8.4 Weersma, Yntema, van Rhijn
167
per square arcsecond. He estimated the starlight in the same units as 20.4 in
the brightest parts of the Milky Way to 22.6 in the Galactic poles.
Nowadays, Yntema’s Earth-light is referred to as ‘airglow’. It originates in
the higher layers of the atmosphere as a result of processes in which molecules
are ionized during the day by sunlight, i.e. electrons are being released, while
they ‘recombine’ again during the night; and by interaction with cosmic rays,
high-energy particles from the Sun or from interstellar space (produced in
supernovae and the like). It is about one and a half magnitudes fainter than
Yntema estimated. There is another component, which Yntema did not identify, and that is called zodiacal light. The name comes from the term zodiac;
it is mainly found in the ecliptic, the orbital plane of the Earth and the other
planets. It is sunlight that is reflected by dust in our planetary system. Its surface
brightness is comparable to that of starlight.
Airglow and zodiacal light still make a good determination of the background starlight from Earth problematic, but now this can be done from
space. Of course then there is no airglow, but still there is zodiacal light. Now,
the dust responsible for this is more common the closer you get to the center
of the Solar System – the Sun. A satellite that goes beyond the orbit of Mars
does not really suffer from it anymore. The spacecraft Pioneer 10 and 11, for
example, which went to investigate the planet Jupiter closely, traveled beyond
Mars. They were launched in 1972 and 1973 respectively and passed Jupiter
at the end of 1973 and the end of 1974. Pioneer 11 also flew past Saturn and
NASA was able to follow these satellites for a long time after they had left the
planetary system. On board these satellites were instruments that charted the
sky from the slowly rolling spacecraft, after they had passed the asteroid belt.
There is then much less dust and hardly any zodiacal light. So they could map
the background starlight, ironically not in order to study it, but to be able to
correct observations of zodiacal light from Earth. I did publish a study of this
material in 1986, in which I used it to investigate the distribution of starlight
in our Galaxy, just as Yntema and Kapteyn had intended long ago.
Yntema remarked at the end of his thesis that the result of his measurements was uncertain, but that a considerable improvement was possible if the
experiment was repeated from a high mountain peak in a dark environment.
This possibility arose during Pieter van Rhijn’s doctoral research. As discussed
in more detail in the next chapter, between 1908 and 1914 Kapteyn traveled
annually to Mount Wilson Observatory in Southern California, and van Rhijn
as a result of this spent some time there in 1913. There he repeated Yntema’s
measurements and the analysis thereof he made public in his dissertation in
1915, although a formal publication of the result in a scientific paper was not
forthcoming until 1919 and 1921, probably because he had too much other
167
per square arcsecond. He estimated the starlight in the same units as 20.4 in
the brightest parts of the Milky Way to 22.6 in the Galactic poles.
Nowadays, Yntema’s Earth-light is referred to as ‘airglow’. It originates in
the higher layers of the atmosphere as a result of processes in which molecules
are ionized during the day by sunlight, i.e. electrons are being released, while
they ‘recombine’ again during the night; and by interaction with cosmic rays,
high-energy particles from the Sun or from interstellar space (produced in
supernovae and the like). It is about one and a half magnitudes fainter than
Yntema estimated. There is another component, which Yntema did not identify, and that is called zodiacal light. The name comes from the term zodiac;
it is mainly found in the ecliptic, the orbital plane of the Earth and the other
planets. It is sunlight that is reflected by dust in our planetary system. Its surface
brightness is comparable to that of starlight.
Airglow and zodiacal light still make a good determination of the background starlight from Earth problematic, but now this can be done from
space. Of course then there is no airglow, but still there is zodiacal light. Now,
the dust responsible for this is more common the closer you get to the center
of the Solar System – the Sun. A satellite that goes beyond the orbit of Mars
does not really suffer from it anymore. The spacecraft Pioneer 10 and 11, for
example, which went to investigate the planet Jupiter closely, traveled beyond
Mars. They were launched in 1972 and 1973 respectively and passed Jupiter
at the end of 1973 and the end of 1974. Pioneer 11 also flew past Saturn and
NASA was able to follow these satellites for a long time after they had left the
planetary system. On board these satellites were instruments that charted the
sky from the slowly rolling spacecraft, after they had passed the asteroid belt.
There is then much less dust and hardly any zodiacal light. So they could map
the background starlight, ironically not in order to study it, but to be able to
correct observations of zodiacal light from Earth. I did publish a study of this
material in 1986, in which I used it to investigate the distribution of starlight
in our Galaxy, just as Yntema and Kapteyn had intended long ago.
Yntema remarked at the end of his thesis that the result of his measurements was uncertain, but that a considerable improvement was possible if the
experiment was repeated from a high mountain peak in a dark environment.
This possibility arose during Pieter van Rhijn’s doctoral research. As discussed
in more detail in the next chapter, between 1908 and 1914 Kapteyn traveled
annually to Mount Wilson Observatory in Southern California, and van Rhijn
as a result of this spent some time there in 1913. There he repeated Yntema’s
measurements and the analysis thereof he made public in his dissertation in
1915, although a formal publication of the result in a scientific paper was not
forthcoming until 1919 and 1921, probably because he had too much other