Editing 1919 in science (section)

==Astronomy==
* April – [[George Ellery Hale]] and collaborators publish their discovery that the magnetic polarity of [[sunspot]] pairs reverses on an 11-year [[solar cycle]] and that the polarity varies by hemisphere, which becomes known as [[Hale's law]].<ref>{{cite journal|last1=Hale|first1=George E.|last2=Ellerman|first2=Ferdinand|last3=Nicholson|first3=S. B.|last4=Joy|first4=A. H.|title=The Magnetic Polarity of Sun-Spots|journal=The Astrophysical Journal|date=April 1919|volume=49|pages=153|doi=10.1086/142452|bibcode=1919ApJ....49..153H|doi-access=free}}</ref><ref>{{cite web|title=Hale's Sunspot Polarity Law|last1=Charbonneau|first1=P.|last2=White|first2=O. R.|date=1995-04-18|publisher=[[High Altitude Observatory]]|url=https://www2.hao.ucar.edu/Education/Sun/hales-sunspot-polarity-law|website=www2.hao.ucar.edu|access-date=2021-08-20|archive-date=2021-08-19|archive-url=https://web.archive.org/web/20210819182851/https://www2.hao.ucar.edu/Education/Sun/hales-sunspot-polarity-law|url-status=live}}</ref>
* The [[International Astronomical Union]] is established in Paris.