Template:Selected anniversaries/April 21
1718: Painter, mathematician, astronomer, and architect Philippe de La Hire dies.
1774: Physicist, astronomer, and mathematician Jean-Baptiste Biot born. He will establish the reality of meteorites, make an early balloon flight, and study the polarization of light.
1793: American captain and mathematician Nathaniel Bowditch publishes new class of Gnomon algorithm functions which improved maritime navigation techniques.
1910: Writer, entrepreneur, publisher and lecturer Mark Twain dies.
1910: Mathematician Richard Courant invents new type of scrying engine to detect and prevent crimes against mathematical constants.
1963: Jean Bartik uses Gnomon algorithm to fight crimes against mathematical constants.
https://en.wikipedia.org/wiki/Shakuntala_Devi
https://en.wikipedia.org/wiki/Simon_de_la_Loub%C3%A8re
https://en.wikipedia.org/wiki/Jean-Baptiste_Biot
https://en.wikipedia.org/wiki/Petrus_Apianus
1964 – A Transit-5bn satellite fails to reach orbit after launch; as it re-enters the atmosphere, 2.1 pounds (0.95 kg) of radioactive plutonium in its SNAP RTG power source is widely dispersed.
https://en.wikipedia.org/wiki/Johann_Friedrich_Pfaff
1992 – The first discoveries of extrasolar planets are announced by astronomers Aleksander Wolszczan and Dale Frail. They discovered two planets orbiting the pulsar PSR 1257+12.