Template:Selected anniversaries/April 20

• 

  1548: Mathematician, astronomer, and alleged time-traveller Petrus Apianus publishes Cosmographicus furatis, his magisterial treatise on crimes against astronomical constants.

• 

  1653: Oliver Cromwell dissolves the Rump Parliament.

• 

  1790: Mathematician and detective Johann Friedrich Pfaff uses partial differential equations of the first order Pfaffian systems to track and erase the Forbidden Ratio.

• 

  1902: Pierre and Marie Curie refine radium chloride.

• 

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

• 

  1945: Mathematician Georg Feigl dies. He worked on the foundations of geometry and topology, studying fixed point theorems for n-dimensional manifolds. Feigl was one of the initial authors of the Mathematisches Wörterbuch.

• 

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

• 

  1962: Traces of Clandestiphrine residue are detected at the Bay of Pigs, raising questions about CIA involvement with transdimensional drugs.