1920 in science

Astronomy and space science

• January 13 – The New York Times ridicules rocket scientist Robert H. Goddard, stating that spaceflight is impossible; in 1969 with Apollo 11 on its way to the Moon, the newspaper will publicly retract this position.
• December 13 – The red giant star Betelgeuse is the first to have its diameter determined by an optical astronomical interferometer, the Michelson stellar interferometer on Mount Wilson Observatory's reflector telescope.

  Biology

• Andrew Douglass proposes dendrochronology dating.
• Approximate date – The HIV pandemic almost certainly originates in Léopoldville, modern-day Kinshasa, the capital of the Belgian Congo.

  History of science and technology

• Newcomen Society founded in the United Kingdom for the study of the history of engineering and technology.

  Medicine

• Frederick Banting and Charles Best co-discover insulin.
• Hans Gerhard Creutzfeldt first describes some of the symptoms of what will become known as Creutzfeldt–Jakob disease.
• Hydrocodone, a narcotic analgesic closely related to codeine, is first synthesized in Germany by Carl Mannich and Helene Löwenheim.
• Black Cross Nurses founded in the United States.

  Meteorology

• Milutin Milanković proposes that long term climatic cycles may be due to changes in the eccentricity of the Earth's orbit and changes in the Earth's obliquity.

  Physics

• Megh Nad Saha states his ionization equation.
• Albert Einstein delivers his .
• Ernest Rutherford predicts the existence of the neutron.
• James Jeans discovers that the dynamical constants of motion determine the distribution function for a system of particles.
• Publication of Alan A. Griffith's analysis of the process of brittle fracture.

  Psychology

• John B. Watson conducts the Little Albert experiment in classical conditioning.

  Technology

• July 25 – The first transatlantic two-way radio broadcast is made.
• September 29 – First domestic radio sets come to stores in the United States – a Westinghouse radio costs $10.
• October – Young Russian physicist Lev Sergeyevich Termen first develops the electronic musical instrument which will become the Theremin.

  Events

• April 26 – Emil Racoviță establishes the world's first institute for the academic study of speleology at the Upper Dacia University in Cluj.
• Publication in Prague of Karel Čapek's drama R.U.R: Rossum's Universal Robots, introducing the word Robot to the world.

  Births

• January 20 – John Maynard Smith, British theoretical evolutionary biologist and geneticist.
• February 7 – An Wang, Chinese-born computer engineer.
• February 3 – Henry Heimlich, American thoracic surgeon.
• March 11 – Nicolaas Bloembergen, Dutch physicist, Nobel Prize laureate.
• March 15 – E. Donnall Thomas, American physician, Nobel Prize laureate.
• March 22 – Katsuko Saruhashi Japanese geochemist.
• March 23 – Barbara Low, English-born biochemist.
• April 6 – Edmond H. Fischer, Swiss American biochemist, Nobel Prize laureate.
• April 9 – Alex Moulton, English mechanical engineer and inventor.
• May 12 – Tang Dingyuan, Chinese physicist.
• June 17 – François Jacob, French biologist, Nobel Prize laureate.
• July 10 – Owen Chamberlain, American physicist, Nobel Prize laureate.
• July 11 – David Challinor, American biologist, naturalist and scientific administrator at the Smithsonian Institution.
• July 25 – Rosalind Franklin, English crystallographer.
• July 30 – Marie Tharp, American geologist.
• August 15 – Eleonore Trefftz, German molecular and nuclear physicist.
• August 22 – Denton Cooley, American cardiac surgeon.
• September 29 – Peter D. Mitchell, English chemist, Nobel Prize laureate.
• October 29 – Baruj Benacerraf, Venezuelan immunologist, Nobel Prize laureate.
• November 13 – Abraham Lilienfeld, American epidemiologist.
• December 6 – George Porter, English chemist, Nobel Prize laureate.

  Deaths

• January 3 – Zygmunt Janiszewski, Polish mathematician.
• January 6 – Hieronymus Georg Zeuthen, Danish mathematician.
• March 13 – Charles Lapworth, English geologist.
• March 26 – William Chester Minor, American surgeon.
• March 31 – Paul Bachmann, German mathematician.
• April 3 – Mary Katharine Brandegee, American botanist.
• April 8 – John Brashear, American astronomer.
• April 9 – Moritz Cantor, German historian of mathematics.
• April 26 – Srinivasa Ramanujan, Indian mathematician.
• June 20
• * Marie Adolphe Carnot, French chemist and mining engineer.
• * John Grigg, New Zealand astronomer.
• August 10 – Ádám Politzer, Hungarian otologist.
• August 12 – Hermann Struve, Russian-born astronomer.
• August 16 – Norman Lockyer, English astronomer.
• August 31 – Wilhelm Wundt, German physiologist and psychologist.
• October 17 – Reginald Farrer, English botanist.
• November 4 – Ludwig Struve, Russian astronomer.
• December 3 – William de Wiveleslie Abney, English astronomer and photographer.