Noted astronomer and skeptic Cornelis (Kees) de Jager died on May 27, 2021, at the age of 100. He was cofounder of the Dutch skeptical organization Skepsis and its first chairman from 1987 to 1997. He was a longtime fellow of the Committee for Skeptical Inquiry.

When Paul Kurtz spoke on January 12, 1987, with a couple of Dutch readers of Skeptical Inquirer about setting up a Dutch skeptical organization, one of the attendants was Kees de Jager. He was freshly retired as professor of astrophysics and space physics and had completed a second stint as president of the Committee on Space Research—one of almost a dozen leading functions in international organizations he had held. He had founded a laboratory for space research that quickly had become a top European institute. It specialized, among other things, in making instruments for observing X-rays from the sun and other stars.

He was also very active in popularizing science, for example lecturing in high schools. So when nine months later on a second such meeting it was announced that De Jager would be chairman of Skepsis, this was a great honor for the skeptical movement.

Kees de Jager (his surname suggests an ancestor towing canal boats) was born April 29, 1921, on the Dutch isle of Texel but spent most of his youth in the Dutch East Indies (now Indonesia), where his father had accepted a post as head of a primary school. De Jager returned to the Netherlands to study science and quickly opted for astronomy. A major part of his time as a student he spent in hiding, because he refused to pledge loyalty to the Germans, who had occupied the Netherlands in 1940. De Jager’s hiding place was in the Utrecht Observatory, which would in 1963 become his home.

His research contributions included the structure and dynamics of the solar atmosphere; the study of the most luminous stars, including determination of their motions; the instability of the atmospheres of such stars; the discovery of the “yellow evolutionary void” in hypergiant star evolution; and the studies of the solar dynamo and of sun-climate relationships, including the main role of UV variation on climate. He had served as director of the Utrecht Observatory, general secretary of the International Astronomical Union, and president of the International Council of Scientific Unions. He also founded the journals Space Science Reviews and Solar Physics.

De Jager’s skeptical activities were mostly organizational, but he did attend various skeptical conferences. Two of his contributions there explained how to extract meaning from random numbers: various measurements on his bicycle and markings and cracks along the hallway in his house. The former yielded various universal constants and the latter, surprisingly, revealed both the end of time (to the nearest minute!) and the location of the Big Bang, which happened to be in the park in front of his home. (His Skeptical Inquirer article about all this, “Adventures in Science and Cyclosophy,” in SI’s Winter 1992 issue, became one of SI’s most popular articles. It is available on SI’s website.)

At yet another conference, he likened science to the profession of his maternal grandfather, who built windmills. Just like a millwright, the practical scientist isn’t much interested in the philosophy of his trade. All the same, De Jager thought that pseudoscientists usually are too much in love with their own theories; they reject and select evidence and mistake correlation for causation.

In 1994, he became the first chairman of the European Council of Skeptical Organisations.

Around the beginning of 1991, he had an idea for explaining the so-called Mars Effect (the claim that sports champions are born a few percentage points more often at particular Mars positions than common citizens). This effect, proposed by French psychologist Michel Gauquelin, was considered the most reliably proven astrological effect, but tests had already resulted in acrimonious debates. De Jager’s idea snowballed, and a part of the 1991 European Skeptical Conference held in Amsterdam was dedicated to this subject. Eventually these efforts petered out in the conviction that there was not a Mars effect but instead a Gauquelin effect. However, the amount of data this effort produced, together with my efforts to edit the conference proceedings, led to my own involvement—with De Jager’s encouragement.

De Jager was also an enthusiastic long-distance runner, and he ran the New York marathon at the age of seventy-five. He remained active in science, even after moving back to his native Texel in 2003. Last year, his season’s greetings were an announcement of a book on the relation between the magnetism of the sun and the climate on earth. His one-hundredth birthday was celebrated by an online event, though he had become too weak to appear. De Jager was exceptional in many respects, but for me he is unforgettable, because he was such a nice man—always kind and cordial and willing to help. He set an example that I try to follow.