Ancient sundials and maps reveal historical geomagnetic declination values



Long records of the evolution of Earth's magnetic field help in the understanding of the field's generating mechanism. Archeomagnetism and palaeomagnetism supply valuable low-resolution information about the field over time ranges of centuries to millions of years. Higher spatial resolution can come from direct measurements of the field, but these are restricted to the past few hundred years.

In some places, such as London, Rome, or Paris, declination and inclination were frequently measured over the past four centuries [Malin and Barracogh, 1981; Cafarella et al., 1992; Alexandrescu et al., 1996]. In the early nineteenth century, measurements of field intensity became available, and the first magnetic observatory recording three field components (declination, inclination, and horizontal intensity) was installed by C.F. Gauss, in Göttingen, Germany, in 1832. Since then, continuous observations of the vector magnetic field have been made by some 200 worldwide observatories, thus making it possible to obtain good, though still incomplete, knowledge of the detailed field morphology and temporal evolution.