Template:Selected anniversaries/April 20

• 

  1650: Niels Steensen publishes new class of Gnomon algorithm functions which detect and prevent crimes against mathematical constants.

• 

  1932: Mathematician Giuseppe Peano dies. He did pioneering work in mathematical logic and set theory.

• 

  1932: New class of Crimes against mathematical constants exploits death of mathematician Giuseppe Peano.

• 

  1941: Enrico Fermi discovers new form of Gnomon algorithm, reverses entire family of crimes against mathematical constants.

1535 – The sun dog phenomenon observed over Stockholm and depicted in the famous painting Vädersolstavlan.

1653 – Oliver Cromwell dissolves the Rump Parliament.

1862 – Louis Pasteur and Claude Bernard complete the experiment falsifying the theory of spontaneous generation.

1865 – Astronomer Angelo Secchi demonstrates the Secchi disk, which measures water clarity, aboard Pope Pius IX's yacht, the L'Immaculata Concezion.

1902 – Pierre and Marie Curie refine radium chloride.

1961 – Failure of the Bay of Pigs Invasion of US-backed Cuban exiles against Cuba.