Jean Picard, first to give an exact measurement of the radius of the Earth

Jean Picard

July 21, 1620, in La Fléche (France) -July 12, 1682, in Paris (France)

French astronomer and priest who studied at the Jesuit College Henry Le Grand, considered one of the best educational centers in France.

In 1644, he moved to live in Paris, where he became a disciple of Pierre Gassendi, helping him with the observations of a solar eclipse on August 21, 1645 and lunar eclipses in 1646 and 1647.

In 1655, he served as professor of astronomy at the Collége de France in Paris, following Gassendi's death that same year.

In 1666, he became a member of the Académie Royale des Sciences, just after its foundation, and from then on he devoted himself to working for the academy.

He introduced important changes that improved numerous observation instruments, such as the design of a micrometer to measure the diameters of celestial objects such as the Sun, the Moon and the planets. In 1667, he added a telescopic sight to the quadrant making it much more useful for observations.

In 1670, he made the first meridian measurement in France, between Sourdon and Malvoisine, which provided a highly accurate measurement of the radius of the Earth. The figure he offered was 6,327.9 km, when the current one is 6,357 km, which means that Picard was only wrong by a margin of 0.44%, regarding the current measurement.

The techniques used for measurement led him to map the Paris region and he joined a project to map France.

The calculation of the radius of the Earth served as the basis for Newton, in 1684, to confirm the truth of his law of universal gravitation.

In 1671, he traveled to Tycho Brahe's observatory (Sweden) to pinpoint his location so that Tycho's observations could be directly compared with others. During his stay, he works with Ole Rømer, observing eclipses of Jupiter's moon Io. He then returns to the Paris observatory (1673) and shortly after Ole Rømer moves to continue observing the moons of Jupiter. It was from the data they both collected that Rømer was able to calculate the speed of light.

It is in the Paris observatory where he contributes to the discovery that the Earth is not a perfect sphere, but rather is flattened at the poles and participates in the measurement of the parallax of Mars.

In 1679, he published the first Astronomical Yearbook in the French language.

In addition to astronomy, Picard was interested in hydraulics, a subject he wrote about and also put his skills to use solving the problem of supplying water to the Versailles fountains.

Interestingly, during his lifetime his astronomical observations were not published, but 60 years after his death.

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