Christiaan Huygens

Christiaan Huygens, Lord of Zeelhem, lived from 14 April 1629 to 8 July 1695. A Dutch mathematician, physicist, engineer, astronomer, and inventor, he is considered a major figure of the Scientific Revolution. In physics, his work in optics and mechanics was groundbreaking. As an astronomer, he examined Saturn’s rings and found its largest moon, Titan. As an inventor and engineer, he made telescopes better and created the pendulum clock, which remained the most precise timekeeping device for nearly three centuries. Huygens also wrote the first modern treatise to idealize a physical problem using mathematical parameters, and his study of light provided the first mathematical, mechanistic account of an invisible physical phenomenon.

Huygens correctly identified the laws of elastic collision in *De Motu Corporum ex Percussione*, finished in 1656 but not published until 1703. In 1659, he used geometry to derive the formula for centrifugal force in classical mechanics, in his work *De Vi Centrifuga*, a decade before Isaac Newton. In optics, he is most famous for his wave theory of light, presented in *Traité de la Lumière* (1690). This theory was initially dismissed in favor of Newton’s corpuscular model, until Augustin-Jean Fresnel adapted Huygens’s idea in 1821 to fully explain how light travels in straight lines and diffracts. Today, this is called the Huygens–Fresnel principle.

Huygens patented the pendulum clock in 1657, and it was manufactured in Paris by Isaac II Thuret. His work on timekeeping led to a deep study of the pendulum in *Horologium Oscillatorium* (1673), one of the most important mechanics texts of the 1600s. Though it includes clock designs, most of the book analyzes pendular motion and the theory of curves. In 1655, Huygens began grinding lenses with his brother Constantijn to build refracting telescopes. He discovered Saturn’s largest moon, Titan, and was the first to explain Saturn’s odd appearance as a thin, flat ring that does not touch the planet and is tilted to the ecliptic. In 1662, he created the Huygenian eyepiece, a telescope with two lenses that reduced light dispersion.

As a mathematician, Huygens developed his theory of evolutes and wrote about games of chance and the problem of points in *Van Rekeningh in Spelen van Gluck*, which Frans van Schooten translated and published as *De Ratiociniis in Ludo Aleae* (1657). His use of expected values, along with that of others, later inspired Jacob Bernoulli’s work on probability.

Huygens was born into a wealthy, influential Dutch family in The Hague on 14 April 1629, the second son of Constantijn Huygens. He was named after his paternal grandfather. His mother, Suzanna van Baerle, died soon after giving birth to his sister. The couple had five children: Constantijn (1628), Christiaan (1629), Lodewijk (1631), Philips (1632), and Suzanna (1637). Constantijn Huygens was a diplomat and advisor to the House of Orange, as well as a poet and musician. He corresponded widely with European intellectuals, including Galileo Galilei, Marin Mersenne, and René Descartes. Christiaan was educated at home until age sixteen and enjoyed playing with miniature mills and machines. His father gave him a liberal education in languages, music, history, geography, mathematics, logic, and rhetoric, along with dancing, fencing, and horse riding. In 1644, his mathematics tutor was Jan Jansz Stampioen, who gave the fifteen-year-old a demanding reading list on modern science. Descartes later admired his geometry skills, and Mersenne called him the “new Archimedes.”

At sixteen, Constantijn sent Christiaan to study law and mathematics at Leiden University, where he enrolled from May 1645 to March 1647. Frans van Schooten Jr., a professor at Leiden’s Engineering School, became private tutor to Christiaan and his older brother Constantijn Jr., replacing Stampioen on Descartes’s advice. Van Schooten updated Huygens’s mathematical education, especially on the work of Viète, Descartes, and Fermat. After two years, starting in March 1647, Huygens continued at the newly founded Orange College in Breda, where his father was a curator. Constantijn Huygens was closely involved with the college, which lasted only until 1669; the rector was André Rivet. Christiaan lived at the home of jurist Johann Henryk Dauber while attending college and took mathematics classes with English lecturer John Pell. His time in Breda ended around when his brother Lodewijk, also a student, dueled with another student. Huygens left Breda after finishing his studies in August 1649 and served briefly as a diplomat on a mission with Henry, Duke of Nassau. After stays in Bentheim and Flensburg in Germany, he visited Copenhagen and Helsingør in Denmark. He hoped to cross the Øresund to see Descartes in Stockholm, but Descartes died before he could.

Although his father wanted Christiaan to become a diplomat, circumstances prevented it. The First Stadtholderless Period, beginning in 1650, removed the House of Orange from power, weakening Constantijn’s influence. Moreover, Constantijn realized his son had no interest in such a career.

Huygens generally wrote in French or Latin. In 1646, while a student at Leiden, he began corresponding with his father’s friend Marin Mersenne, who died soon after in 1648. Mersenne wrote to Constantijn about his son’s mathematical talent and, on 3 January, flatteringly compared him to Archimedes.