The splitting of a spectral line due to a magnetic field. It is named after the Dutch physicist Pieter Zeeman (1865–1943). The Zeeman effect is widely used for determining magnetic fields in astronomical objects, notably the Sun. However, field strengths are often insufficient to cause actual line splitting (sunspot fields, of about 0.4 tesla, are an exception); instead, line broadening occurs. The strengths of weak fields can be deduced by measuring the polarization of each line wing, which is the principle of the solar magnetograph, an instrument for measuring magnetic fields.