July 12, 1682 – Death of Jean Picard, first to give an exact measurement of the Earth's radius

Sundial by Jean Picard, on the pediment of the Sorbonne.

French astronomer and priest who began his career as a disciple of Pierre Gassendi whom he helped with the observations of a solar eclipse on August 21, 1645 and lunar eclipses in 1646 and 1647.

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

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

In 1670, he made the first measurement of a meridian in France, between Sourdon and Malvoisine, giving a highly accurate measurement of the Earth's radius. 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%, compared to the current measurement.

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

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

In 1671, he traveled to the Tycho Brahe observatory (Sweden) to pinpoint its 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 returned to the Paris observatory (1673) and shortly after Ole Rømer moved to continue observing Jupiter's moons. It was from the data they both collected that Rømer was able to calculate the speed of light.

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

If you want to know more about this scientist, click on the following link: Jean Picard

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