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 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 sextant he used to find the meridian had a radius of six feet, and was equipped with a micrometer to enable minute adjustments.

[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.

Picard made his observations using the precision pendulum clock that Dutch physicist Christiaan Huygens had recently developed.