Cornelis "Kees" de Jager (; 29 April 1921 – 27 May 2021) was a Dutch astronomer who specialized in predicting
solar variation to assess the Sun's impact on future climate. He was the General Secretary of the
IAU
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
from 1967 to 1973 and former director of the observatory at
Utrecht
Utrecht ( ; ; ) is the List of cities in the Netherlands by province, fourth-largest city of the Netherlands, as well as the capital and the most populous city of the Provinces of the Netherlands, province of Utrecht (province), Utrecht. The ...
. He was a fellow with the
Committee for Skeptical Inquiry
The Committee for Skeptical Inquiry (CSI), formerly known as the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), is a program within the U.S. non-profit organization Center for Inquiry (CFI), which seeks to " ...
and played an important role in the European
skeptical movement as the first chairman of both
Stichting Skepsis and the
European Council of Skeptical Organisations.
Personal life and education
Born in
Den Burg on the Dutch island of
Texel, de Jager spent his school years in the
Dutch East Indies
The Dutch East Indies, also known as the Netherlands East Indies (; ), was a Dutch Empire, Dutch colony with territory mostly comprising the modern state of Indonesia, which Proclamation of Indonesian Independence, declared independence on 17 Au ...
.
In 1939, De Jager heard Professor Minnaert speak. De Jager said "I was so fascinated by what he said, that I decided right then and there to study Astronomy." From 1939 to 1945, he studied
mathematics
Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
,
physics
Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
and
astronomy
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
at
Utrecht University
Utrecht University (UU; , formerly ''Rijksuniversiteit Utrecht'') is a public university, public research university in Utrecht, Netherlands. Established , it is one of the oldest universities in the Netherlands. In 2023, it had an enrollment of ...
.
On 13 October 1952, he obtained his
PhD
A Doctor of Philosophy (PhD, DPhil; or ) is a terminal degree that usually denotes the highest level of academic achievement in a given discipline and is awarded following a course of graduate study and original research. The name of the deg ...
with a thesis called "The Hydrogen Spectrum of the Sun".
His supervisor was
Marcel Minnaert
Marcel Gilles Jozef Minnaert (12 February 1893 – 26 October 1970) was a Belgian-Dutch astronomer. He was born in Bruges and died in Utrecht. He is notable for his contributions to astronomy and physics and for a popular book on meteorologic ...
.
De Jager died where he was born, in Den Burg (Texel), on 27 May 2021,
less than a month after he became a
centenarian
A centenarian is a person who has reached the age of 100. Because life expectancies at birth worldwide are well below 100, the term is invariably associated with longevity. The United Nations estimated that there were 316,600 living centenarian ...
.
Solar and stellar research
De Jager did work on stars and
solar physics
Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics.
Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...
, in relation to which he was a founding editor of the journal ''
Solar Physics
Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics.
Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...
''. In 1980, he was
principal investigator of the Hard X-ray Imaging Spectrometer (HXIS) on board the
Solar Maximum Mission
The Solar Maximum Mission satellite (or SolarMax) was designed to investigate Solar phenomena, particularly solar flares. It was launched on February 14, 1980. The SMM was the first satellite based on the Multimission Modular Spacecraft bus man ...
satellite.
His work on
solar flare
A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and ot ...
s was often done in collaboration with
Zdeněk Švestka
Zdeněk Švestka (30 September 1925 – 2 March 2013) was a Czech astronomer. For several decades he was the world's leading expert on solar flares. He studied mathematics and physics at Charles University, Prague, until graduating in 1948. Togeth ...
.
From 1978 onward, de Jager did noted work on the most luminous stars, known as
hypergiant
A hypergiant ( luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MK ...
s. From 1960 to 1986, de Jager was a professor at Utrecht University.
Sun–climate relations
De Jager's later research focused on predicting
solar variation to assess the Sun's impact on future climate.
Solar activity
Solar phenomena are natural phenomena which occur within the Stellar atmosphere, atmosphere of the Sun. They take many forms, including solar wind, Solar radio emission, radio wave flux, solar flares, coronal mass ejections, Stellar corona#Coron ...
is usually defined by the Sun's toroidal magnetic field, the field component parallel to the solar equator. Sunspots are one expression of this component. De Jager introduced the
poloidal
The terms toroidal and poloidal refer to directions relative to a torus of reference. They describe a three-dimensional coordinate system in which the poloidal direction follows a small circular ring around the surface, while the toroidal direct ...
field of the Sun, which connects its two poles, as a factor of possibly similar importance. He used
proxies for both components and took 19-year
running average
In statistics, a moving average (rolling average or running average or moving mean or rolling mean) is a calculation to analyze data points by creating a series of averages of different selections of the full data set. Variations include: simple ...
s to eliminate all effects that last only one or two
solar cycle
The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of Modern Maximum, variations in the number of observed sunspots on the Sun ...
s. Next he plotted both components in a diagram, thus creating an experimental
phase portrait
In mathematics, a phase portrait is a geometric representation of the orbits of a dynamical system in the phase plane. Each set of initial conditions is represented by a different point or curve.
Phase portraits are an invaluable tool in st ...
. The track of the two components went from low to high activity around 1923. Around 2006 the same point has been passed in the opposite direction.
Thus solar activity in the 21st century is expected to be lower than it was for most of the 20th century. A reduction in solar activity means less energy input to the Earth as part of the
Earth's energy budget
Earth's energy budget (or Earth's energy balance) is the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into con ...
, partially countering
climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
.
De Jager postulated that solar magnetic activity is the most significant contributor to
tropospheric
The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary s ...
temperatures, with polar activity also being significant, and that with the subtraction of these factors from temperatures recorded over the preceding 400 years, peaks and dips in temperature could be observed, accounting for recent increase in global warming.
Similar theories have been rejected by other climate scientists as solar activity and global temperatures have diverged since 1975, with energy output from the Sun decreasing and Earth temperatures still increasing. Additionally, warming caused exclusively by the Sun would result in an even warming throughout the atmosphere, rather than the observed cooling in the
upper atmosphere
Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes:
* The mesosphere, which on Earth lies between th ...
and warming in the
lower atmosphere associated with
greenhouse gas
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
es.
Cyclosophy

Expanding on a 1990 paper presentation at the International Skeptics Conference, de Jager published an article for ''
Skeptical Inquirer
''Skeptical Inquirer'' (S.I.) is a bimonthly American general-audience magazine published by the Committee for Skeptical Inquiry (CSI) with the subtitle "The Magazine for Science and Reason". The magazine initially focused on investigating clai ...
'' where he parodies
numerology
Numerology (known before the 20th century as arithmancy) is the belief in an occult, divine or mystical relationship between a number and one or more coinciding events. It is also the study of the numerical value, via an alphanumeric system, ...
. In ''Adventures in Science and Cyclosophy'', de Jager claims that many times pseudoscientific reasoning ignores coincidences dealing with the relationship between objects when there are unlimited data points. He states that measurements surrounding the
Great Pyramids
The Giza pyramid complex (also called the Giza necropolis) in Egypt is home to the Great Pyramid, the pyramid of Khafre, and the pyramid of Menkaure, along with their associated pyramid complexes and the Great Sphinx. All were built during the ...
have been used to show a relationship with astronomy. To do so, he explains, anyone can use the
law of large numbers
In probability theory, the law of large numbers is a mathematical law that states that the average of the results obtained from a large number of independent random samples converges to the true value, if it exists. More formally, the law o ...
to relate to anything one would want, to try and prove there is some connection. As an illustration, he uses the example of his
bicycle
A bicycle, also called a pedal cycle, bike, push-bike or cycle, is a human-powered transport, human-powered or motorized bicycle, motor-assisted, bicycle pedal, pedal-driven, single-track vehicle, with two bicycle wheel, wheels attached to a ...
and the cosmos. Enthusiasts in this formula have created a website that allows visitors to submit data to replicate de Jager's experiment.
According to
Kendrick Frazier
Kendrick Crosby Frazier (March 19, 1942 – November 7, 2022) was an American science writer and longtime editor of '' Skeptical Inquirer'' magazine. He was also a former editor of ''Science News'', author or editor of ten books, and a Fellow o ...
, who attended the 1998 Second World Skeptics Congress in
Heidelberg
Heidelberg (; ; ) is the List of cities in Baden-Württemberg by population, fifth-largest city in the States of Germany, German state of Baden-Württemberg, and with a population of about 163,000, of which roughly a quarter consists of studen ...
, Germany, de Jager's "dead-pan" description of how he took measurements throughout his house showing the "absurdities of those who attach great mystical significance to measurements of the Great Pyramid" had the audience "in stitches". Apparently "his home is in an astronomical observatory, a location, he said, 'that may be very close to the cosmos and well receptive to its incredible powers.'"
Other activities

He was the General Secretary of the
IAU
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
from 1967 to 1973 and former director of the observatory at
Utrecht
Utrecht ( ; ; ) is the List of cities in the Netherlands by province, fourth-largest city of the Netherlands, as well as the capital and the most populous city of the Provinces of the Netherlands, province of Utrecht (province), Utrecht. The ...
. In 1981, de Jager became a founding member of the
World Cultural Council
The World Cultural Council is an international organization whose goals are to promote cultural values, goodwill and philanthropy among individuals. The organization founded in 1982 and based in Mexico, has held a yearly award ceremony since 198 ...
. He was the first chairman of
Stichting Skepsis from 1987 to 1998, the first chairman of the
European Council of Skeptical Organisations from 1994 to 2001, and was also a
Committee for Skeptical Inquiry
The Committee for Skeptical Inquiry (CSI), formerly known as the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), is a program within the U.S. non-profit organization Center for Inquiry (CFI), which seeks to " ...
fellow.
De Jager joined his CSI peers by signing the "Deniers are not Skeptics" petition that asks the media to stop referring to
climate change denier
Climate change denial (also global warming denial) is a form of science denial characterized by rejecting, refusing to acknowledge, disputing, or fighting the scientific consensus on climate change. Those promoting denial commonly use rhetor ...
s as skeptics, with the petition stating "proper skepticism promotes scientific inquiry, critical investigation, and the use of reason in examining controversial and extraordinary claims", not "rejection of ideas without objective consideration".
He spoke on astrology at the World Skeptics Congress in 1996.
Honours and awards
In 1969, he became a member of the
Royal Netherlands Academy of Arts and Sciences
The Royal Netherlands Academy of Arts and Sciences (, KNAW) is an organization dedicated to the advancement of science and literature in the Netherlands. The academy is housed in the Trippenhuis in Amsterdam.
In addition to various advisory a ...
. In 1990, he was elected a member of
Academia Europaea
The Academia Europaea is a pan-European Academy of humanities, letters, law, and sciences.
The Academia was founded in 1988 as a functioning Europe-wide Academy that encompasses all fields of scholarly inquiry. It acts as co-ordinator of Europe ...
.
* 1974
Karl Schwarzschild Medal The Karl Schwarzschild Medal, named after the astrophysicist Karl Schwarzschild, is an award presented by the Astronomische Gesellschaft (German Astronomical Society) to eminent astronomers and astrophysicists.
Recipients
SourceGerman Astronomical ...
* 1984
Prix Jules Janssen
The Prix Jules Janssen is the highest award of the Société astronomique de France (SAF), the French astronomical society.
This annual prize is given to a professional French astronomer or to an astronomer of another nationality in recognition ...
* 1988
Gold Medal of the Royal Astronomical Society
The Gold Medal of the Royal Astronomical Society is the highest award given by the Royal Astronomical Society (RAS). The RAS Council have "complete freedom as to the grounds on which it is awarded" and it can be awarded for any reason. Past awar ...
for Astronomy
* 1988
George Ellery Hale Prize The George Ellery Hale Prize, or Hale Prize, is awarded annually by the Solar Physics Division of the American Astronomical Society for outstanding contributions over an extended period of time to the field of solar astronomy. The prize is named in ...
by the
Solar Physics Division The Solar Physics Division of the American Astronomical Society (AAS/SPD) or (AAS-SPD), often referred to as simply the "Solar Physics Division" (SPD), is the primary trade organization of solar physicists in the U.S. It exists for the advancement ...
of the
American Astronomical Society
The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC. The primary objective of the AAS is to promote the adv ...
* 1990
Committee for Skeptical Inquiry
The Committee for Skeptical Inquiry (CSI), formerly known as the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), is a program within the U.S. non-profit organization Center for Inquiry (CFI), which seeks to " ...
''In Praise of Reason'' Award presented to de Jager for his "notable contributions to science and his vigorous criticism of pseudoscience."
The asteroid
3798 de Jager was named for him.
In 2021, Springer Nature established the ''Kees de Jager Prize'' in honor of the founding editor of the journals ''
Solar Physics
Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics.
Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...
'' and ''
Space Science Reviews
''Space Science Reviews'' is a peer-reviewed, scientific journal of space science
Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dime ...
'' as well as the book series ''Astrophysics and Space Science Library''. The prize will be awarded annually to the best article in the journal ''Solar Physics''.
References
External links
Cornelis de Jager homepage (archive)
{{DEFAULTSORT:De Jager, Kees
1921 births
2021 deaths
Dutch men centenarians
People from Texel
20th-century Dutch astronomers
Academic staff of Utrecht University
Dutch skeptics
Utrecht University alumni
Foreign fellows of the Indian National Science Academy
Founding members of the World Cultural Council
Members of Academia Europaea
Members of the Royal Netherlands Academy of Arts and Sciences
Recipients of the Gold Medal of the Royal Astronomical Society