Jean Picard sundial on the pediment of the Sorbonne
|Died||12 July 1682 (aged 61)|
|Mesure de la Terre|
He is principally notable for his accurate measure of the size of the Earth, based on a careful survey of one degree of latitude along the Paris Meridian.
Picard was the first person to measure the size of the Earth to a reasonable degree of accuracy in a survey conducted in 1669–70, for which he is honored with a pyramid at Juvisy-sur-Orge. Guided by Maurolycus's methodology and Snellius's mathematics for doing so, Picard achieved this by measuring one degree of latitude along the Paris Meridian using triangulation along thirteen triangles stretching from Paris to the clocktower of Sourdon, near Amiens.
His measurements produced a result of 110.46 km for one degree of latitude, which gives a corresponding terrestrial radius of 6328.9 km. Isaac Newton was to use this value in his theory of universal gravitation.
The polar radius has now been measured at just over 6357 km. This was an error only 0.44% less than the modern value. This was another example of advances in astronomy and its tools making possible advances in cartography.
Picard was the first to attach a telescope with crosswires (developed by William Gascoigne) to a quadrant, and one of the first to use a micrometer screw on his instruments. The quadrant he used to determine the size of the Earth had a radius of 38 inches and was graduated to quarter-minutes. The sextant he used to find the meridian had a radius of six feet, and was equipped with a micrometer to enable minute adjustments. These equipment improvements made the margin of error only ten seconds, as opposed to Tycho Brahe's four minutes of error. This made his measurements 24 times as accurate.
Picard collaborated and corresponded with many scientists, including Isaac Newton, Christiaan Huygens, Ole Rømer, Rasmus Bartholin, Johann Hudde, and even his main competitor, Giovanni Cassini, although Cassini was often less than willing to return the gesture. These correspondences led to Picard's contributions to areas of science outside the field of geodesy, such as the aberration of light he observed while in Uraniborg, or his discovery of mercurial phosphorescence upon his observance of the faint glowing of a barometer. This discovery led to Newton's studies of light's visible spectrum.
Picard also developed what became the standard method for measuring the right ascension of a celestial object. In this method, the observer records the time at which the object crosses the observer's meridian. Picard made his observations using the precision pendulum clock that Dutch physicist Christiaan Huygens had recently developed.
- Débarbat, Suzanne; Wilson, Curtis (2003). "The Galilean satellites of Jupiter from Galileo to Cassini, Roemer and Bradley". In Taton, R.; Wilson, C.; Hoskin, Michael (eds.). Planetary Astronomy from the Renaissance to the Rise of Astrophysics.Part A: Tycho Brahe to Newton'. Cambridge University Press. pp. 150–151. ISBN 9780521542050.
- Picard, Jean (1729). "Voyage D'Uranibourg ou Observations Astronomiques faites en Dannemarck". Mémoires de l'Académie Royale des Sciences (in French). 7 (1): 223–264.
- Johann (or Jan) van Waveren Hudde (1628–1704), mayor of Amsterdam, mathematician, lens maker. See:
- "Experience faire à l'Observatoire sur la Barometre simple touchant un nouveau Phenomene qu'on y a découvert" (Experiment done at the [Paris] observatory on a simple barometer concerning a new phenomenon that was discovered there), Le Journal des Sçavans [later: Journal des Savants ], page 112 (Paris edition) or page 126 (Amsterdam edition) (25 May 1676). Available on-line (in French) at: Gallica. See also: "Sur la lumière du baromètre" [On the light of the barometer], Histoire de l'Académie Royale des sciences de Paris, pages 202–203 (1694). For further information, see: Barometric light.
- Picard did not conceive the method of measuring a celestial body's right ascension by recording the time at which the body crossed the observer’s meridian. According to French astronomer Camille Guillaume Bigourdan (1851-1932), the French astronomers Adrien Auzout (1622-1691) and Jacques Buot (or Buhot) (<1623-1678), the Dutch physicist Christiaan Huygens (1629-1695), the Czech physician/astronomer Hagecius (1525-1600) had all suggested the method ; even the ancient Greek astronomer Hipparchus (190 B.C.E.-120 B.C.E.) had hinted at it. However, the method had never been put into practice because it required both a telescope in place of the traditional sight of a quadrant and a very accurate clock. Picard was the first astronomer to actually employ the method. [G. Bigourdan (1917) "Sur l'emplacement et les coordonées de l'Observatoire de la porte Montmartre" (On the site and coordinates of the observatory by the Montmartre gate), Comptes rendus, vol. 164, pages 537-543.] In October 1669, Picard sent, to the Royal Academy of Sciences in Paris, a report of his celestial observations during the preceding year, which included the observation of two bright stars, Regulus and Arcturus, while the sun was still in the sky. The report was recorded in the Registres des Procès-verbaux de l‘Académie des Sciences. On reading the report, it becomes apparent that Picard had been using clocks to determine the right ascension of stars. French astronomer Pierre Charles Le Monnier (1715-1799) records an extract of Picard’s report and then remarks: "Cette Observation est remarquable, étant inoüi qu'on eût jamais pris la Hauteur Méridienne des Etoiles fixes non seulment en plein Soleil, mais pas même encore dans la force du Crépuscle ; desorte qu'il est maintenant facile (continue M. Picard) de trouver immédiatement les Ascensions droites des Etoiles fixes non seulment par les Horloges à Pendule, mais aussi par l'Observation du Vertical du Soleil au mème temps qu'on observera la hauteur Méridienne d'une Etoile fixe." (This observation is remarkable, it being unheard of that one has ever taken the meridian altitude of fixed stars not only in full sun, but still not in the force of twilight; so it is now easy (continues Mr. Picard) to find immediately the right ascensions of the fixed stars not only by pendulum clocks but also by observation of the vertical of the sun at the same time that one observes the meridian altitude of a fixed star.) [Pierre-Charles Le Monnier, Histoire céleste, ou Recueil de toutes les observations astronomiques faites par ordre du Roi … (Paris, France: Briasson, 1741), page 40.]
- Wolf, Abraham, A History of Science, Technology, and Philosophy in the 16th and 17th Centuries, vol. 2 (London, England: George Allen and Unwin, 1950), page 172.
- O'Connor, John J.; Robertson, Edmund F., "Jean Picard", MacTutor History of Mathematics archive, University of St Andrews.
- Herbermann, Charles, ed. (1913). Catholic Encyclopedia. New York: Robert Appleton Company. .
- А.А.Гурштейн "Звезды Парижа" 2016. 
- Digital exhibition about Jean Picard (in french) / Observatoire de Paris