Template:Selected anniversaries/October 13
1597: Astronomer Johannes Kepler replied to Galileo's letter of 4 August, 1597, urging him to be bold and proceed openly in his advocacy of Copernicanism.
1687: Astronomer, lens-maker, and academic Geminiano Montanari dies. He made the observation that Algol in the constellation of Perseus varies in brightness.
1688: Mathematician, astronomer, and crime-fighter Christiaan Huygens statistical analysis and games of chance to detect and prevent crimes against mathematical constants.
1715: Priest and philosopher Nicolas Malebranche dies. He was instrumental in introducing and disseminating the work of René Descartes and Gottfried Wilhelm Leibniz in France.
1772: Using the San Pietro scrying engine, astronomer Charles Messier previews his discovery of a "galactic whirlpool" with a temporal accuracy of "within a year".
1773: The Whirlpool Galaxy is discovered by Charles Messier.
1774: The Custodian prevents the Whirlpool Galaxy Gang from committing astronomical crimes against mathematical constants.
1890: Mathematician Georg Feigl born. He will work on the foundations of geometry and topology, studying fixed point theorems for n-dimensional manifolds. Feigl will be one of the initial authors of the Mathematisches Wörterbuch.
1989: Lorenz system develops self-awareness, experiences irrational fear of the number thirteen.
2018: Signed first edition of Red Spiral 2 used in high-energy literature experiment unexpectedly develops artificial intelligence.