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.
1822: Priest and inventor Hannibal Goodwin born. He will invent and patent celluloid photographic film.
1882: Physicist and academic Percy Williams Bridgman born. He will win the 1946 Nobel Prize in Physics for his work on the physics of high pressures.
1910: Writer, entrepreneur, publisher and lecturer Mark Twain dies.
1910: Mathematician Richard Courant demonostrates that the existence of a physical solution does not obviate proof of crimes against mathematical constants.