**Christiaan Huygens** was a Dutch mathematician who patented the first pendulum clock, which greatly increased the accuracy of time measurement. He laid the foundations of mechanics and also worked on astronomy and probability.

- It was through him that Christiaan was to gain access to the top scientific circles of the times.
- Tutored at home by private teachers until he was 16 years old, Christiaan learned geometry, how to make mechanical models and social skills such as playing the lute.
- His mathematical education was clearly influenced by Descartes who was an occasional visitor at the Huygens' home and took a great interest in the mathematical progress of the young Christiaan.
- Christiaan Huygens studied law and mathematics at the University of Leiden from 1645 until 1647.
- Although John Pell was a teacher at Breda about this time, he seems to have had little contact with Huygens.
- Mersenne challenged Huygens to solve a number of problems including the shape of the rope supported from its ends.
- In 1649 Huygens went to Denmark as part of a diplomatic team and hoped to continue to Stockholm to visit Descartes but the weather did not allow him to make this journey.
- Huygens's first publications in 1651 and 1654 considered mathematical problems.
- Huygens' 1654 work De Circuli Magnitudine Inventa Ⓣ(Finding the magnitude of the circle) was a more major work on similar topics.
- Huygens soon turned his attention to lens grinding and telescope construction.
- Using one of his own lenses, Huygens detected, in 1655, the first moon of Saturn.
- He informed the mathematicians in Paris including Boulliau of his discovery and in turn Huygens learnt of the work on probability carried out in a correspondence between Pascal and Fermat.
- On his return to Holland Huygens wrote a small work De Ratiociniis in Ludo Aleae on the calculus of probabilities, the first printed work on the subject.
- Boulliau had failed to detect Saturn's moon Titan so Huygens realised that he was using an inferior telescope.
- By 1656 Huygens was able to confirm his ring theory to Boulliau and the results were reported to the Paris group.
- In Systema Saturnium (1659), Huygens explained the phases and changes in the shape of the ring.
- Some, including the Jesuit Fabri, attacked not only Huygens theories but also his observations.
- However by 1665 even Fabri was persuaded to accept Huygens' ring theory as improving telescopes confirmed his observations.
- Work in astronomy required accurate timekeeping and this prompted Huygens to tackle this problem.
- Huygens believed that a pendulum swinging in a large are would be more useful at sea and he invented the cycloidal pendulum with this in mind.
- As a result of this Huygens, Hooke, Halley and Wren formulated the inverse-square law of gravitational attraction.
- Huygens returned to Paris in 1660 and went to meetings of various scientific societies there.
- In 1661 Huygens visited London, particularly to find out more about the newly forming Royal Society meeting at that time in Gresham College.
- The Duke and Duchess of York came to observe the Moon and Saturn through Huygens' telescope.
- While in London Huygens saw Boyle's vacuum pump and he was impressed.
- Huygens was elected to the Royal Society of London in 1663.
- At this time Huygens patented his design of pendulum clock with the solution of the longitude problem in mind.
- Huygens wrote to Hooke doubting this approach which he felt would be unduly affected by temperature changes.
- Despite this Huygens did begin to experiment with clocks regulated by springs, but their accuracy was poorer than his pendulum clocks.
- Huygens accepted an invitation from Colbert in 1666 to become part of the Académie Royale des Sciences.
- After meetings were held with Roberval, Carcavi, Auzout, Frenicle de Bessy, Auzout and Buot in Colbert's library the Society moved to the Bibliothèque du Roi where Huygens took up residence.
- Huygens' work on the collision of elastic bodies showed the error Descartes' laws of impact and his memoir on the topic was sent to the Royal Society in 1668.
- The Royal Society had posed a question on impact and Huygens proved by experiment that the momentum in a fixed direction before the collision of two bodies is equal to the momentum in that direction after the collision.
- Circular motion was a topic which Huygens took up at this time but he also continued to think about Descartes' theory of gravity based on vortices.
- He seems to have shown signs of being unhappy with Descartes' theory around this time but he still addressed the Académie on this topic in 1669 although after his address Roberval and Mariotte argued strongly, and correctly, against Descartes' theory and this may have influenced Huygens.
- From his youth Huygens' health had never been robust and in 1670 he had a serious illness which resulted in him leaving Paris for Holland.
- By 1671 Huygens returned to Paris.
- However in 1672 Louis XIV invaded the Low Countries and Huygens found himself in the extremely difficult position of being in an important position in Paris at a time France was at war with his own country.
- In 1672 Huygens and Leibniz met in Paris and thereafter Leibniz was a frequent visitor to the Académie.
- Leibniz owes much to Huygens from whom he learnt much of his mathematics.
- In this same year Huygens learnt of Newton's work on the telescope and on light.
- His own work, Horologium Oscillatorium sive de motu pendulorum Ⓣ(The pendulum clock and the motion of the pendulum) appeared in 1673 and showed that Huygens had moved far from Descartes' influence.
- In it Huygens proves that the cycloid is tautochronous, an important theoretical result but one which had little practical application to the pendulum.
- Huygens describes the descent of bodies in a vacuum, either vertically or along curves.
- Huygens attempts for the first time in this work to study the dynamics of bodies rather than particles.
- Papin worked as an assistant to Huygens around this time and after he left to work with Boyle, Huygens was joined by Tschirnhaus.
- Another bout of illness in 1676 saw Huygens return to the Hague again.
- By 1678 Huygens had returned to Paris.
- In that year his Traité de la lumiere Ⓣ(Treatise on light) appeared, in it Huygens argued in favour of a wave theory of light.
- Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
- La Hire, who had always argued against foreigners in the Académie, sent his best wishes to Huygens but he clearly hoped that he would not return so that he might himself might acquire his position.
- The longitude problem had remained a constant cause for Huygens to continue work on clocks all his life.
- Huygens missed having people around him with whom he could discuss scientific topics.
- In England Huygens met Newton, Boyle and others in the Royal Society.
- In some sense of course Huygens was right, how can one believe that two distant masses attract one another when there is nothing between them, nothing in Newton's theory explains how one mass can possible even know the other mass is there.
- In the final years of his life Huygens composed one of the earliest discussions of extraterrestrial life, published after his death as the Cosmotheoros Ⓣ(Theory of the cosmos) (1698).
- Huygens described the 31-tone equal temperament in Lettre touchant le cycle harmonique Ⓣ(Letter about the harmonic cycle).
- Huygens was the greatest mechanist of the seventeenth century.
- the ideas of mass, weight, momentum, force, and work were finally clarified in Huygens' treatment of the phenomena of impact, centripetal force and the first dynamical system ever studied - the compound pendulum.

Born 14 April 1629, The Hague, Netherlands. Died 8 July 1695, The Hague, Netherlands.

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