|Died||12 July 1682 (aged 61)|
|Notable work||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 an arc measurement 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 he was 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:
- Picard, Jean (1729). "Voyage d'Uranibourg, ou observations astronomiques faites en Dannemarck" [Uranibourg voyage or astronomical observations made in Denmark]. Mémoires de l'Académie royale des sciences (in French). 7: 193–230. From pp. 215-216: Picard stated that Tycho couldn't accurately determine the position of Polaris because he lacked a telescope. However "… il y a un obstacle de la part de l'Etoile Polaire … paroît plus proche de Pole d'environ 20" qu'elle n'étoit un an auparavant." ( … there is an obstacle on the part of the star Polaris, which from one season to another suffers certain variations that Tycho had not noticed, and that I've observed for about ten years. That is, although the star Polaris annually approaches the pole by about 20", it happens nevertheless that towards the month of April the meridian height and the inferior height of that star become less by some seconds than it had appeared at the preceding winter solstice; instead, it should be greater by 5": then in the month of August and September its superior meridian height is found roughly such as it had been observed in winter, and even sometimes greater, although it should be diminished by 10 to 15"; but finally towards the end of a year, everything is compensated, such that Polaris appears closer to the pole by about 20" than it was a year before.) Picard concluded that the variation in the position of Polaris wasn't due to refraction by the atmosphere. However "… pour dire la verité, je n'ai encore rien pû m'imaginer qui me satisfît là-dessus …" ( … to tell the truth, I still couldn't imagine anything that would satisfy me [regarding] that [i.e., the variations in the position of Polaris] … )
- (Staff) (1676). "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]. Journal des Sçavans (Paris edition) (in French): 112–113. From pp. 112–113: "On sçait que le Barometre simple n'est autre chose qu'un tuyau de verre … toutes les circonstances qu'on y découvrira." (One knows that the simple barometer is nothing more than a glass tube [that is] hermetically sealed at the top and open at the bottom, in which there is mercury which usually stands at a certain height, the remainder [of the tube] above being void. Mr. Picard has one of them at the observatory [in Paris] which in the dark — when one shakes it enough to make the mercury jiggle — makes sparks and throws a certain flickering light which fills all of the part of the tube that's void: but it happens during each swing only in the void and only during the descent of the mercury. One has tried to perform the same experiment on various other barometers of the same composition ; but so far one has succeeded with only [this] one. As one has resolved to examine the thing in every way, we will give at greater length all the circumstances of this as one discovers them.)
- Reprinted in: (Staff) (1676). "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]. Journal des Sçavans (Amsterdam edition) (in French): 132.
- (Staff) (1694). "Sur la lumière du baromètre" [On the light of the barometer]. Histoire de l'Académie Royale des Sciences de Paris (in French). 2: 202–203. From p. 202: "Vers l'année 1676, M. Picard faisant transporter son Baromètre, … il ne s'en trouva aucun qui fit de la lumière." (Towards the year 1676, [while] Mr.. Picard [was] transporting his barometer from the observatory [in Paris] to the port of Saint Michel during the night, he perceived a light in the part of the tube where the mercury was moving; this phenomenon surprising him, he immediately announced it to the [Journal des] Sçavans, and those who had barometers having examined them, they found nothing which made light.) By the time of Picard's death (1682), his barometer had lost its ability to produce light. However, after Philippe de La Hire (1640–1718) restored Picard's barometer, it once again produced light. Cassini (1625–1712) also owned a barometer that produced light.
- See also: 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