15 - The English instruments and the star catalogue by Eustachio Zanotti.

In 1735 Manfredi decided to initiate a new series of observations, using Lusverg’s mural semicircle [file 16], adjusted for maximum accuracy and fitted with a new lens and eyepiece, designed "by Dr. Francesco Vandelli professor of military architecture at the Institute" (134). The results however were not satisfactory, probably because of the accuracy that recent observational techniques had achieved and that the instruments of Lusverg, built several decades before, could not match (135): "The Observatory needs better instruments, something that the experience of eight years or more has increasingly taught us" (136).

The assignation of funds to the Istituto delle Scienze by Pope Clement XII (1652-1740) in 1738 provided Manfredi with an opportunity to ask for a new series of instruments. The private correspondence between Manfredi and the papal archiater Leprotti and the official correspondence between the Administrators of the Institute, the Bolognese Senators and the Bolognese ambassador in Rome allow us to follow the affair in some detail (137).

The intervention of cardinal Davia, Leprotti and the afore mentioned English ambassador in Rome, Thomas Derham, helped conclude negotiations with the papal court.

"...and the other remaining 1000 S[cudi] are for buying in England certain instruments, and tools, absent from the rooms of Astronomy and Experimental Physics", the Bolognese ambassador in Rome wrote the Administrators on February 8, 1738.

Derham took charge of the order in England and, as we learn from a letter of Manfredi to Leprotti dated 9 August, the Royal Society also intervened in the matter of their construction, thanks to the ties Manfredi, through his studies on aberration, had struck up with that Society, to which he became associated from 1729.

From the very start the name of the Englishman George Graham emerged as the craftsman to commission the instruments from.

Graham was the successor of the celebrated watch-maker Thomas Tompion (1683-1713)- builder of the quadrant used by Hooke - whose workshop he had taken over in Fleet Street. In this laboratory - whose sign was At the Dial and One Crown - Graham had come up with numerous inventions that had benefited mechanical watch-making greatly and, around 1710-1715, had started work on astronomical instruments, bringing a series of improvements to them. He had in fact built the instruments Halley had renewed the Greenwich Observatory with and the ones Bradley had used to observe the aberration of the fixed stars; he had also designed the model of the mural quadrant which, with a few alterations, would remain the basic instrument for every observatory for the whole century. The prototype of this instrument, installed in Greenwich in 1725, became widely known because of its minute description in the 1738 work, A compleat system of optics, by Robert Smith (1689-1768), professor in Astronomy at Cambridge.

Manfredi could not have known this book which was translated in German in 1755 and French in 1767, but the fame of the English craftsman had undoubtedly convinced him of the need to acquire the new instruments for his laboratory.

Any early hopes he entertained of receiving the new instruments in good time were, however, quickly dashed for reasons of both manufacturing times and increased costs (138). He in fact did not live to see completion of the deal to purchase the English instruments. The worsening of his stone condition (Urinary bladder lithiasis), which for years had tormented him, caused his death on February 14 1739, just a month after the death of Thomas Derham.

The Bolognese ambassador in Rome went on with the transaction and on March 25 of that year informed the administrators of the Institute that the instruments would be ready inside two months. The instruments - mural quadrant, transit telescope, and moveable astronomical quadrant - are on exhibit in this museum [files 14, 17, 19 and 35) and, among the letters to the Istituto delle Scienze, the original invoice has been found, dated June 1, 1739, and signed by the craftsman who built them, the Jonathan Sisson who is considered successor to the Graham tradition and the most important English scientific instrument maker - together with his son Jeremiah - of the mid-XVIIIth century.

The actual price was well above the 1000 scudi that had been talked about originally: the 245 pounds amounted to 5500 scudi and all record had been lost of how much had been agreed between Derham and the English craftsmen. A document (139) tells us that the instruments were shipped from London only in May of 1740, after the Zagnoni Bank in Bologna had charged, in a letter of January 27 of the same year, "Mr. Gistard and Alexander Compagni traders in that market" to see to the payment and that the relative draft, contested on February 27 by the said Compagni traders, had been honored with "money obtained with some difficulty from other transactions"

The instruments, the document goes on, were "loaded on the ship called - Expedition - of the Englishman Captain Salcott; and they left England; shortly after it is known the ship was forced to return to port because of the appearance from the Lands of France of the French fleet; and the latest news is that the ship with this cargo then made the trip with a large mercantile convoy escorted by war ships, and that they all reached Gibraltar some time ago, but that only a part of the convoy headed off on the journey to Leghorn, the other merchant ships remaining in Gibraltar for lack of escort; and that among these remaining in Spain is the ship with the cargo bound for the Institute."

Eventually the ship arrived in Leghorn and, towards the end of February 1741, the instruments reached Bologna. There to receive them was Eustachio Zanotti who, in the first pages of the seventh volume of the observation logbooks - indicated also as "first volume regarding the use of the new English instruments" - wrote:

"...at last the new astronomical instruments made in England arrived in Bologna, instruments which had already been ordered years before when the celebrated Mr. Eustachio Manfredi, first Astronomer of this Institute, was alive. The boxes were opened in the rooms of the Assunteria where in the days that followed recognition was made of all the pieces and their uses and how best to assemble each instrument, in which state it remained for some time in the said rooms to give the administrators of the Institute and the Professors plenty of time to satisfy their curiosity and contemplate an exceptional piece of work:...

On May 20 the new instruments were moved to the chambers of the Observatory,...

Now we can say the Observatory is as well equipped as it was poorly provided for in the past when one of the best astronomers Italy and indeed Europe has ever had was alive."

More than a year was needed to prepare the meridian observation room, from which Lusverg’s old semicircle had been removed, for the instruments. Modifications were made to the support wall for the mural instrument, making it shorter and at the same time more solid. "A pedestal with two doric style columns or pillars in a proportion that would satisfy needs as well as elegance" (140) was built to support the transit telescope. The lantern that illuminated the string meridian was renewed and, for the sake of ornamentation, a meridian line in marble and brass with analemmatic dial for describing the Time equation was added; this latter was the work of Ercole Lelli and is still to be seen on the floor of the room [file 3] (141).

In August 1742 the instruments were finally installed and the operations of rectification and adjustment, that the archive records report in minutest detail, got under way.

The three instruments represented the best equipment one could hope for in those days, equipment that until the end of the century would remain standard for positional astronomy: it is no coincidence that this was the type of instrument - identical down to the smallest detail - that Diderot and d’Alembert included in their Enciclopédie.

Their designer, Jonathan Sisson (1690?-1747), was collaborator and successor to Graham and it is known he was still working with him in 1743, a fact that leads us to suppose that Graham’s name too might be associated with the Bolognese instruments. These latter appear to be the oldest bearing Sisson’s name and the invoice for the mural quadrant, entirely in brass, suggests we bring the date forward for the use of a single metal in instrument making from 1743 to 1739, and credit Sisson and Graham with introducing this technique rather than John Bird (1709-1776) and James Bradley as Maurice Daumas maintains (142). Traditionally, in fact, these instruments had a brass limb and iron frame - as can be seen in Lusverg’s great mural semicircle - whereas using a single metal avoided the distorsions produced in a bimetaelic structure by variations in temperature.

More than a year went by before Zanotti considered the instruments "rectified enough", even though in the years that followed the stability of the transit telescope left a good deal to be desired, as reported in the records.

It was finally only on September 26, 1749, that the series of observations began which would give rise to a catalogue of 413 stars included in the Zodiac belt and other major stars spread across the heavens. This can be considered the first catalogue of stars based on modern criteria and was published by Zanotti in the 1750 reprint of Manfredi’s Introductio in Ephemerides. The observations went on until August 14, 1750, and were complemented by a re-computation of the latitude of Bologna - which was found to be 44°29’54", just 1.2 arcseconds more than the actual value - and determination of the first point of Aries, i.e. the intersection of the equator and ecliptic corresponding to the vernal equinox.

In the observations, Eustachio Zanotti manned the mural quadrant and Giovanni Angelo Brunelli (?-1791) the transit telescope. It was this latter scholar who provided the first experimental proof that radiant heat diminishes with the square of the distance from the source - as indeed light does - and who was later to become mathematician to the King of Portugal who gave him the brief of drawing up the map of Brazil (143).

They were assisted in their observations by abbot Petronio Matteucci (?-1800) who was to become director of the Bologna Specola after the death of Zanotti.

The stars in the catalogue were all observed twice and the measurements accepted only when the differences did not exceed half a second in transit times and eight arcseconds in elevation. The positions were all reduced to the equinox of 1750, taking into account the precession of the equinoxes and Bradley aberration: it was in this way the first catalogue take account of this latter effect.

Of no less importance, as was mentioned, was the computation of the position of the vernal equinox, a problem that was traditionally dealt with by first trying to identify the epoch of the solstices and equinoxes and then establishing the position of the stars relative to the Sun at the spring equinox. Before the introduction of the pendulum clock and telescope, this second measure was somewhat uncertain, particularly because of the difficulty of accurately transferring the time from day to night and the impossibility of observing the stars by day (144).

With the introduction of these instruments - especially the pendulum clock designed by Christiaan Huygens and built by Salomon de Coster at Scheveningen in 1657 - both operations became possible, albeit fraught with difficulties. In the best tables available in the mid-XVIIIth century, those of Cassini and Halley, the epochs of the solstices differed by up to 11 minutes!

Manfredi "haec tabularum incommoda non tulit" and came up with a new method (145). In practice he substituted for "epochs", which are affected by the inequalities of motion of the Sun, the measurement of angles. While the mean epoch, between the moments when the Sun reaches the same elevation before and after the solstice, differs, because of the unequal movement of the Sun, from the epoch of the solstice itself, the mean difference in right ascension between a determined star and the Sun, measured when the Sun has equal elevation, is in effect equal to the difference in right ascension obtaining on the day of the solstice. Adding to this 90°, in the case of the summer solstice, or 270° for the winter solstice, one arrives at the right ascension of the star.

Experimented by Manfredi in 1734, this method was applied by Zanotti in 1749. Introducing, as we said, the added refinement of correcting the differences in right ascension for effects caused by Bradley aberration, the uncertainty surrounding the position of the vernal equinox was reduced to about 5 arcseconds, the equivalent of 2 minutes of time in the epoch of the equinoxes.

As Zanotti pointed out in the Commentarii, not only had Manfredi provided a method for computing the equinoxes, he had also "opened new avenues for the study of the theory of the Sun". It had in fact become possible to follow the motion of the Sun itself in right ascension more accurately than simply obtaining elevations from observations alone, given that atmospheric refraction was no longer part of the equation and the dependency of the value used for the inclination of the ecliptic was fairly small.

This was the last of the major contributions the Bolognese astronomers made to positional astronomy. Attention turned instead to celestial physics, and the mechanics and theory of gravitation, though studies underwent a progressive impoverishment that probably went hand in hand with the decadence that had set in in Bologna in the period before the Napoleonic era.


 
  1. - Arch. Dip. Astron. Bologna, Registri delle osservazioni, vol. V alla data 26 maggio 1735.
  2. - Arch. Dip. Astron. Bologna, Registri delle osservazioni, vol. V alla data 6 ottobre 1735.
  3. - Arch. Dip. Astron. Bologna, Registri delle osservazioni, vol. V alla data 24 settembre 1735.
  4. - Sunto del Chirografo papale del 1 marzo 1738, Arch. Stato Bologna, Fondo assunteria d'Istituto, Registri, vol 35, p.181. Biblioteca Comunale Archiginnasio Bologna, Fondo speciale Manfredi, III. Lettere all'Istituto delle Scienze, vol. 1734-1755 e copialettere Istituto delle Scienze, vol. 1734-1755, Arch. Stato Bologna.
  5. - Manfredi a Leprotti, in data 10 settembre 1738, Biblioteca Comunale Archiginnasio Bologna, Fondo speciale Manfredi Lettere all'Istituto delle Scienze, in data 23 settembre 1738, Arch. Stato Bologna.
  6. - Arch. Stato Bologna, Fondo Assunteria d'Istituto, Div. 5, n. 4, documento in data 10 dicembre 1740.
  7. - Arch. Dip. Astron. Bologna, Registri delle osservazioni, vol VII alla data 16 agosto 1742.
  8. - De Bononiensi scientiarum et artium Instituto atque Academia Commentarii: 1745, Tomo II, Bologna, p. 40.
  9. - M. Daumas: 1953, Les instruments scientifiques aux XVIIe et XVIIIe sičcles, Presses Universitaires de France, Paris, p. 235 e 305.
  10. - G. A. Brunelli: 1745, Calor a flamma propagatur eadem proportione, qua lux, in De Bononiensi scientiarum et artium Instituto atque Academia Commentarii, Tomo II, pars I, Bologna, p. 367. Annotazione di E. Zanotti in Arch. Dip. Astron. Bologna, Registri delle Osservazioni, vol.VII alla data 21 giugno 1750.
  11. - F. Bňnoli: 1991, l'Astronomia, n. 106, p. 17.
  12. - E. Zanotti, De Solstitiorum tempore definiendo, in De Bononiensi scientiarum et artium Instituto atque Academia Commentarii, Tomo II, pars I, Bologna, p. 437.