Yrjö Väisälä (Finnish: [ˈyrjø ˈʋæi̯sælæ] ; 6 September 1891 – 21 July 1971) was a Finnish astronomer and physicist.
His main contributions were in the field of optics. He was also active in geodetics, astronomy and optical metrology. He had an affectionate nickname of Wizard of Tuorla (Observatory/Optics laboratory), and a book with the same title in Finnish describes his works. His discoveries include 128 asteroids and 3 comets.
His brothers were mathematician Kalle Väisälä (1893–1968) and meteorologist Vilho Väisälä (1889–1969). His daughter Marja Väisälä (1916–2011) was an astronomer and discoverer of minor planets.
Väisälä was a fervent supporter of Esperanto, presiding over the Internacia Scienca Asocio Esperantista ("International Association of Esperanto Scientists") in 1968.
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