Jacques du Chevreul

Jacques du Chevreul was born 1595 in Cotounces which is a commune in the Manche department in Normandy in north-western France. He was born a year earlier than René Descartes, and died a year before Descartes as well. Unlike Descartes, du Chevreul was always associated with a university, specifically the University of Paris and the College of Harcourt.[2] He was the son of a magistrate, a civil officer or judge who administers the law. Descartes’ father was also a magistrate, adding to their similarities. Jacques du Chevreul's father held a Master's of Law from Poitiers.[2] Although his father studied law, Du Chevreul decided to take his career in a different direction.

He started his education in humanities and philosophy in which he received a master's degree (Master of Arts) from the University of Paris in 1616. After receiving his master's degree, unlike many of his peers, he decided to continue his education in the higher faculty of theology and received the degree of Bachelor of Divinity in 1619.[1] Du Chevreul held Aristotelian views on mathematics and began a teaching career in 1620 at Harcourt, which is another commune in the Normandy region of northern France.[2] He taught primarily philosophy, but also some mathematics.[2][1] The duration of the philosophy course was two years, and it consisted of a year of Logic and Ethics followed by another year of Metaphysics and Physics.[1] From the 1620s to the 1640s, Jacques du Chevreul was a professor at the University of Paris.[4] In the years 1623 through 1626 and 1633 through 1634, du Chevreul taught the Logic and Ethics curriculum.[4] In the years 1628 through 1629 and 1634 through 1635, du Chevreul taught the Metaphysics and Physics curriculum.[4] Two years prior to his death, du Chevreul also worked at the Collège Royal as a professor of philosophy.[1] Jacques du Chevreul died in the year 1649 in Paris, France.[4]

Title page of Sphaera published in Paris in 1629. Reproduced from an original copy in History of Science Collections, University of Oklahoma Libraries.

Although he did not publish his philosophy lectures, he did publish two mathematical texts. The mathematical curriculum in the early 17th century was divided into the quadrivium, the four liberal arts of arithmetic, geometry, music, and astronomy together with optics and other mathematically based disciplines.[2] During his time at the University of Paris, du Chevreul published two mathematical works, Arithmetica in 1622 and Sphaera in 1623.[1] The basis of astronomical teaching at the time was the textbook genre known as the Sphere.[1]

The seventeenth century, a two-dimensional model drawn by du Chevreul was meant to be grasped as a solid, however the technology was not available to make a three-dimensional layout.  Jacques du Chevreul's eccentric-epicycle geocentric template of the universe had elements like previous astronomers Ptolemy and Aristotle and modifications based on Galileo's telescopic observations.[3] The model, created in 1623 had, unlike other geocentric models of its time, sunspots rotating about the sun, in the form of the Bourbon Planets suggested by Jean Tarde. Planets Mercury, Venus, Mars, Jupiter, and Saturn were also in rotation around the earth.[1] Two planets Jupiter and Saturn, had each been given moons surrounding them, with Jupiter having four and Saturn given two. The sunspots that du Chevreul incorporated into his depiction caused mixed interpretations between professions.[3] Some mathematicians did not acknowledge such claims while schoolmen widely around France had followed Tarde's suggestion and thought the sunspots were small planets, as in du Chevreul's depiction.[3] In the traditional view of that time no modifications he made to his earlier model needed to be significantly changed although Jacques did make changes to Mercury and Venus planet phases, the moons of Jupiter, sunspots, and the handles that surrounded the planet Saturn.[1] Du Chevreul also accounted for the lunar spots/sunspots as celestial matter condensations and rarefactions.[3] Through the modifications, du Chevreul's depiction of the universe maintained its spherical form, its central earth, and epicyclic-eccentric model.[3]

In this image, Jacques du Chevreul displays the earth, the moon, and the planets in a 3 o'clock Copernican arrangement. Opposite of the earth, but in the same orb, at 9 o'clock, du Chevreul adds the counter-earth required by original Pythagoreans. Reproduced from an original copy in History of Science Collections, University of Oklahoma Libraries.

Du Chevreul argued that the shape of the Earth was spherical for many reasons in Sphaera:

1. Its fitness: A sphere has a larger capacity than other shapes and it can rotate around its axis because it is well accommodated to circular motion. The Earth is massive and contains so much within it. It also rotates on its axis. Therefore, it must be a sphere.[1]
2. Its necessity: If the world were not spherical, a vacuum would have to exist. Vacuums do not exist in earth's nature; therefore it must be a shape that is impenetrable.[1]
3. The nature of things: Based on natural events such as gravity, the movement of the earth's crust, and meteorological events such as cloud formation, it can be concluded that the Earth must have a spherical shape.[1]
4. The appearances: Stars are fixed, and yet each has a unique distance from the earth. Additionally, stars rise and set during the night, and their distance with respect to the horizon changes as one changes his/her position on the Earth.[1]
5. During a lunar eclipse, the Earth's shadow is circular in shape.[1]

Du Chevreul rejected the Copernican theories on the system of the universe for many reasons. First, he believed that the Earth was at the center of the universe and did not orbit. He used the teachings of Plato, Aristotle, and other philosophers and astrologers who asserted this geocentric idea and cited Plato in the Timaeus and Phaedo and Aristotle in the De caelo.[1] Second, he looked to passages in the Bible for theological support. One passage stated that when God created the Earth, he did not intend for it to move. Another passage referenced the miracle witnessed by the biblical character Joshua in which God halted the movement of the sun and the moon. The use of Biblical Scriptures was consistent with the 1616 condemnation of Copernicanism by the Catholic Church as well as du Chevreul's own views on religion and astronomy.[1] Du Chevreul also argued that Copernicus's theories were inconsistent with the physics concerning the motion of a simple body because Copernicus believed the simple body- the Earth- would have two motions however simple bodies can only have a single and simple motion. Even though he claimed that Earth was at the center of the universe, du Chevreul also claimed that Venus and Mercury orbited the Sun. The root of this theory was his claim that spots seen on the Sun are actually Venus and Mercury rather than the fact that they have phases like the moon.[1]

On the heavens, particularly on the question of whether or not they can change, du Chevreul took a probabilistic approach, and theorized that the brightness of the stars indicated how dense and how rare they were. He classified stars into two categories: stars that did not move (fixed) and stars that did move (wandering). He classified the planets, the Sun, and the Moon as wandering stars. Following in the steps of Aristotle and Plato, he confirmed that there are seven wandering stars: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon. However, he also counted the four Medician stars that orbit Jupiter, which were discovered by Galileo, and he counted the two planets that orbit Saturn. Therefore, he counted thirteen planets in all.[1]

du Chevreul believed that Aristotle's observations should in general be taken into consideration; however there were some points where du Chevreul disagreed with him such as the number of levels in the heavens.[1][4] According to Aristotle, along with Plato, the number of levels of the heavens was at least eight, which included Saturn, Venus, Mercury, Jupiter, Mars, the Sun, the Moon, and the stars. du Chevreul had a different theory in which he counted only five levels: Saturn, Jupiter, Mars, the Sun, and the Moon. Unlike his predecessors who either argued that Venus and Mercury were above the Sun or below it, du Chevreul asserted all of the above. Since Venus and Mercury orbit the Sun, du Chevreul deduced, they could be found both above and below it. Thus, Venus and Mercury were located within the Sun's level and did not have levels of their own. The following are du Chevreul's levels of the heavens in order, with Earth located at the center:

1. The Moon
2. The Sun, in which Mercury and Venus circulate around the sun
3. Mars
4. Jupiter, in which the four Medicean stars circulate around Jupiter
5. Saturn, in which two planets circulate around Saturn

Keeping in line with biblical scriptures, du Chevreul claimed that above all these levels was the celestial waters and where God resigned, known as the Empyrean Heaven.[1]

Jacques du Chevreul's observations were in line with Galileo's observations as he accepted all of the 1610-1613 telescope findings. These findings were: more stars, four moons around Jupiter, the “handles” of Saturn from Starry Messenger (1610), as well as “sunspots” and phases of Venus from Letters on Sunspots (1613). Instead interpreting the “handles” of Saturn as two moons that orbit around Saturn and sunspots as small planets orbiting the Sun. Jacques du Chevreul modified Aristotelianism to account for Galileo's observations, however does not specifically credit Galileo. Du Chevreul also accepted Galileo's other observations that cooperated with Aristotelian astronomy, as seen in the chapter covering eccentric and epicyclic orbs, De ordine partium. Eccentrics are crucial because the parallax, their distances from the center of the world, of planets are changeable, or vary as shown through astronomical observations. Epicycles are also necessary because planets can slow down or speed up, as seen through astronomical observations.[1]

Jacques du Chevreul agreed with Galileo's observations with one exception. Galileo discovered that the moon had mountains and valleys that were similar to those of the earth. du Chevreul's response to this was that if the moon did have mountains and valleys, then the moon would also have areas of empty space, which nature does not allow. Instead, du Chevreul believed that what Galileo had saw were actually the varying densities of the moon and Galileo did not make a direct observation, instead making an inference based on a false assumption.[1] Just as a star's brightness indicates its density and rarity, so does the variance of brightness on the moon.

Another astronomer's observations that Jacques du Chevreul rejected were those of Tycho Brahe. He not only rejected the Tychonic description of the heavens, but he also rejected Brahe's theory that Mars was below the Sun. Jacques de Chevreul argued that Mars cannot be located below the Sun, because for that to occur the heavens would have to be permeable which directly contradicts the appearances. He continued on in his section on the matter of the world and denied the language that Tycho followers used. For example, the verbiage that the stars wander in the heavens like fish swimming in water was contrary to what Jacques du Chevreul believed.[1] Additionally, to accept Brahe's theories of the universe was to reject eccentrics and epicycles, as well as the planetary heavens being liquid and permeable, which du Chevreul was not willing to do as it entailed making more adjustments to traditional astronomy. He argued for the existence of eccentrics because observations had shown that as planets orbited, their distance from the center of the universe varied. Epicycles, he claimed, revealed the idea that the planets vary in their speed as they orbit around the center of the universe.[1] His model of epicycles and eccentrics was three dimensional and spherical.[2] This was consistent with the Aristotelean and Ptolemaic models which were popular in Paris during the 1600s.[3] In Paris during the 1640s and 1650s, the Tychonic system became popular and thus the universe became a three heaven model.[7] This consisted of the planetary heaven being liquid and permeable, the solid firmament (or sphere of fixed stars), and lastly the Empyrean.[1]

Jacques du Chevreul produced a unique take on astronomy by maintaining the most traditional Aristotelian astronomy possible while formally rejecting the Tychonic system. He also integrated and adjusted for the observations being made during the time, particularly from Galileo.[8] He incorporated the moons of Jupiter and Saturn, the phases of Venus and Mercury, and sunspots into his system while keeping epicycles and eccentrics. While he rejected the Copernican system due to its contradiction to both physics and the Scriptures, he did not reject the arrangement of the Earth and the Moon around the Sun as it was like a terrestrial orb with the Moon contained similarly to an epicycle with the four elements.[1]