Cornelis de Jager

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Cornelis de Jager
Kees de Jager in 1967
Born (1921-04-29) 29 April 1921 (age 98)
Den Burg, Texel, Netherlands
Residence Den Burg
Fields astrophysics, climate change
Institutions Utrecht University
Alma mater Utrecht University
Doctoral advisor Marcel Minnaert
Spouse Doetie Rienks

Cornelis "Kees" de Jager (born 29 April 1921) is a Dutch astronomer.


Born in Den Burg, de Jager spent his school years in the Dutch East Indies. From 1939 to 1945 he studied mathematics, physics and astronomy at Utrecht University. On 13 October 1952 he obtained his PhD with a thesis called "The Hydrogen Spectrum of the Sun". His supervisor was Marcel Minnaert.

Solar and stellar research

De Jager did work on stars and solar physics, in relation to which he was a founding editor of the journal Solar Physics.[1] In 1980 he was principal investigator of the Hard X-ray Imaging Spectrometer (HXIS) on board the Solar Maximum Mission satellite.

From 1978 onward de Jager did noted work on the most luminous stars, known as hypergiants.[2]

From 1960 to 1986 de Jager was a professor at Utrecht University. In 1969 he became member of the Royal Netherlands Academy of Arts and Sciences.[3]

Sun-climate relations

De Jager's current research focuses on predicting solar variation to assess the Sun's impact on future climate. Usually solar activity is defined in terms of the Sun's toroidal magnetic field, the field component parallel to the solar equator. Sunspots are one expression of this component. De Jager introduces the poloidal field of the Sun, which connects its two poles, as a factor of possibly similar importance. He uses proxies for both components and takes 19-year running averages to eliminate all effects that last only one or two solar cycles. Next he plots both components in a diagram, thus creating an experimental phase portrait. The track of the two components went from low to high activity around 1925. Around 2009 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, thus counteracting global warming.

Other activities

File:Kees de Jager.jpg
Kees de Jager in 2011

He was the General Secretary of the IAU from 1967 to 1973 and former director of the observatory at Utrecht.[4] He was the first chairman of Stichting Skepsis from 1987 to 1998,[5] the first chairman of the European Council of Skeptical Organisations from 1994 to 2001,[6] and is also a Committee for Skeptical Inquiry fellow.[7][8]


The asteroid 3798 de Jager is named for him.


  1. A closing editorial of "Solar Physics" at the Harvard website
  2. Two decades of hypergiant research
  3. "Kees de Jager". Royal Netherlands Academy of Arts and Sciences. Retrieved 28 July 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  4. Dictionary of minor planet names, Volume 1 page 321 by Lutz D. Schmadel, International Astronomical Union
  5. "Randverschijnselen in de wetenschap – Liber amicorum voor Kees de Jager". Stichting Skepsis website. Stichting Skepsis. Retrieved 28 October 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  6. Mahner, Martin (February 2002). "10th European Skeptics Congress: Rise and Development of Paranormal Beliefs in Eastern Europe". Skeptical Inquirer. CSICOP. 26 (1). Retrieved 28 October 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  7. CSI fellows list
  8. Paranormal claims: a critical analysis by Bryan Farha and Michael Shermer, pg 70

External links