24
2 Boarding School and University
Proposition XV reads:The average of the proper motions of stars of different
apparent brightnesses is not inversely proportional to their distance.This is fascinating; Kapteyn spent his whole life working on the distribution of the distances
of stars, and this shows that he had already thought about this in his student
days. In the next chapter we will see that William Herschel (1738–1822)—
John Herschel’s father—had assumed that all stars were intrinsically equally
bright. Then their apparent brightnesses in the sky would be an indication
of their distances. But stars are not at all equally bright intrinsically and have
a wide range of ‘luminosities’. The proper motion is the slow change of the
position of a star in the sky as a result of its motion through space. If stars on
average would move at the same speed, then the proper motion would be an
indication, statistically at least, of their distances. The smaller the proper motion, the further away (grosso modo) the star is from us. Now, what Kapteyn
was stating is that in fact it is more complicated. He must have researched the
astronomical literature on this subject and must have already thought about
the distribution of the stars in space, the subject to which he devoted his
astronomical life.
So we see that for Kapteyn in Utrecht there was no opportunity to write
a PhD thesis in astronomy, because at just this time the professor’s chair in
that disciple had been vacant. But he nevertheless developed a great interest in
astronomy and must even have done extensive observational and theoretical
work in it. In his thesis he expressed his gratitude for his training to Grinwis
and Buys Ballot as his teachers; although Hoek was still a professor in the first
years of his studies, Kapteyn apparently did not benefit much from him and
did not think it was necessary to thank him.
2.4 Catherina Elisabeth Kalshoven
During the last years of his studies in Utrecht, Kapteyn became acquainted with
the Kalshoven family and his future wife Catharina Elisabeth (Elise) (1879–
1945). He came to know her probably through Elise’s half-sister Jacqueline
Kalshoven (1846–1926). She was married to Simon Brouwer (1833–1920), a
notary who probably knew Kapteyn from the student fraternity. The Brouwer
family lived in Maarssen, about 10 km to the north-west of Utrecht, and the
Kalshovens lived there as well. Elise’s mother, Henriëtte Mariëtte Augustine
Albertine Kalshoven–Frieseman (1822-1895), was married to Jacobus Wilhelmus Kalshoven (1813–1869) in 1853 and had two daughters and a son with
him; Elise was the oldest. Kalshoven was a manufacturer and seller of pianos;
he had previously been married to Catharina Elisabeth Brandt (1814–1845)
2 Boarding School and University
Proposition XV reads:The average of the proper motions of stars of different
apparent brightnesses is not inversely proportional to their distance.This is fascinating; Kapteyn spent his whole life working on the distribution of the distances
of stars, and this shows that he had already thought about this in his student
days. In the next chapter we will see that William Herschel (1738–1822)—
John Herschel’s father—had assumed that all stars were intrinsically equally
bright. Then their apparent brightnesses in the sky would be an indication
of their distances. But stars are not at all equally bright intrinsically and have
a wide range of ‘luminosities’. The proper motion is the slow change of the
position of a star in the sky as a result of its motion through space. If stars on
average would move at the same speed, then the proper motion would be an
indication, statistically at least, of their distances. The smaller the proper motion, the further away (grosso modo) the star is from us. Now, what Kapteyn
was stating is that in fact it is more complicated. He must have researched the
astronomical literature on this subject and must have already thought about
the distribution of the stars in space, the subject to which he devoted his
astronomical life.
So we see that for Kapteyn in Utrecht there was no opportunity to write
a PhD thesis in astronomy, because at just this time the professor’s chair in
that disciple had been vacant. But he nevertheless developed a great interest in
astronomy and must even have done extensive observational and theoretical
work in it. In his thesis he expressed his gratitude for his training to Grinwis
and Buys Ballot as his teachers; although Hoek was still a professor in the first
years of his studies, Kapteyn apparently did not benefit much from him and
did not think it was necessary to thank him.
2.4 Catherina Elisabeth Kalshoven
During the last years of his studies in Utrecht, Kapteyn became acquainted with
the Kalshoven family and his future wife Catharina Elisabeth (Elise) (1879–
1945). He came to know her probably through Elise’s half-sister Jacqueline
Kalshoven (1846–1926). She was married to Simon Brouwer (1833–1920), a
notary who probably knew Kapteyn from the student fraternity. The Brouwer
family lived in Maarssen, about 10 km to the north-west of Utrecht, and the
Kalshovens lived there as well. Elise’s mother, Henriëtte Mariëtte Augustine
Albertine Kalshoven–Frieseman (1822-1895), was married to Jacobus Wilhelmus Kalshoven (1813–1869) in 1853 and had two daughters and a son with
him; Elise was the oldest. Kalshoven was a manufacturer and seller of pianos;
he had previously been married to Catharina Elisabeth Brandt (1814–1845)