3.1 Astronomy of the Solar System
31
Sun could continue to shine at the rate it does. This calculation, made by
William Thomson, 1st Baron Kelvin (1824–1907) and Hermann Ludwig
Ferdinand von Helmholtz (1821–1894) and therefore named the KelvinHelmholtz contraction time, yields 30 million years. So, on that basis, it
was assumed that the Sun could be at most millions of years old. That was
much older than would follow from the dating of the Creation at 4004BC
by bishop James Ussher (or Usher; 1581-1656) on the basis of Biblical
chronologies.
3.2 Higher Parts of the Heavens
In 1875 people had a very good idea of the structure of the Solar System.
Seen from Earth, the Sun describes an orbit in the sky, completing a full
circle in a year’s time. This orbit in the sky is called the ecliptic, and the
constellations through which the Sun moves (if you could see them at that
moment) are those of the zodiac, which we know from astrology. The orbits
of the planets and asteroids are all close to the orbital plane of the Earth, so
in the sky they are always quite close to the ecliptic. The study of the ‘fixed
stars’ was much less developed than that of the planets. Because the stars
can stand anywhere in the sky and do not care about the ecliptic, Kapteyn
would later refer to this as the ‘Higher Parts of Heaven’ (in his speech as
outgoing Rector Magnificus in 1891).
Most of the astronomers’ research effort around 1875 went to the Planetary System. An inventory of scientific publications 2 from that period shows
the following. About 35% concerned the orbits of objects in the Solar System
and 25% the studies of meteors, the Sun and Moon (including eclipses) and
the physical properties of planets. Nearly 20% was about instrumentation,
photography, observational and mathematical techniques and astronomy
related sciences. Only about 20% of the literature was devoted to the stars
and nebulae outside our planetary system. At the time that Kapteyn entered
university, almost nothing was known about the physical properties of fixed
stars, nor about the origin of the light they emit, or about their formation
and evolution. But since antiquity there had been catalogs describing their
positions in the sky and their apparent brightness. Before I say more about
this, I have to explain first how astronomers catalog the positions of stars
and quantify brightness.
We represent the sky as a sphere that we look at from the inside. The
position of a star in the sky is measured in the same way as the geographical
2 See Sect. 3.8 and Table 3.1 in my Jacobus Cornelius Kapteyn; Born investigator of the Heavens [5].
31
Sun could continue to shine at the rate it does. This calculation, made by
William Thomson, 1st Baron Kelvin (1824–1907) and Hermann Ludwig
Ferdinand von Helmholtz (1821–1894) and therefore named the KelvinHelmholtz contraction time, yields 30 million years. So, on that basis, it
was assumed that the Sun could be at most millions of years old. That was
much older than would follow from the dating of the Creation at 4004BC
by bishop James Ussher (or Usher; 1581-1656) on the basis of Biblical
chronologies.
3.2 Higher Parts of the Heavens
In 1875 people had a very good idea of the structure of the Solar System.
Seen from Earth, the Sun describes an orbit in the sky, completing a full
circle in a year’s time. This orbit in the sky is called the ecliptic, and the
constellations through which the Sun moves (if you could see them at that
moment) are those of the zodiac, which we know from astrology. The orbits
of the planets and asteroids are all close to the orbital plane of the Earth, so
in the sky they are always quite close to the ecliptic. The study of the ‘fixed
stars’ was much less developed than that of the planets. Because the stars
can stand anywhere in the sky and do not care about the ecliptic, Kapteyn
would later refer to this as the ‘Higher Parts of Heaven’ (in his speech as
outgoing Rector Magnificus in 1891).
Most of the astronomers’ research effort around 1875 went to the Planetary System. An inventory of scientific publications 2 from that period shows
the following. About 35% concerned the orbits of objects in the Solar System
and 25% the studies of meteors, the Sun and Moon (including eclipses) and
the physical properties of planets. Nearly 20% was about instrumentation,
photography, observational and mathematical techniques and astronomy
related sciences. Only about 20% of the literature was devoted to the stars
and nebulae outside our planetary system. At the time that Kapteyn entered
university, almost nothing was known about the physical properties of fixed
stars, nor about the origin of the light they emit, or about their formation
and evolution. But since antiquity there had been catalogs describing their
positions in the sky and their apparent brightness. Before I say more about
this, I have to explain first how astronomers catalog the positions of stars
and quantify brightness.
We represent the sky as a sphere that we look at from the inside. The
position of a star in the sky is measured in the same way as the geographical
2 See Sect. 3.8 and Table 3.1 in my Jacobus Cornelius Kapteyn; Born investigator of the Heavens [5].