On this day:
1632 – Galileo’s Dialogue Concerning the Two Chief World Systems is published.
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 translated into Latin as Systema cosmicum (English: Cosmic System) in 1635 by Matthias Bernegger. The book was dedicated to Galileo’s patron, Ferdinando II de’ Medici, Grand Duke of Tuscany, who received the first printed copy on February 22, 1632.
In the Copernican system, the Earth and other planets orbit the Sun, while in the Ptolemaic system, everything in the Universe circles around the Earth. The Dialogue was published in Florence under a formal license from the Inquisition. In 1633, Galileo was found to be “vehemently suspect of heresy” based on the book, which was then placed on the Index of Forbidden Books, from which it was not removed until 1835 (after the theories it discussed had been permitted in print in 1822). In an action that was not announced at the time, the publication of anything else he had written or ever might write was also banned in Catholic countries.
While writing the book, Galileo referred to it as his Dialogue on the Tides, and when the manuscript went to the Inquisition for approval, the title was Dialogue on the Ebb and Flow of the Sea. He was ordered to remove all mention of tides from the title and to change the preface because granting approval to such a title would look like approval of his theory of the tides using the motion of the Earth as proof. As a result, the formal title on the title page is Dialogue, which is followed by Galileo’s name, academic posts, and followed by a long subtitle. The name by which the work is now known was extracted by the printer from the description on the title page when permission was given to reprint it with an approved preface by a Catholic theologian in 1744. This must be kept in mind when discussing Galileo’s motives for writing the book. Although the book is presented formally as a consideration of both systems (as it needed to be in order to be published at all), there is no question that the Copernican side gets the better of the argument.
The book is presented as a series of discussions, over a span of four days, among two philosophers and a layman:
Salviati argues for the Copernican position and presents some of Galileo’s views directly, calling him the “Academician” in honor of Galileo’s membership in the Accademia dei Lincei. He is named after Galileo’s friend Filippo Salviati (1582–1614).
Sagredo is an intelligent layman who is initially neutral. He is named after Galileo’s friend Giovanni Francesco Sagredo (1571–1620).
Simplicio, a dedicated follower of Ptolemy and Aristotle, presents the traditional views and the arguments against the Copernican position. He is supposedly named after Simplicius of Cilicia, a sixth-century commentator on Aristotle, but it was suspected the name was a double entendre, as the Italian for “simple” (as in “simple minded”) is “semplice”. Simplicio is modeled on two contemporary conservative philosophers, Lodovico delle Colombe (Italian) (1565–1616?), Galileo’s fiercest detractor, and Cesare Cremonini (1550–1631), a Paduan colleague who had refused to look through the telescope. Colombe was the leader of a group of Florentine opponents of Galileo’s, which some of the latter’s friends referred to as “the pigeon league”.
The discussion is not narrowly limited to astronomical topics, but ranges over much of contemporary science. Some of this is to show what Galileo considered good science, such as the discussion of William Gilbert’s work on magnetism. Other parts are important to the debate, answering erroneous arguments against the Earth’s motion.
A classic argument against earth motion is the lack of speed sensations of the earth surface, though it moves, by the earth’s rotation, at about 1700 km/h at the equator. In this category there is a thought experiment in which a man is below decks on a ship and cannot tell whether the ship is docked or is moving smoothly through the water: he observes water dripping from a bottle, fish swimming in a tank, butterflies flying, and so on; and their behavior is just the same whether the ship is moving or not. This is a classic exposition of the Inertial frame of reference and refutes the objection that if we were moving hundreds of kilometres an hour as the Earth rotated, anything that one dropped would rapidly fall behind and drift to the west.
The bulk of Galileo’s arguments may be divided into three classes:
Rebuttals to the objections raised by traditional philosophers; for example, the thought experiment on the ship.
Observations that are incompatible with the Ptolemaic model: the phases of Venus, for instance, which simply couldn’t happen, or the apparent motions of sunspots, which could only be explained in the Ptolemaic or Tychonic systems as resulting from an implausibly complicated precession of the Sun’s axis of rotation.
Arguments showing that the elegant unified theory of the Heavens that the philosophers held, which was believed to prove that the Earth was stationary, was incorrect; for instance, the mountains of the Moon, the moons of Jupiter, and the very existence of sunspots, none of which was part of the old astronomy.
Generally, these arguments have held up well in terms of the knowledge of the next four centuries. Just how convincing they ought to have been to an impartial reader in 1632 remains a contentious issue.
Galileo attempted a fourth class of argument:
Direct physical argument for the Earth’s motion, by means of an explanation of tides.
As an account of the causation of tides or a proof of the Earth’s motion, it is a failure. The fundamental argument is internally inconsistent and actually leads to the conclusion that tides do not exist. But, Galileo was fond of the argument and devoted the “Fourth Day” of the discussion to it.
The degree of its failure is—like nearly anything having to do with Galileo—a matter of controversy. On the one hand, the whole thing has recently been described in print as “cockamamie.” On the other hand, Einstein used a rather different description:
It was Galileo’s longing for a mechanical proof of the motion of the earth which misled him into formulating a wrong theory of the tides. The fascinating arguments in the last conversation would hardly have been accepted as proof by Galileo, had his temperament not got the better of him. [Emphasis added]
Born on this day:
1796 – Adolphe Quetelet, Belgian mathematician, astronomer, and sociologist (d. 1874)
Lambert Adolphe Jacques Quetelet (French: [kətlɛ]; 22 February 1796 – 17 February 1874) ForMemRS was a Belgian astronomer, mathematician, statistician and sociologist. He founded and directed the Brussels Observatory and was influential in introducing statistical methods to the social sciences. His name is sometimes spelled with an accent as Quételet. He developed the body mass index scale.
Adolphe was born in Ghent (which, at the time was a part of the new French Republic), the son of François-Augustin-Jacques-Henri Quetelet, a Frenchman and Anne Françoise Vandervelde, a Flemish woman. His father, François, was born at Ham, Picardy, and being of a somewhat adventurous spirit, he crossed the English Channel and became both a British citizen and the secretary of a Scottish nobleman. In that capacity, he traveled with his employer on the Continent, particularly spending time in Italy. At about 31, he settled in Ghent and was employed by the city, where Adolphe was born the fifth of nine children, several of whom died in childhood.
Francois died when Adolphe was only seven years old. Adolphe studied at the Ghent lycée, where he started teaching mathematics in 1815 at the age of 19. In 1819 he moved to the Athenaeum in Brussels and in the same year he completed his dissertation (De quibusdam locis geometricis, necnon de curva focal – Of some new properties of the focal distance and some other curves).
Quetelet received a doctorate in mathematics in 1819 from the University of Ghent. Shortly thereafter, the young man set out to convince government officials and private donors to build an astronomical observatory in Brussels; he succeeded in 1828. He became a member of the Royal Academy in 1820. He lectured at the museum for sciences and letters and at the Belgian Military School. In 1825 he became correspondent of the Royal Institute of the Netherlands, in 1827 he became member. From 1841 to 1851 he was supernumerair’ associate in the Institute, and when it became Royal Netherlands Academy of Arts and Sciences he became foreign member. In 1850, he was elected a foreign member of the Royal Swedish Academy of Sciences.
Quetelet also founded several statistical journals and societies, and was especially interested in creating international cooperation among statisticians. He encouraged the creation of a statistical section of the British Association for the Advancement of Science (BA), which later became the Royal Statistical Society, of which he became the first overseas member.
In 1855 Quetelet suffered from apoplexy, which diminished but did not end his scientific activity. He died in Brussels on 17 February 1874, and is buried in the Brussels Cemetery.
His scientific research encompassed a wide range of different scientific disciplines: meteorology, astronomy, mathematics, statistics, demography, sociology, criminology and history of science. He made significant contributions to scientific development, but he also wrote several monographs directed to the general public. He founded the Royal Observatory of Belgium, founded or co-founded several national and international statistical societies and scientific journals, and presided over the first series of the International Statistical Congresses. Quetelet was a liberal and an anticlerical, but not an atheist or materialist nor a socialist.
The new science of probability and statistics was mainly used in astronomy at the time, where it was essential to account for measurement errors around means. This was done using the method of least squares. Quetelet was among the first to apply statistics to social science, planning what he called “social physics”. He was keenly aware of the overwhelming complexity of social phenomena, and the many variables that needed measurement. His goal was to understand the statistical laws underlying such phenomena as crime rates, marriage rates or suicide rates. He wanted to explain the values of these variables by other social factors. These ideas were rather controversial among other scientists at the time who held that it contradicted the concept of freedom of choice.
His most influential book was Sur l’homme et le développement de ses facultés, ou Essai de physique sociale, published in 1835 (In English translation, it is titled Treatise on Man, but a literal translation would be “On Man and the Development of his Faculties, or Essays on Social Physics”). In it, he outlines the project of a social physics and describes his concept of the “average man” (l’homme moyen) who is characterized by the mean values of measured variables that follow a normal distribution. He collected data about many such variables.
When Auguste Comte discovered that Quetelet had appropriated the term ‘social physics’, which Comte had originally introduced, Comte found it necessary to invent the term ‘sociologie’ (sociology) because he disagreed with Quetelet’s collection of statistics.
Quetelet was an influential figure in criminology. Along with Andre-Michel Guerry, he helped to establish the cartographic school and positivist schools of criminology which made extensive use of statistical techniques. Through statistical analysis, Quetelet gained insight into the relationships between crime and other social factors. Among his findings were strong relationships between age and crime, as well as gender and crime. Other influential factors he found included climate, poverty, education, and alcohol consumption, with his research findings published in Of the Development of the Propensity to Crime.
In his 1835 text on social physics, in which he presented his theory of human variance around the average, with human traits being distributed according to a normal curve, he proposed that normal variation provided a basis for the idea that populations produce sufficient variation for artificial or natural selection to operate.
In terms of influence over later public health agendas, one of Quetelet’s lasting legacies was the establishment of a simple measure for classifying people’s weight relative to an ideal for their height. His proposal, the body mass index (or Quetelet index), has endured with minor variations to the present day. Anthropometric data is used in modern applications and referenced in the development of every consumer-based product.