Obituary: Sergio Cortesi

On 18 December 2021, Sergio Cortesi passed away at the age of 89. He was one of the giants amidst solar observers, and essential in the continuity and stability of the International Sunspot Number. The text underneath was taken from SolarNews (NSO/Michele Bianda). A documentary on the solar observations done at Specola Solare Ticinese was realized in 2020 by RSI/Superalbum (Valeria Bruni) and can be found at (Italian). For his contributions, minor planet 91428 was named after him.

Sergio Cortesi, born 1932, passed away on December 18, 2021.

Sergio Cortesi worked at Specola Solare Ticinese, since it was was built in 1957 as an external station south of the Alps, belonging to the Swiss Federal Observatory of ETH Zurich, under the direction of Prof. Max Waldmeier. Sergio did the observations to determine the sunspot number and sent the data, until 1980, to ETH Zurich, where the official Zurich Sunspot Number was determined (which is nowadays referred to as the Solar Sunspot Number SSN). After the retirement of Waldmeier in 1980, ETH decided to discontinue this service, and the new director, Prof. Jan Olof Stenflo, was able to find a solution by transferring the role to the Observatoire Royale de Bruxelles, an institute directed by Prof. André Koeckelenbergh. Since 1981 Specola Solare, from then on managed by a local association, became the pilot station of Brussels.

Sergio Cortesi was the key person who could guarantee the continuity and stability of the SSN and minimize discontinuities in the transition, thanks to his profound experience and knowledge that he had acquired while working with Waldmeier. Engineer by education, Sergio showed a deep interest in natural sciences, in particular in astronomy. Thanks to the employment at Specola Solare his hobby became his work. He transferred his know-how to his successor, Marco Cagnotti, but after his retirement he still continued to make sunspot drawings, to observe the sun for a total of 64 years (his last drawing was performed on December 4, 2021), and to provide historical support, in particular by giving fundamental information during the transfer of crucial documents to the ETH library. His engagement in outreach was also extensive, including popular teaching and presiding over the local astronomical society.

Sergio was known for his very friendly character and deep humanity. He will be greatly missed.


Credits: tipress



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