3.2 Higher Parts of the Heavens
39
Fig. 3.6 Positional measurements of stars were performed with meridian circles. This is
the one of the Sterrewacht Leiden [38]
the measured proper motions of at first 12, later 36 stars. The position of the
Apex, which he derived, corresponds remarkably well to modern determinations. The Sun moves in the direction of the constellation Hercules (about
15 ◦ from the bright star Vega in the constellation Lyra). The speed of the
Sun relative to the stars in its neighborhood can only be determined (statistically, if we have large numbers of stars) if we know the radial velocity of
those stars. Such radial velocities of stars can be measured using the effect
that light or sound gets a higher frequency (or smaller wavelength) when
an object moves towards us, and vice versa. This effect was discovered in
1842 by Andreas Doppler (1803–1853). The velocity of a star follows from
the shift of spectral lines: dark lines of known stationary wavelength in the
spectrum. Spectroscopy of stars, however, was only possible towards the
end of the nineteenth century, and then only for the brightest stars. Then it
became clear then that the velocity of the Sun relative to the collection of
stars in the the solar neighborhood is 17–18 km/s.
Another concept that we will encounter frequently is that of parallax; I
refer to Appendix A.2 for a more technical discussion. Ever since the introduction of the Copernican or heliocentric model of the Planetary System,
the aim had been to observe the reflection the motion of the Earth around
39
Fig. 3.6 Positional measurements of stars were performed with meridian circles. This is
the one of the Sterrewacht Leiden [38]
the measured proper motions of at first 12, later 36 stars. The position of the
Apex, which he derived, corresponds remarkably well to modern determinations. The Sun moves in the direction of the constellation Hercules (about
15 ◦ from the bright star Vega in the constellation Lyra). The speed of the
Sun relative to the stars in its neighborhood can only be determined (statistically, if we have large numbers of stars) if we know the radial velocity of
those stars. Such radial velocities of stars can be measured using the effect
that light or sound gets a higher frequency (or smaller wavelength) when
an object moves towards us, and vice versa. This effect was discovered in
1842 by Andreas Doppler (1803–1853). The velocity of a star follows from
the shift of spectral lines: dark lines of known stationary wavelength in the
spectrum. Spectroscopy of stars, however, was only possible towards the
end of the nineteenth century, and then only for the brightest stars. Then it
became clear then that the velocity of the Sun relative to the collection of
stars in the the solar neighborhood is 17–18 km/s.
Another concept that we will encounter frequently is that of parallax; I
refer to Appendix A.2 for a more technical discussion. Ever since the introduction of the Copernican or heliocentric model of the Planetary System,
the aim had been to observe the reflection the motion of the Earth around