Jacobus Cornelius Kapteyn (19 January 1851 – 18 June 1922) was a Dutch

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

. He carried out extensive studies of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

. He found that the apparent movement of stars was not randomly distributed but had two preferential directions: the two star streams. This discovery was later reinterpreted as evidence for galactic rotation. Kapteyn also suggested that these stellar velocities could be used to find the amount of non-luminous matter in the galaxy, which his student, Jan Oort, measured in 1932, referring to it as "invisible matter".

Biography

Kapteyn was born in Barneveld to Gerrit J. and Elisabeth C. (née Koomans) Kapteyn, and went to the

University of Utrecht Utrecht University (UU; , formerly ''Rijksuniversiteit Utrecht'') is a public research university in Utrecht, Netherlands. Established , it is one of the oldest universities in the Netherlands. In 2023, it had an enrollment of 39,769 students, a ...

to study

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 ...

and

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 ...

in 1868. In 1875, after having finished his

thesis A thesis (: theses), or dissertation (abbreviated diss.), is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.International Standard ISO 7144: D ...

, he worked for three years at the Leiden Observatory, before becoming the first

Professor Professor (commonly abbreviated as Prof.) is an Academy, academic rank at university, universities and other tertiary education, post-secondary education and research institutions in most countries. Literally, ''professor'' derives from Latin ...

of Astronomy and Theoretical

Mechanics Mechanics () is the area of physics concerned with the relationships between force, matter, and motion among Physical object, physical objects. Forces applied to objects may result in Displacement (vector), displacements, which are changes of ...

at the

University of Groningen The University of Groningen (abbreviated as UG; , abbreviated as RUG) is a Public university#Continental Europe, public research university of more than 30,000 students in the city of Groningen (city), Groningen, Netherlands. Founded in 1614, th ...

, where he remained until his

retirement Retirement is the withdrawal from one's position or occupation or from one's active working life. A person may also semi-retire by reducing work hours or workload. Many people choose to retire when they are elderly or incapable of doing their j ...

in 1921. In 1888 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 ...

. Between 1896 and 1900, lacking an observatory, he volunteered to measure

photographic plate Photographic plates preceded film as the primary medium for capturing images in photography. These plates, made of metal or glass and coated with a light-sensitive emulsion, were integral to early photographic processes such as heliography, d ...

s taken by David Gill, who was conducting a photographic survey of southern hemisphere stars at the Royal Observatory at the Cape of Good Hope. The results of this collaboration was the publication of '' Cape Photographic Durchmusterung'', a catalog listing positions and magnitudes for 454,875

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s in the Southern Hemisphere. In 1897, as part of the above work, he discovered

Kapteyn's Star Kapteyn's Star is a class M1 red subdwarf about 12.83 light-years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With an apparent magnitude of nearly 9, it is visible through binocul ...

. It had the highest

proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...

of any star known until the discovery of

Barnard's Star Barnard's Star is a small red dwarf star in the constellation of Ophiuchus. At a distance of from Earth, it is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and is the c ...

in 1916. In 1904, studying the

s of stars, Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way Galaxy, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort. In 1906, Kapteyn launched a plan for a major study of the distribution of stars in the Galaxy, using counts of stars in different directions. The plan involved measuring the

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

, and

of stars in 206 zones. This enormous project was the first coordinated statistical analysis in astronomy and involved the cooperation of over forty different observatories. He was awarded the

James Craig Watson Medal image:Watson_medal_NAS.gif, 400px, James Craig Watson Medal The James Craig Watson Medal was established by the bequest of James Craig Watson, an astronomer the University of Michigan between 1863 and 1879, and is awarded every 1-4 years by the U.S. ...

in 1913. Kapteyn later retired in 1921 at the age of seventy, but on the request of his former student and director of Leiden Observatory

Willem de Sitter Willem de Sitter (6May 187220November 1934) was a Dutch mathematician, physicist, and astronomer. He is known for the de Sitter universe is a cosmological model, which was named after him. Life and work Born in Sneek, Netherlands, de Sitte ...

, Kapteyn went back to Leiden to assist in upgrading the observatory to contemporary astronomical standards. His life's work, ''First attempt at a theory of the arrangement and motion of the sidereal system'', was published in 1922, and described a lens-shaped island universe of which the density decreased away from the center, now known as the '' Kapteyn Universe''. In his model the Galaxy was thought to be 40,000

light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...

s in size, the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

being relatively close (2,000 light years) to its center. The model was valid at high galactic latitudes but failed in the

galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...

because of the lack of knowledge of interstellar absorption. It was only after Kapteyn's death, in

Amsterdam Amsterdam ( , ; ; ) is the capital of the Netherlands, capital and Municipalities of the Netherlands, largest city of the Kingdom of the Netherlands. It has a population of 933,680 in June 2024 within the city proper, 1,457,018 in the City Re ...

, that Robert Trumpler determined that the amount of

interstellar reddening In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trum ...

was actually much greater than had been assumed. This discovery increased the estimate of the galaxy's size to 100,000 light years, with the Sun replaced to a distance of 30,000 light years from the

Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

. The astronomy institute of the

is named after Kapteyn. A street in the city of Groningen is also named after Kapteyn: the J.C. Kapteynlaan. And the

Isaac Newton Group of Telescopes The Isaac Newton Group of Telescopes or ING consists of three optical telescopes: the William Herschel Telescope, the Isaac Newton Telescope, and the Jacobus Kapteyn Telescope, operated by a collaboration between the UK Science and Technology ...

La Palma La Palma (, ), also known as ''La isla bonita'' () and historically San Miguel de La Palma, is the most northwesterly island of the Canary Islands, a Spanish autonomous community and archipelago in Macaronesia in the North Atlantic Ocean. La Pa ...

in the

Canary Islands The Canary Islands (; ) or Canaries are an archipelago in the Atlantic Ocean and the southernmost Autonomous communities of Spain, Autonomous Community of Spain. They are located in the northwest of Africa, with the closest point to the cont ...

named the Jacobus Kapteyn Telescope (JKT) after him. His daughter Henrietta (1881-1956) married astronomer Ejnar Hertzsprung.

Honours

Awards *

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 ...

(1902) *

(1913) *

Bruce Medal The Catherine Wolfe Bruce Gold Medal is awarded every year by the Astronomical Society of the Pacific for outstanding lifetime contributions to astronomy. It is named after Catherine Wolfe Bruce, an American patroness of astronomy, and was ...

(1913) Named after him * Kapteyn (crater) on the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

Asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

818 Kapteynia *

Kapteyn Astronomical Institute The Kapteyn Astronomical Institute is the department of astronomy of the University of Groningen in the Netherlands. The institute is named after its founder, Jacobus Cornelius Kapteyn, who lived from 1851 to 1922. Jacobus Kapteyn was appointed pr ...

at the University of Groningen * Jacobus Kapteyn Telescope (JKT) at La Palma, one of the

* Kapteyn Package, Astronomical package for Python

References

External links

History of the Kapteyn Institute
{{DEFAULTSORT:Kapteyn, Jacobus 1851 births 1922 deaths People from Barneveld Dutch astronomers Utrecht University alumni Academic staff of the University of Groningen Recipients of the Bruce Medal Recipients of the Gold Medal of the Royal Astronomical Society Foreign members of the Royal Society Members of the Royal Netherlands Academy of Arts and Sciences Honorary Fellows of the Royal Society of Edinburgh Corresponding members of the Saint Petersburg Academy of Sciences