7.6 Plan of Selected Areas
143
Fig. 7.13 David Gill. This picture comes from the Album Amicorum, presented to H.G.
van de Sande Bakhuyzen on the occasion of his retirement as professor of astronomy
and director of the Sterrewacht Leiden in 1908 [47]
to flourish, and he saw his friend as the great man, as president of the British
Association and host.
Kapteyn finally published his Plan in 1906, after he had received pledges of
cooperation from the most important persons. Figure 7.14 shows the distribution of the 206 Selected Areas across the sky. This seems more regular than
it in fact is; the exact locations were chosen to avoid bright stars and other
extended objects such as nebulae and the like. The publication specified the
data that Kapteyn proposed to be measured. In addition to accurate magnitudes for 200,000 stars in two wavelength regions (so that there would also be
information about the colors), these were the parallaxes and proper motions
for 20,000 of these stars, as well as spectra, so that the type of star could be
determined, and for as many stars as possible radial velocities. The dimensions