Wesselink proceeded to Leiden University where he was appointed to an assistantship . He pursued research into variable stars . He was trained by prominent astronomers including Ejnar Hertzsprung , Willem de Sitter and Jan Woltjer . He made photographic observations of the brightness of the Sun during the total eclipse of 19 June 1936 outside of the total phase in an attempt to measure how the surface brightness of the Sun varies across its disk
total eclipse of 19 June 1936
Solar eclipse of June 19, 1936
Adriaan Wesselink