Articles | Volume 7, issue 2
Hist. Geo Space. Sci., 7, 73–77, 2016

Special issue: History of geophysical institutes and observatories

Hist. Geo Space. Sci., 7, 73–77, 2016

Short note 14 Sep 2016

Short note | 14 Sep 2016

After some 350 years – zero declination again in Paris

Mioara Mandea1 and Jean-Louis Le Mouël2 Mioara Mandea and Jean-Louis Le Mouël
  • 1CNES – Centre National d'Etudes Spatiales, 2 Place Maurice Quentin, 75039 Paris CEDEX 01, France
  • 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR7154 – CNRS, 1 rue Jussieu, 75238 Paris CEDEX 05, France

Abstract. The main part of the geomagnetic field – produced by a dynamo process in the Earth's outer core – changes its direction and strength in time, over timescales from months to centuries, even millennia. Its temporal variations, known as secular variation and secular acceleration, are crucial ingredients for understanding the physics of the deep Earth. Very long series of measurements therefore play an important role. Here, we provide an updated series of geomagnetic declination in Paris, shortly after a very special occasion: its value has reached zero after some 350 years of westerly values. Indeed, during October and November 2013, the declination at the Chambon la Forêt geomagnetic observatory changed from westerly to easterly values, the agonic line then passing through this place. We take this occasion to emphasize the importance of long series of continuous measurements.