110
6 Laboratory and Statistical Astronomy
introduced the Einstein–de Sitter model of the Universe. However, de Sitter
himself seems to have considered his work on Jupiter’s satellite system as more
significant.
De Sitter was next appointed as Kapteyn’s assistant. He started with what
he had actually gone for to the Cape. Kapteyn had noted that on the photographic plates for the CPD not only relatively more stars could be seen in
the Milky Way than at higher Galactic latitudes, but that this effect was even
more pronounced than in the Durchmusterungs made visually and with meridian circles (the southern extensions of the Bonner Durchmusterung, including
those of the Córdoba Observatorio Astronómico in Argentinië, which had
been started in the meantime). Because photographic plates are more sensitive
to blue light than the eye, this would mean that there should be relatively
more blue stars in the Milky Way. Kapteyn had already presented an extensive
study on this subject for the KNAW in 1892. One approach would be to use
new photographic emulsions (so-called isochromatic emulsions), which had a
color-response more similar to that of the human eye. Gill was already using
these to take plates in areas far away from the Milky Way. De Sitter then measured these up. The result was published in 1900 as a second contribution to
the Publications of the Astronomical Laboratory at Groningen. The conclusion
was not very conclusive; it seemed that there were systematic errors in the
visual estimated magnitudes. De Sitter then investigated this in detail using
more photometric and visual observations done at the Cape. In the meantime
Robert Innes had obtained more of such observations, that de Sitter had not
been able to do because the weather had been bad during his stay there. In the
Volume XII of the Publications of the Astronomical Laboratory at Groningen in
1904, he confirmed Kapteyn’s findings, but also showed that the matter was
actually more complicated because it appeared that the number of blue stars
fluctuated at least as much along the Milky Way as it did perpendicular to it.
As Kapteyn’s assistant, de Sitter was closely involved in measuring the plates
Anders Donner had obtained in Helsingfors to measure parallaxes and proper
motions. Between 1902 and 1908, when Willem de Sitter left for Leiden, the
Publications of the Astronomical Laboratory at Groningen contained four large
volumes about such observations in which Willem de Sitter had been involved.
6.5 Secular Parallax
While the production of the CPD was still in progress, Kapteyn already took
an extensive interest in determining the distribution of the stars in space. We
are in the fortunate circumstance that he was asked in 1907 by David Gill to
6 Laboratory and Statistical Astronomy
introduced the Einstein–de Sitter model of the Universe. However, de Sitter
himself seems to have considered his work on Jupiter’s satellite system as more
significant.
De Sitter was next appointed as Kapteyn’s assistant. He started with what
he had actually gone for to the Cape. Kapteyn had noted that on the photographic plates for the CPD not only relatively more stars could be seen in
the Milky Way than at higher Galactic latitudes, but that this effect was even
more pronounced than in the Durchmusterungs made visually and with meridian circles (the southern extensions of the Bonner Durchmusterung, including
those of the Córdoba Observatorio Astronómico in Argentinië, which had
been started in the meantime). Because photographic plates are more sensitive
to blue light than the eye, this would mean that there should be relatively
more blue stars in the Milky Way. Kapteyn had already presented an extensive
study on this subject for the KNAW in 1892. One approach would be to use
new photographic emulsions (so-called isochromatic emulsions), which had a
color-response more similar to that of the human eye. Gill was already using
these to take plates in areas far away from the Milky Way. De Sitter then measured these up. The result was published in 1900 as a second contribution to
the Publications of the Astronomical Laboratory at Groningen. The conclusion
was not very conclusive; it seemed that there were systematic errors in the
visual estimated magnitudes. De Sitter then investigated this in detail using
more photometric and visual observations done at the Cape. In the meantime
Robert Innes had obtained more of such observations, that de Sitter had not
been able to do because the weather had been bad during his stay there. In the
Volume XII of the Publications of the Astronomical Laboratory at Groningen in
1904, he confirmed Kapteyn’s findings, but also showed that the matter was
actually more complicated because it appeared that the number of blue stars
fluctuated at least as much along the Milky Way as it did perpendicular to it.
As Kapteyn’s assistant, de Sitter was closely involved in measuring the plates
Anders Donner had obtained in Helsingfors to measure parallaxes and proper
motions. Between 1902 and 1908, when Willem de Sitter left for Leiden, the
Publications of the Astronomical Laboratory at Groningen contained four large
volumes about such observations in which Willem de Sitter had been involved.
6.5 Secular Parallax
While the production of the CPD was still in progress, Kapteyn already took
an extensive interest in determining the distribution of the stars in space. We
are in the fortunate circumstance that he was asked in 1907 by David Gill to