Monday, March 16, 2009

Father Giuseppe (Joseph) Piazzi

Father Giuseppe (Joseph) Piazzi, a priest of the Theatine order, is known as "the Columbus of the Lesser Planets" - and his work assisted the great Bessel in demonstrating that the earth indeed revolves around the sun. (But not, dear Galileo, in circles - why didn't you listen to that nice Mr. Kepler?) Oh yes, we will hear about Galileo. But today we'll meet Fr. Piazzi.
"The first day of the century", says the celebrated astronomer Frederick William Bessel, "was marked by a brilliant discovery: Piazzi of Palermo on the first day of January 1801 found a new planet, 'Ceres'. His discovery was a by-product of a great and admirable undertaking, the determination, namely, by a long series of observations, of the positions of some 7,000 fixed stars." Bessel proceeds to explain the significance for astronomical science of this determination of the latitude and longitude of so many stars with the greatest possible accuracy, and recounts the laborious efforts of Tycho Brahe and his successors to arrive at the same result. "Piazzi", he continues, "had striven strenuously to secure the erection of an observatory at Palermo, and to equip it with splendid instruments, the work of the never-to-be-forgotten Ramsden; and when he had succeeded in this he stepped at once to the head of astronomical science. He published in 1803 after incalculable labour a catalogue of the positions of some 7,000 stars, and so resolute was his determination to secure the most accurate results attainable with his instruments that he repeated all his observations, and in 1814 was able to publish a second and much improved edition of his catalogue. Here was in truth a worthy beginning of the century. The appetite for thorough observation was roused from the slumber in which it had lain since the death of Bradley."

At first sight the figures mentioned by Bessel may not seem very formidable. Some may not consider it a colossal undertaking to direct a telescope on one after another of 7,000 stars, and read off their positions on the graduated circle. But the life of Piazzi supplies an effective reply to such a criticism. During the course of the observations he believed that he had found the long-sought parallax of certain fixed stars that is, the displacement which the position of the stars must undergo from the fact that in the course of the earth's annual movement they are observed from widely separated points of observation. On closer examination, however, he discovered that it was not the stars but his telescope that had suffered displacement, in all probability from the fact that the tower on which the telescope rested received more heat from the sun on one side that on the other. This will give a hint of the minute accuracy requisite in astronomical observations. Further, every instrument has its trifling defects, which import an element of error into all observations, and is further liable to disturbance by the most trivial occurrences, so that to arrive at practically useful results there is needed not only inexhaustible patience and persistence, but also acute insight, so as to discover the various sources of error and either devise a telescope that will make them inoperative, or correct the results mathematically. After the practical work of observation the astronomer must be prepared for weary hours at his desk. Every star must be observed several times, for no one observation ever corresponds precisely with another: and to the list of figures thus obtained, the Calculus of Probabilities must be applied so as to arrive at the closest possible approximation to the truth.

No one could appreciate better than Bessel the difficulty of compiling such a catalogue, for he himself stood in the first rank of observers, and had reduced to workable form Bradley's essay in the same direction, which contained only single observations. "Nearly all the Flamsteed stars", he writes, "were observed five times, so that I had to reduce to shape a total of more than 25,000 observations.... On Piazzi's catalogue which contains still more observations than Bradley's two astronomers were engaged and the task occupied many years: I am now convinced that that gigantic work has been estimated rather below than above its value."

Bessel drew a rich yield from the works of Piazzi and Bradley. When he had resolved on his plan for the revision of Bradley's catalogue he wrote to Olbers: "Thanks to the known dexterity of Bradley, and the excellent instruments of the Greenwich Observatory, Bradley's catalogue yields little in point of accuracy to Piazzi's. An interesting feat, surely, to attain the same accuracy in 1750 as in 1800!" The collation and revision of the two catalogues helped, in the event, to bring to light a very interesting fact: the self-movement of the fixed stars. It showed "that nearly one half of the stars contained in both catalogues (numbering 2959) possess a self-movement amounting to the tenth of a second annually, or, in certain cases to more", The greatest movement was that exhibited by a star of the fifth magnitude Nr. 61 of the Swan [Also called 61 Cygni, see below]: it was as considerable as five seconds a year. Bessel selected this double star on which to renew the search for the long-sought parallax. His labours were successful. He determined the parallax, about the third of a second, and with it the distance of one of the nearest fixed stars. "Nearest" is, however, hardly a word to accentuate in this connection. Its distance from the sun is according to Bessel about 657,700 times the radius of the earth's orbit. Light takes ten years to travel that distance: a train covering 200 miles a day would take two hundred million years).

These discoveries count, it is true, to the credit of Bessel, not of Piazzi, but it is clear that but for Piazzi's work they could never have been made. Bessel himself speaks elsewhere of the "invaluable services" 2 of the Italian astronomer, and always mentions his name with the most marked respect. And a greater than Bessel, Gauss himself, honoured Piazzi so far as to call him his first-born son Joseph.

It was his practice of observing each star more than once that led up to the significant discovery of the first asteroid, Ceres. On January 1st he noted down the position of a small star, on January 2nd he made a second observation of it but found figures different from the first. Either then, one of the observations was incorrect, or the star possessed a movement of its own. Further investigation decided in favour of the latter alternative.

Piazzi was a member of the Order of Theatines founded by St. Cajetan of Thiene. Born in 1746 at Veltlin, he made his first studies at Milan and there entered the Order. He received a decisive impulse towards the cultivation of science from the two editors and expounders of Newton, Thomas Leseur (+ 1770), Professor at the Sapienza, and Francis Jacquier (+ 1788), Professor at the Roman College, both of whom belonged to the Order of the Minims of St. Francis of Paula. [the same order as Fr. Mersenne whom we studied previously!] Having completed his course in Philosophy at Genoa and Ravenna, and in Mathematics at Malta, he was appointed Professor of Dogmatic Theology at Rome where he formed a lifelong friendship with his colleague Barnabas Chiaramonti, afterwards Pope Pius VII. On the advice of Jacquier, Piazzi in 1780 accepted the post of Mathematical Professor at Palermo, and thus made his entry into the field in which he was to attain such distinction. He at once bestirred himself to give the scientific Faculty an impulse in a new direction. He was supported by the Government in his efforts, and especially in his project of erecting at Palermo an Observatory of the first class. Journeys to France and England brought him into association with the first scientists of the 18th century, and gave him an opportunity of appreciating the scientific needs of the day. After his return the Observatory was erected in 1786, and Piazzi entered into occupation of it. Under the rule of the Bourbons he received a call to Naples where he died in 1826.
[Kneller, Christianity and the Leaders of Modern Science]
About that star 61 Cygni: as Fr. Kneller notes, Bessel tried to figure its parallax because Fr. Piazzi had noted its very high "proper motion" - that is, it was actually moving with respect to the other stars. According to Burnham's Celestial Handbook, Piazzi found it to be moving at 5.22 seconds of arc every year, and called it "the Flying Star". In 1838 Bessel suceeded in measuring its parallax, 0.29 seconds of arc, which he calculated to be 10.3 light years. BCH says its presently accepted distance is 11.1 light years, the fourth nearest to Earth (after Alpha Centauri, Sirius, Epsilon Eridani). [BCH 768-9] Bear in mind that this shift in its position, confirmed by many other correlated measurements since then, finally proved the Earth was in motion around the sun, which had hitherto been only a hypothesis. (But we'll hear about all that eventually, when we do Galileo.)

There is another piece to the story of the discovery of Ceres, also told by Fr. Kneller, as part of the history of a very great man:
On New Year's Night 1801, at Palermo, Piazzi discovered the first of what we now know to be a number of small planets between Mars and Jupiter. But before the observer had ascertained the positions of the planet necessary, by the methods of computation then practiced, to determine accurately its path, it approached so near the sun that it was lost [about 7 weeks later] from sight in the sun's radiance. Thus the planet had no sooner been discovered than it was lost again, for there was no hope of locating it unless astronomers knew in what quarter of the heavens to search for it, and this could not be known without a knowledge of its path. At this juncture there came to the aid of Astronomy a young mathematician of only four and twenty: he brought a completely new method by which, from the meagre observations of Piazzi, the path of the vanished planet could be deduced. In the spot indicated by him Ceres was re-discovered by Olbers on Jan. 1, 1802. The youthful scientist, who thus leaped into a European reputation, was Karl Frederick Gauss, one of the foremost mathematicians of all time.
[Kneller, Christianity and the Leaders of Modern Science]
Also see here.


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