On the discovery of the Zeeman effect on the Sun and in the laboratory

The origin of the discoveries, both on the Sun and in the laboratory, of the action of a magnetic field on spectral lines, the so-called Zeeman effect, is discussed. The talk embraces the period from 1866, the first date of which the speaker is aware of observed evidences about the widening of spectral lines in sunspots (as compared to those formed in the photosphere), until 1908, the year in which the magnetic field in sunspots is definitely discovered. The interval between 1896-97, and 1908 is mainly dealt with from an astrophysical standpoint, although there are plenty of important contributions from laboratory experiments. The reason is twofold: on the one hand, the significant role played by the Zeeman effect on the development of quantum mechanics has suggested major historical studies that have already appeared in the literature and that are mainly concerned with laboratory -but not with astrophysical- spectroscopy; on the other hand, the understanding of the sizeable delay between Zeeman’s and Hale’s discoveries (12 years) seems to be of concern after accounting for the fact that the findings by the first author were soon brought to the notice of the astrophysical community.