Discourse on Comets
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The ''Discourse on Comets'' (''Discorso delle Comete'') was a pamphlet published in 1619 with
Mario Guiducci Mario Guiducci (Florence 18 March 1583 - Florence 5 November 1646) was an Italian scholar and writer. A friend and colleague of Galileo, he collaborated with him on the '' Discourse on Comets'' in 1618. Early life Mario Guiducci was born in the ...
as the named author, though in reality it was mostly the work of
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He wa ...
. In it Galileo conjectured that comets were not physical bodies but atmospheric effects like the
aurora borealis An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
.


Galileo's interest in comets

Three comets were seen in Europe in 1618. The first appeared in October, the second in mid-November, and the third and brightest at the end of November. After publishing ''
Letters on Sunspots '' Letters on Sunspots '' (''Istoria e Dimostrazioni intorno alle Macchie Solari'') was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it, Galileo outlined his recent observation of dar ...
'' in 1613, Galileo had largely stopped working on telescopic astronomy and he was to publish nothing further based on the observation and recording of data for astronomical events. In 1616 the heliocentric views of
Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulated ...
were declared formally heretical and Galileo was warned by
Cardinal Bellarmine Robert Bellarmine, SJ ( it, Roberto Francesco Romolo Bellarmino; 4 October 1542 – 17 September 1621) was an Italian Jesuit and a cardinal of the Catholic Church. He was canonized a saint in 1930 and named Doctor of the Church, one of only ...
to neither teach nor defend them. After this he was silent on astronomical matters for several years. However Virginio Cesarini wrote to him asking for his views on the 1618 comets, as did
Archduke Leopold of Austria Leopold I (Leopold Ignaz Joseph Balthasar Franz Felician; hu, I. Lipót; 9 June 1640 – 5 May 1705) was Holy Roman Emperor, King of Hungary, Croatia, and Bohemia. The second son of Ferdinand III, Holy Roman Emperor, by his first wife, Maria ...
and Domenico Bonsi, who wrote to him that the court mathematicians of
Louis XIII of France Louis XIII (; sometimes called the Just; 27 September 1601 – 14 May 1643) was King of France from 1610 until his death in 1643 and King of Navarre (as Louis II) from 1610 to 1620, when the crown of Navarre was merged with the French crown ...
all wanted to know his opinion on the phenomenon. Galileo had not observed the comets as he was unwell in the autumn of 1618. However he learned that the
Collegio Romano The Roman College ( la, Collegium Romanum, it, Collegio Romano) was a school established by St. Ignatius of Loyola in 1551, just 11 years after he founded the Society of Jesus (Jesuits). It quickly grew to include classes from elementary school t ...
had held four lectures on the comets, respectively by a theologian, a mathematician, a philosopher and a rhetorician. The mathematician was
Orazio Grassi Orazio Grassi, S.J. (b. Savona 1 May 1583 – d. Rome 23 July 1654), was an Italian Jesuit priest, who is best noted as a mathematician, astronomer and architect. He was one of the authors in controversy with Galileo Galilei on the nature of ...
, a pupil of Maelcote and Grienberger. Grassi argued that the absence of parallax meant that the comets must be very distant from the Earth, and he suggested that they existed beyond the Moon. Soon afterwards the lecture was published in Rome as an anonymous pamphlet entitled "De Tribus Cometis Anni MDCXVIII". In March 1619, Galileo received a letter from
Giovanni Battista Rinuccini Giovanni Battista Rinuccini (1592–1653) was an Italian Roman Catholic archbishop in the mid-seventeenth century. He was a noted legal scholar and became chamberlain to Pope Gregory XV. In 1625 Pope Urban VIII made him the Archbishop of Fermo ...
alerting him to the fact that some people outside the Jesuit order were claiming that Grassi's lecture on comets provided a definitive proof that Copernicus' ideas were wrong. The evident threat to Copernicus, whom Galileo could no longer defend, prompted him to attack the Tychonic ideas now popular among the Jesuits with particular force. Galileo received a copy of Grassi's lecture and was very angered by it. The notes he scribbled in the margin of his copy are full of insults - 'pezzo d'asinaccio' ('piece of utter stupidity'), 'bufolaccio' ('buffoon'), 'villan poltrone' ('wicked idiot'), 'balordone' ('bumbling idiot'). He decided to respond to it through his friend Mario Guiducci, who wanted a topic for his planned address to the Accademia Fiorentina. Guiducci read the ''Discourse on Comets'' at the Accademia Fiorentina in May 1619 and it was published the next month. In public, Galileo insisted that Guiducci, and not he, was the author of the ''Discourse on Comets''. Despite Galileo's public protestations, there is no doubt whatever that he was the main author of the ''Discourse on Comets''. The manuscript is largely in Galileo's handwriting, and the sections in Guiducci's hand have been revised and corrected by Galileo.


Conjecture

Grassi responded to the ''Discourse on Comets'' as if it was propounding a theory about their origin, but Galileo made clear that he was not doing this. Rather, as in his
Letter to the Grand Duchess Christina The "Letter to The Grand Duchess Christina" is an essay written in 1615 by Galileo Galilei. The intention of this letter was to accommodate Copernicanism with the doctrines of the Catholic Church. Galileo tried to use the ideas of Church Fathers ...
four years previously, his purpose was to insist that the burden of proof lay with those who had ideas that did not accord with his own. He intended to criticise pedantic thinkers who believed they had easily found a definitive answer to something, disregarding the fact that nature may have many possible ways of producing the same effect. He conceded that he knew very little about comets; his point was to expose those who were convinced that they knew the answers. The Discourse does not attempt to offer clear proofs of Galileo's conjectures, (unlike his ''
Letters on Sunspots '' Letters on Sunspots '' (''Istoria e Dimostrazioni intorno alle Macchie Solari'') was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it, Galileo outlined his recent observation of dar ...
'' or ''Discourse on Floating Bodies''); instead it focuses on arguments which undermine Grassi's contentions, forcing him to examine the phenomenon of comets more thoroughly and produce more substantial evidence for his argument that they are real.


Arguments in the ''Discourse''

The ''Discourse on Comets'', although formally a response to Grassi, was a rebuttal of the arguments made by
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was ...
. It advanced the proposition that the absence of parallax observable with comets was due not to their great distance from the Earth, but to the fact that they were not real objects; they were probably atmospheric effects. Galileo (through Guiducci) also argued against Tycho's case for comets having uniform, circular paths. Instead, he maintained, their paths were straight. Throughout, he expressed surprise that mathematicians at the Collegio (where Guiducci had been educated) could have adopted Tycho's positions so uncritically when his arguments were so poor. As well as attacking Grassi, the ''Discourse'' also continued an earlier dispute with
Christoph Scheiner Christoph Scheiner SJ (25 July 1573 (or 1575) – 18 June 1650) was a Jesuit priest, physicist and astronomer in Ingolstadt. Biography Augsburg/Dillingen: 1591–1605 Scheiner was born in Markt Wald near Mindelheim in Swabia, earlier markgrav ...
about sunspots, belittling the illustrations in Scheiner's book as 'badly coloured and poorly drawn.' Against classical authority: The ''Discourse'' opens with a review of the opinions on comets of Aristotle, Anaxagoras,
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...
,
Hippocrates of Chios Hippocrates of Chios ( grc-gre, Ἱπποκράτης ὁ Χῖος; c. 470 – c. 410 BC) was an ancient Greek mathematician, geometer, and astronomer. He was born on the isle of Chios, where he was originally a merchant. After some misadve ...
and
Seneca the Younger Lucius Annaeus Seneca the Younger (; 65 AD), usually known mononymously as Seneca, was a Stoic philosopher of Ancient Rome, a statesman, dramatist, and, in one work, satirist, from the post-Augustan age of Latin literature. Seneca was born in ...
. By showing how one contradicts the other, and indeed how Aristotle contradicts even himself, Galileo sought "to inculcate a certain skepticism and distrust of dogmatic authority, to encourage observation and mathematical analysis in preference to philosophical speculation, and to emphasise the vast extent of the unknown in comparison with the little men had gained as certain knowledge." Against the assumption that parallax can measure all visible objects: He cites phenomena such as haloes, rainbows and parhelia, none of which have parallax, and then refers to Pythagoras in suggesting that comets may be an optical illusion caused by light being reflected by a vertically rising column of vapour. Against misunderstanding of the telescope: Galileo refutes the claim by Grassi that when looking through a telescope one sees 'nearby objects are enlarged very much, and more distant ones less and less in proportion to their greater distance.' He demonstrates at considerable length that this is untrue, and urges the scholars of the Collegio Romano to correct such a serious fault in their understanding. Against Tycho: The final part of the ''Discourse'' is an assault on Tycho and his arguments. First he points out the implausibility of there being a celestial sphere devoted to comets, as they move in different directions and at different speeds. Next he argues from the apparent motion and speed of comets that they are more likely to travel in straight lines than in a circle, as Tycho had suggested.


Contrasts in Galileo's views

In the ''Discourse on Comets'' Galileo argued positions different to those in some of his other works. * Smoothness of heavenly bodies: The standard
cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
of Galileo's time, based on Aristotle's Physics, held that the stars and planets were perfectly smooth. Galileo had upset this view by proving that the Moon had mountains and valleys on its surface, and that the Sun had spots. Nevertheless, in ''Discourse on Comets'' Galileo argued that, on the supposition that the heavenly spheres were perfect, smooth and polished, Aristotle must be wrong in suggesting that comets were caused by friction fires in the celestial spheres. Grassi pointed this out in his reply to Galileo, ''Libra astronomica ac philosophica''. * Planetary rotation and atmospheric phenomena: In ''Letters on Sunspots'' Galileo claimed that sunspots were similar to clouds on Earth, carried around by the rotation of the Sun. In contrast, in ''Discourse on Comets'' Galileo conjectured that the atmospheric conditions that might produce comets as an optical effect were static and were not carried round by the Earth's motion. * The origins of comets: Although in the ''Discourse on Comets'' Galileo argued that the celestial origin of comets was by no means certain, he had argued the opposite case in August, 1612, in his Second Letter on Sunspots. Here he referred to Tycho's theory of the origin of comets to support his view that mutability in the heavens was not confined to the regions below the Moon: "As if to remove all doubt from our minds, a host of observations come to us to teach us that the comets are generated in the celestial regions." However, by 1632, in his
Dialogue Concerning the Two Chief World Systems The ''Dialogue Concerning the Two Chief World Systems'' (''Dialogo sopra i due massimi sistemi del mondo'') is a 1632 Italian-language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was tran ...
, Galileo was taking a different view: "As far as the comets are concerned, I, for my part, care little whether they are generated below or above the moon, nor have I ever set much store in Tycho's verbosity." * Artistotelian thinking: Since 1611, scholars of the Jesuit order had been bound by their General
Claudio Acquaviva Claudio Acquaviva, SJ (14 September 1543 – 31 January 1615) was an Italian Jesuit priest. Elected in 1581 as the fifth Superior General of the Society of Jesus, he has been referred to as the second founder of the Jesuit order. Early life an ...
to defend Aristotle's views on natural philosophy. In many instances (e.g. in ''Sidereus Nuncius'' and ''Letters on Sunspots'') Galileo argued against the received Aristotelian model and Jesuits who defended it. However, in the ''Discourse on Comets'' Galileo defended a view which posed no challenge to the main tenets of Aristotelian cosmology, while Grassi had in fact departed from the strictly Aristotelian view and embraced Tycho Brahe. Galileo adopted this line of argument in order to highlight the weaknesses of Tycho's system as a replacement for
Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...
.


Continuing dispute

The
Jesuit order , image = Ihs-logo.svg , image_size = 175px , caption = ChristogramOfficial seal of the Jesuits , abbreviation = SJ , nickname = Jesuits , formation = , founders = ...
of which Grassi was a member was very angry at the expression of Galileo's view in the ''Discourse on Comets.'' The pamphlet was a major factor in the alienation of the Jesuits from Galileo, who had previously been broadly supportive of his ideas, even despite his attacks on
Christoph Scheiner Christoph Scheiner SJ (25 July 1573 (or 1575) – 18 June 1650) was a Jesuit priest, physicist and astronomer in Ingolstadt. Biography Augsburg/Dillingen: 1591–1605 Scheiner was born in Markt Wald near Mindelheim in Swabia, earlier markgrav ...
. While Guiducci and Galileo were working in the ''Discourse'', a second anonymous Jesuit pamphlet appeared in
Milan Milan ( , , Lombard: ; it, Milano ) is a city in northern Italy, capital of Lombardy, and the second-most populous city proper in Italy after Rome. The city proper has a population of about 1.4 million, while its metropolitan city h ...
- ''Assemblea Celeste Radunata Nuovamente in Parnasso Sopra la Nuova Cometa''. This argued for the new model of the universe proposed by Tycho Brahe and against the traditional
cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
of Aristotle. Guiducci and Galileo collaborated on a response to this as well, which set out the arguments for a heliocentric model. The debate continued when, in
Perugia Perugia (, , ; lat, Perusia) is the capital city of Umbria in central Italy, crossed by the River Tiber, and of the province of Perugia. The city is located about north of Rome and southeast of Florence. It covers a high hilltop and part o ...
later in 1619, Grassi published a reply to the ''Discourse'' in ''La Libra Astronomica ac Philosophica'' under the pen-name Lotario Sarsi Sigensano. This work dismissed Guiducci as a mere 'copyist' for Galileo, and attacked Galileo's ideas directly. While the
Accademia dei Lincei The Accademia dei Lincei (; literally the "Academy of the Lynx-Eyed", but anglicised as the Lincean Academy) is one of the oldest and most prestigious European scientific institutions, located at the Palazzo Corsini on the Via della Lungara in Rom ...
were considering what tone a reply from Galileo ought to take, Guiducci replied directly to Grassi in the Spring of 1620. The reply was formally addressed to another Jesuit, Father Tarquinio Galluzzi, his old rhetoric master. Guiducci countered the various arguments Grassi had put forward against Galileo, describing some of Grassi's experiments as 'full of errors and not without a hint of fraud.' Guiducci concluded with an attempt to reconcile experimental evidence with theological arguments, but firmly asserted the primacy of data gathered through observation. Galileo was very pleased with Guiducci's efforts, proposing him for membership of the Accademia dei Lincei in May 1621 (although he did not actually become a member until 1625). Galileo's final response in the dispute with Grassi was ''Il Saggiatore'' (''
The Assayer ''The Assayer'' ( it, Il Saggiatore) was a book published in Rome by Galileo Galilei in October 1623 and is generally considered to be one of the pioneering works of the scientific method, first broaching the idea that the book of nature is to be ...
'') which he published in 1623. Grassi replied in 1626 with ''Ratio ponderum librae et simbellae'' which focused on doctrinal issues rather than scientific questions. Having defeated Grassi on the points he considered important, Galileo declined to publish anything further on the topic.


See also

* Observational history of comets *
Copernican Revolution The Copernican Revolution was the paradigm shift from the Ptolemaic model of the heavens, which described the cosmos as having Earth stationary at the center of the universe, to the heliocentric model with the Sun at the center of the Solar Sys ...


External links


Orazio Grassi's ''De Tribus Cometis Anni MDCXVIII'' (Rome, 1618)Guiducci & Galilei's ''Discorso delle Comete'' (Florence, 1619)Orazio Grassi's ''Libra Astronomica ac Philosophica'' (Perugia, 1619)Guiducci's ''Lettera al m.r.p. Tarquinio Galluzzi della Compagnia di Giesù'' (Florence 1620)


References

{{Galileo Galilei 1619 books Galileo Galilei 1619 in science Comets Jesuit scientists Catholic Church and science