Portal:History of science/Selected anniversaries/July 21
- 1620 - Birth of Jean Picard, French astronomer (d. 1682)
- 1810 - Birth of Henri Victor Regnault, French chemist (d. 1878)
- 1880 - Birth of Milan Rastislav Štefánik, Slovak politician and astronomer (d. 1919)
- 1923 - Birth of Rudolph A. Marcus, Canadian chemist, Nobel laureate
- 1925 - Scopes Trial: In Dayton, Tennessee, high school biology teacher John T. Scopes is found guilty of teaching evolution in class and fined $100
- 1969 - Neil Armstrong and Edwin "Buzz" Aldrin become the first men to walk on the Moon, during the Apollo 11 mission
- 2004 - Death of Edward B. Lewis, American geneticist, Nobel Prize in Physiology or Medicine laureate (b. 1918)