7.5 Parallaxes Again
141
the distance. Karl Friedrich Küstner had previously measured the radial velocity
of three stars in the Hyades (in these cases almost exactly 40 km/s), and from
Boss’ determination of the Apex for the Hyades followed a spatial velocity of
the Hyades relative to us of 45 km/s and a parallax of 0 .0253. This method
of distance determination for a nearby cluster is called the ‘moving cluster
method’. The Hyades today are still vital for the distance scale in the Universe,
because as the nearest cluster it is a reference point for more distant star clusters.
But there was more work to be done. Kapteyn himself did extensive research
in order to improve the accuracy of the precession constant. As we have seen,
the coordinates of stars change as a result of precession; the spinning motion
of the rotation axis of the Earth. As a result the vernal equinox (both equinoxes
of course) moves along the equator and it was absolutely necessary to know
how fast this change occurred (the precession constant) in order to be able to
measure proper motions, which are after all based on changing positions on the
sky. In addition, in order to be able to use secular parallaxes, it was necessary
to split the proper motions of stars into the component in the direction ApexAntapex, and the one perpendicular to it. Kapteyn did not like quick and
dirty work and, if necessary, was prepared to take on enormous amounts of
calculation, which was done by assistants (calculators). For the Bradley-stars he
calculated those components for six different combinations of the precession
constant and the position of the Apex. Weersma completed his PhD thesis in
1908 as part of this project. It was entitled A determination of the Apex of the
solar motion according to the method of Bravais.
Weersma did more important work in support of Kapteyn’s program, but
left astronomy in 1912. He was very much interested in philosophy and in
the long run astronomy was no longer interesting enough for him. In order
to have more time to better follow his interests, he became a mathematics
teacher. He converted himself to socialism and wrote books on Marxism and
other philosophical themes involving dialectics and logic.
All this work of his PhD students and assistants was part of the preparation
for Kapteyn’s eventual attempt to solve the problem of the construction of
heavens. But first of all, what was needed was a reliable and extensive inventory
of the stars in the sky, including their properties. Kapteyn had thought about
this at length before he went to St. Louis.
7.6 Plan of Selected Areas
Not long after the turn of the century Kapteyn had already realized that it
was necessary to systematically determine the distribution and properties of
141
the distance. Karl Friedrich Küstner had previously measured the radial velocity
of three stars in the Hyades (in these cases almost exactly 40 km/s), and from
Boss’ determination of the Apex for the Hyades followed a spatial velocity of
the Hyades relative to us of 45 km/s and a parallax of 0 .0253. This method
of distance determination for a nearby cluster is called the ‘moving cluster
method’. The Hyades today are still vital for the distance scale in the Universe,
because as the nearest cluster it is a reference point for more distant star clusters.
But there was more work to be done. Kapteyn himself did extensive research
in order to improve the accuracy of the precession constant. As we have seen,
the coordinates of stars change as a result of precession; the spinning motion
of the rotation axis of the Earth. As a result the vernal equinox (both equinoxes
of course) moves along the equator and it was absolutely necessary to know
how fast this change occurred (the precession constant) in order to be able to
measure proper motions, which are after all based on changing positions on the
sky. In addition, in order to be able to use secular parallaxes, it was necessary
to split the proper motions of stars into the component in the direction ApexAntapex, and the one perpendicular to it. Kapteyn did not like quick and
dirty work and, if necessary, was prepared to take on enormous amounts of
calculation, which was done by assistants (calculators). For the Bradley-stars he
calculated those components for six different combinations of the precession
constant and the position of the Apex. Weersma completed his PhD thesis in
1908 as part of this project. It was entitled A determination of the Apex of the
solar motion according to the method of Bravais.
Weersma did more important work in support of Kapteyn’s program, but
left astronomy in 1912. He was very much interested in philosophy and in
the long run astronomy was no longer interesting enough for him. In order
to have more time to better follow his interests, he became a mathematics
teacher. He converted himself to socialism and wrote books on Marxism and
other philosophical themes involving dialectics and logic.
All this work of his PhD students and assistants was part of the preparation
for Kapteyn’s eventual attempt to solve the problem of the construction of
heavens. But first of all, what was needed was a reliable and extensive inventory
of the stars in the sky, including their properties. Kapteyn had thought about
this at length before he went to St. Louis.
7.6 Plan of Selected Areas
Not long after the turn of the century Kapteyn had already realized that it
was necessary to systematically determine the distribution and properties of