If you find having a favourite map projection a delightfully geeky quirk, you're going to want to raise a toast in honour of Jean-Félix Picard. Geodetics and cartography owe a debt of gratitude to the 17th century scientist who made the first accurate measurement of the Earth's size.

Jean-Félix Picard was born on this day in 1620 in La Flèche, France. He's worth celebrating because in 1671, he published Mesure de la Terre, one of the first accurate measurements of the Earth's size. He managed to measure the size of the Earth to within fractions of our current best-estimates using triangulation, mathematics, and a whole lot of patience.

Using triangulation to measure the width of a river in the 16th century. Source: Hulsius

Picard's not-so-secret technique was blending the precision of measuring distances with triangulation and the mathematical wizardry of algebra and geometry.

He started by very carefully measuring the arc length of a single degree in latitude with excruciating precision with thirteen triangles along the meridian between Paris and a clock tower in Amien. He dropped that into the relationship between radius and arc length for circles, jiggling around measured arc length and the subtended angle to extract the radius of our planet.

Here's the crazy bit: he measured the arc length of 1 degree latitude as 110.46 kilometers, thus calculated the terrestrial radius of 6,328.9 kilometers. Our current best measurement of the polar radius* is 6,356.7 kilometers. That's 0.44% error, otherwise known as "Impressive."