Template:Selected anniversaries/April 21
1552: Mathematician and astronomer Petrus Apianus dies. His works on cosmography, Astronomicum Caesareum (1540) and Cosmographicus liber (1524), were extremely influential in his time.
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.
1822: Priest and inventor Hannibal Goodwin born. He will invent and patent rolled celluloid photographic film.
1900: The Waking of the Slate ceremony is louder than ever.
1934: The "Surgeon's Photograph", the most famous photo allegedly showing the Loch Ness Monster, is published in the Daily Mail. (It will be revealed as a hoax in 1999.)
1965: Physicist and academic Edward Victor Appleton dies. Appleton made pioneering contributions to radiophysics, and was awarded the Nobel Prize in Physics in 1947 for his seminal work proving the existence of the ionosphere during experiments carried out in 1924.