Sur la période des maxima d’activité solaire

This is the publication of a pli cacheté (sealed letter) submitted to the French Academy of Sciences in 1918 by Jean Malburet (amateur astronomer, 1879–1955). The subject is “On the period of solar activity maxima”. If not reclaimed for 100 years, such sealed letters are opened and analysed by members of the Academy. This letter was considered sufficiently interesting to be published in Comptes rendus Geoscience, along with a descriptive comment of its contents and some of the subsequent history of the ideas put forward in that sealed contribution. In 1918, J. Malburet proposed that “triple syzygies” (alignments) of the planets Jupiter, Venus, and the Earth (with some smaller influence of Mercury) caused tides on the Sun that imposed on our star the observed periodicity (or rather quasi-periodicity) of 11 years. Thanks to contacts with the author’s grandson, two subsequent (1918 and 1925) papers were found, whereas Nicola Scafetta (pers. comm.) pointed out a 1927 criticism of the paper by H.M. Losh. The topic is still an active one today, as witnessed for example by papers by Scafetta, 2012a, Scafetta, 2012b, Scafetta, 2016). The handwritten text of the sealed letter has been typed for easier access to readers and is placed after a short analysis. French has been used throughout this as this is the language of the letter.