Alexis Clairaut

Alexis Clairaut (1713–1765) was a French mathematician and astronomer — and a jaw-dropping child prodigy. He was reading advanced geometry as a small boy, and presented original research to the Paris Academy at an age when most of us are still learning long division.

The claim to fame

Clairaut's theorem is a lovely fact about partial derivatives: if a function is nice and smooth, it doesn't matter which variable you differentiate by first — the mixed partial derivatives come out equal either way. He also proved a famous result about the shape of the Earth, showing how its spin makes it bulge at the equator.

One of the great scientific arguments of the 1700s was over the Earth's true shape: was it squashed at the poles, as Newton predicted, or stretched? To settle it, Clairaut joined a daring expedition to the freezing north of Lapland to measure the planet with painstaking care. The data came back on Newton's side — the Earth really does bulge at the middle. Later Clairaut also nailed the return date of Halley's Comet by grinding through months of gruelling hand calculation with a small team. A mathematician who took his sums out into the ice and the sky.