a military engineer in the Balkans in the service of the imperial army and had there been faced with the problem of astronomically determining the geographic coordinates of certain localities where military campaigns were being conducted. It should also be remembered that the work done in Bologna, mid-way through the century, by Gian Domenico Cassini, and in particular his observations of the eclipses of Jupiter’s satellites which had enabled him to develop a practical method for determining geographical longitude, had undoubtedly left its mark on Bolognese cultural and scientific circles and especially on the young Manfredi.
In 1711 Manfredi wrote to Marsili who was visiting Rome to solicit the Pope on behalf of the Bolognese Institute:
"The main benefit to be drawn from astronomical observations is the reform of Geography. It is badly needed in Italy where the maps of Magini are all wrong in their scale and much worse than those that are being published daily in Italy and especially in Rome. The job of correcting Italian geography requires the fixed residence of an Astronomer in an Observatory, as in fact will be the case of Your Grace, and also requires that one or two other Astronomers will travel round the beaches, and main points, to finish the work in a couple of years...".(108)
Work along these lines had in fact already been done: the difference in longitude between Bologna and Paris had been accurately computed, simple and transportable instruments had been designed to measure noon in different localities by the method of corresponding altitudes (109), a list of longitudes obtained by occultations of stars from the Moon, using a method devised by Manfredi, had been published in the Mèmories de l’Académie Royale des Sciences (110).
Despite this, however, the programme was not implemented: it was probably too ambitious for the times and for the political conditions obtaining in Italy. A few initiatives did however survive that can be seen as linked to it in some way.
The most important was the compilation and printing of the Ephemerides Bononienses (111), which for decades remained the most extensive and complete among the many produced in Europe, and which, having reached a wide audience, helped make the Bolognese institution famous.
Compiled by Manfredi with important help from his sisters Teresa and Maddalena and using the so-called "Cassinian" tables - produced in Paris and obtained following solicitation by Marsili - the Effemeridi Bolognesi contained everything that could be useful for astronomically computing the geographical coordinates of places.
At the end of the manuscript Introductio in Ephemerides, we read the following note, missing in the printed edition: "I began the ephemerides of December 1712 in Bologna. With many interruptions they were continued in the following years with the help of my two sisters Maddalena and Teresa, and of Mr Giuseppe Nadi, and a while after of Mr Cesare Parisij... The table of longitudes and latitudes was calculated by my sister Maddalena around 1702 - 1703".(112) Moreover, in the copy of the original tables preserved in the Estense Library (Manoscritti Campori 2428) we read the following hand-written note by Manfredi "originals of Cassini tables sent me at different times by Mr Maraldi with the knowledge of Mr Cassini".(113)
For the first time the calculations were developed to such a point that the work could be done even by non specialists - and here we see the mark of the illuminist ideas of the craftsman, Manfredi, and of the maecenas, Marsili and their joint project of opening scientific discoveries to the public - offering detailed synopses of the ephemerides of the Moon and planets and all kinds of information on solar and lunar eclipses, as well as Jupiter’s satellites, and on the occultations of stars from the Moon. Furthermore, they were accompanied by an introductory volume - the above mentioned Introductio in Ephemerides - which described their use in detail and how the different astronomical operations should be performed in the defined places.
As was pointed out at the beginning, the Bolognese astronomers had always managed to combine personal and "philosophical" interests with these "practical" and, as it were, "institutional" tasks.
Interest in the Galilean "problem of the Systems of the World", i.e. the still heated debate on the geocentric and heliocentric systems, and the emerging Newtonian physics was at the time enormous. We need only recall that in 1722 Marsili, already sixty-four and in poor health, wanted to go all the way to England to pay homage to the then president of the Royal Society, Isaac Newton (1643-1727), by whom he was in fact solemnly received as an old member. The Bolognese Observatory would benefit greatly from these contacts (114). Before this date, in the mid-XVIIth century, Gian Domenico Cassini had built and used the meridian in San Petronio to demonstrate - successfully (see also chapter 10) - the non uniform nature of the relative motion of the Earth and the Sun, demolishing once and for all the Aristotelian celestial physics of uniform motion (115).
What was still missing, however, at the end of the XVIIth century was definitive proof of the Earth’s motion around the Sun, and indeed around itself.
Proof of this latter, as we shall see, would have to wait till Giovan Battista Guglielmini’s experiments in Bologna at the end of the 1700s on the fall of bodies, and those of Léon Foucault (1819-1868) in Paris in the mid-XIXth century on the simple pendulum.
An experiment, however, which since early Antiquity had been viewed as crucial for verifying the Earth’s motion around the Sun was the observation of the parallax of the fixed stars: the apparent change in the position of the stars during the year due to the Earth’s rotation around the Sun - a change, however, which, because of the enormous distance of the stars, was very small and hence difficult to measure.
With the new instruments of the late XVIIth century - that used the telescope as sighting instrument on sextants and other astronomical positional instruments - and the introduction of pendulum clocks, increasingly accurate in measuring transit times, it seemed the moment had come to make such measurements.
Many astronomers did in fact engage in this kind of observation, using different techniques and obtaining discordant results. Galileo had computed the annual parallax of the fixed stars at around three arcmins, comparing the apparent diameter of the Sun with the apparent diameter of the stars, that he thought he could measure with the telescope. Christiaan Huygens (1629-1695), on the basis of an estimate of the difference in brightness between Sirius and the Sun, had assumed a parallax value for this star of about ten arcsecs. Using the reflected light of the planets to estimate the difference in size between the stars and the Sun, Newton had arrived at a value of thirty-six hundredths of an arcsec for the nearest stars, a value which is not that far off the real one.
We have to wait till 1838 when Friedrich Wilhelm Bessel (1784-1846) measured the angular distance between the star 61 Cygni and two faint nearby stars, observing them from Königsberg for six consecutive months, i.e. until the Earth and with it the observer had passed from one extremity to another of the orbital diameter. The instrument used by Bessel was a heliometer, built by Joseph Fraunhofer (1787-1846) according to the new technique of splitting the object glass in half along the optical axis so as to produce in the field of the same instrument two images of the object observed. This instrument was used to measure the diameters of planets and the Sun (hence the name) and of small angular distances, superimposing the images of two nearby stars by displacing the two halves of the object glass. Measurement of this displacement in millimetres, after suitable calibration, provided the angular distance between the two stars [file 41].
Bessel’s observations led to the computation of an angle, for 61 Cygni, of thirty-one hundredths of an arcsec. Today’s parallax value for the star measured by Bessel is twenty-nine hundredths of an arcsec. Today we know that the star nearest to us is Proxima Centauri whose parallax is sixty-six hundredths of an arcsec. Indeed, as the definition of stellar parallax tells us, the higher its value, the nearer the star. We do not know of any stars today with a parallax value higher than that of Proxima Centauri, part of a three star system whose other two stars, slightly farther away, are Â Centauri A and B.
Going back to the afore-mentioned XVIIth-century attempts at measuring the parallax, it is difficult to say to what extent these estimates were known and appreciated by astronomers of the time. Some, for example, viewed the problem very schematically and terms of commensurability between the diameter of the terrestial orbit and the orbe magno, the sphere of the fixed stars recalling Archimede’s text that referred to the heliocentric system conceived by Aristarchus in the third century BC (116).
What is certain is that towards the end of the XVIIth century, various astronomers were hunting after it, including Robert Hooke (1635-1703), Ole Römer (1644-1710), John Flamsteed (1646-1719) and Jacques Cassini (1677-1756). During these tricky observations indications began to emerge, non too comprehensible, of movement of the fixed stars away from their mean positions. Today we can say that these effects were, at least in part, due to light aberration and variations produced by pressure and atmospheric temperature on refraction. This latter effect was certainly behind the variations in altitude of the Pole Star, already observed in 1671 by Jean Picard (1620-1682) at Uranjeborg while he was trying to improve on measurement of the geographical site coordinates of Tycho Brahe’s famous observatory. At first these "aberrations" were taken as indication of parallactic effects even if it soon became clear their internal agreement was poor and they did not fit in with expectations for such motion.
Confirmation of Newtonian celestial physics would have led, via Kepler’s laws that can be deduced from it, to confirmation of the Copernican system. This explains the interest Manfredi showed (117) in the question of the perturbations in planetary motion.
Another method for proving the diurnal rotation of the Earth was to determine its flattening, due to the effects of centrifugal force, as first suggested by Huygens. Initial results to this end, however, were somewhat contradictory and much time and effort was needed to settle the issue. The Bolognese astronomers also contributed to this question when Eustachio Zanotti (1709-1782) - assistant at the Bologna Observatory from 1729 and Manfredi’s successor as director - suggested using observations of star occultations from the Moon to define the shape of the geoid (118).
The contribution of Bolognese astronomy to these issues will be rapidly illustrated in the pages that follow, paying particular attention to Manfredi’s studies on aberration and to the compilation, by Zanotti, of the first modern star catalogue, based on the mean positions of the stars.