Jean Picard

Last updated
Jean-Felix Picard
CS Sorbonne Picard.jpg
Jean Picard sundial on the pediment of the Sorbonne
Born(1620-07-21)21 July 1620
Died12 July 1682(1682-07-12) (aged 61)
Occupation Astronomer
Notable work
Mesure de la Terre

Jean-Félix Picard (21 July 1620 12 July 1682) was a French astronomer and priest born in La Flèche, where he studied at the Jesuit Collège Royal Henry-Le-Grand. He died in Paris, France.


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 166970, 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.

Other work

In 1670-71, Picard travelled to the site of Tycho Brahe's Danish observatory, Uraniborg, in order to assess its longitude accurately so that Tycho's readings could be compared to others [1] [2] .

Picard collaborated and corresponded with many scientists, including Isaac Newton, Christiaan Huygens, Ole Rømer, Rasmus Bartholin, Johann Hudde, [3] 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. [4] 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. [5] [6] 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.


See also


  1. 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.
  2. Picard, Jean (1729). "Voyage D'Uranibourg ou Observations Astronomiques faites en Dannemarck". Memoires de l'Academie Royale des Sciences (in French). 7 (1): 223–264.
  3. Johann (or Jan) van Waveren Hudde (1628–1704), mayor of Amsterdam, mathematician, lens maker. See:
  4. "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 202203 (1694). For further information, see: Barometric light.
  5. 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.]
  6. 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.

Related Research Articles

Nicolas-Louis de Lacaille French astronomer

Abbé Nicolas-Louis de Lacaille, formerly sometimes spelled de la Caille, was a French astronomer and geodesist who named 14 out of the 88 constellations. From 1750 to 1754 he studied the sky at the Cape of Good Hope in present-day South Africa. Lacaille observed over 10,000 stars using just a half-inch refractor.

Ole Rømer Danish astronomer

Ole Christensen Rømer was a Danish astronomer who, in 1676, made the first quantitative measurements of the speed of light.

Christiaan Huygens 17th-century Dutch mathematician and natural philosopher

Christiaan Huygens, also spelled Huyghens, was a Dutch physicist, mathematician, astronomer and inventor, who is widely regarded as one of the greatest scientists of all time and a major figure in the scientific revolution. In physics, Huygens made groundbreaking contributions in optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan. As an inventor, he improved the design of the telescope with the invention of the Huygenian eyepiece. His most famous invention, however, was the pendulum clock in 1656, which was a breakthrough in timekeeping and became the most accurate timekeeper for almost 300 years. Huygens was an outstanding mathematician and, because he was the first to transfer mathematical inquiry to describe unobservable physical phenomena, he has been called the first theoretical physicist and the founder of modern mathematical physics.

Uraniborg Historic observatory

Uraniborg was a Danish astronomical observatory and alchemy laboratory established and operated by Tycho Brahe. It was built c. 1576 – c. 1580 on Hven, an island in the Øresund between Zealand and Scania, Sweden, which was part of Denmark at the time. It was expanded with the underground facility Stjerneborg on an adjacent site.

Paris Observatory Foremost astronomical observatory of France

The Paris Observatory, a research institution of PSL University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Bank of the Seine in central Paris, but most of the staff work on a satellite campus in Meudon, a suburb southwest of Paris.

Jacques Cassini French astronomer

Jacques Cassini was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini.

César-François Cassini de Thury French cartographer and astronomer

César-François Cassini de Thury, also called Cassini III or Cassini de Thury, was a French astronomer and cartographer.

Philippe de La Hire French mathematician and astronomer

Philippe de La Hire was a French painter, mathematician, astronomer, and architect. According to Bernard le Bovier de Fontenelle he was an "academy unto himself".

Giacomo Filippo Maraldi was a French-Italian astronomer and mathematician. His name is also given as Jacques Philippe Maraldi. Born in Perinaldo he was the nephew of Giovanni Cassini, and worked most of his life at the Paris Observatory. He also is the uncle of Jean-Dominique Maraldi.

Constantinople Observatory of Taqi ad-Din astronomical observatory built in 1577

The Constantinople observatory of Taqi ad-Din, founded in Constantinople by Taqi ad-Din Muhammad ibn Ma'ruf in 1577, was one of the largest astronomical observatories in medieval world. However, it only existed for a few years and was destroyed in 1580.

Barometric light is a name for the light that is emitted by a mercury-filled barometer tube when the tube is shaken. The discovery of this phenomenon in 1675 revealed the possibility of electric lighting.

Gabriel Mouton was a French abbot and scientist. He was a doctor of theology from Lyon, but was also interested in mathematics and astronomy. His 1670 book, the Observationes diametrorum solis et lunae apparentium, proposed a natural standard of length based on the circumference of the Earth, divided decimally. It was influential in the adoption of the metric system in 1799.

Paris meridian Meridian line running through the Paris Observatory in Paris, France

The Paris meridian is a meridian line running through the Paris Observatory in Paris, France – now longitude 2°20′14.03″ East. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world. The "Paris meridian arc" or "French meridian arc" is the name of the meridian arc measured along the Paris meridian.

Giovanni Domenico Cassini Italian/French mathematician, astronomer, engineer, and astrologer

GiovanniDomenico Cassini, also known as Jean-Dominique Cassini was an Italian mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard state. Cassini is known for his work in the fields of astronomy and engineering. Cassini discovered four satellites of the planet Saturn and noted the division of the rings of Saturn; the Cassini Division was named after him. Giovanni Domenico Cassini was also the first of his family to begin work on the project of creating a topographic map of France.

Seconds pendulum Pendulum whose period is precisely two seconds

A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz. A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force combined with the pendulum's mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one complete cycle, a left swing and a right swing, is called the period. The period depends on the length of the pendulum, and also to a slight degree on its weight distribution and the amplitude (width) of the pendulum's swing.

Mural instrument

A mural instrument is an angle measuring device mounted on or built into a wall. For astronomical purposes, these walls were oriented so they lie precisely on the meridian. A mural instrument that measured angles from 0 to 90 degrees was called a mural quadrant. They were utilized as astronomical devices in ancient Egypt and ancient Greece. Edmond Halley, due to the lack of an assistant and only one vertical wire in his transit, confined himself to the use of a mural quadrant built by George Graham after its erection in 1725 at the Royal Observatory, Greenwich. Bradley's first observation with that quadrant was made on 15 June 1742.

24101 Cassini, provisional designation 1999 VA9, is an eccentric background asteroid from the middle region of the asteroid belt, approximately 7 kilometers in diameter. It was discovered on 9 November 1999, by American amateur astronomer Charles Juels at the Fountain Hills Observatory (678) in Arizona, United States. It was named after Italian–French astronomer Giovanni Cassini.

Rømers determination of the speed of light 1676 experiment which showed that light has finite speed

Rømer's determination of the speed of light was the demonstration in 1676 that light has a finite speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710), who was working at the Royal Observatory in Paris at the time.

History of the metre origins and previous definitions of the SI base unit for measurement

The history of the metre starts with the scientific revolution that began with Nicolaus Copernicus's work in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision in use, they also preferred a decimal system to ease their calculations.

Triangulation (surveying)

In surveying, triangulation is the process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly as in trilateration. The point can then be fixed as the third point of a triangle with one known side and two known angles.