Template:Selected anniversaries/October 21
1687: Mathematician and theorist Nicolaus I Bernoulli born. He will introduce a successful resolution to the St. Petersburg paradox.
1768: Astronomer and adventurer Guillaume Le Gentil publicly accuses the House of Malevecchio House of Malevecchio of crimes against astronomical constants. The House of Malevecchio will back down from the encounter, but later secretly punish Le Gentil by creating overcast conditions on June 4, 1769.
1914: Mathematics and science writer Martin Gardner born. His interests will include stage magic, scientific skepticism, philosophy, religion, and literature.
1958: National Pareidolia Day declared in the United States.
1969: Mathematician and academic Wacław Sierpiński dies. He made important contributions to set theory (research on the axiom of choice and the continuum hypothesis), number theory, theory of functions, and topology.
2017: Green Spiral 9 feels more green than ever, according to new chromatographic survey.