Birthday of René Descartes (1596)

René Descartes invented the Cartesian coordinate system — arguably the most important idea in the history of mathematics after the invention of zero. By assigning x and y coordinates to every point, curves become equations and equations become curves. The circle x² + y² = r² became something you could calculate with. Geometry and algebra, separated for 2,000 years, were unified. In the same year he published this — 1637 — he also published La Géométrie, introduced the convention of using letters from the end of the alphabet (x, y, z) for unknowns, and came close to inventing analytic geometry in its entirety.