Shen Kuo's Compass Declination

The compass does not point to true north

First published: Shen Kuo, *Mengxi Bitan* (*Dream Pool Essays*), ch. 24, article 437 (1088).

A magnetised needle, suspended or floated, does not point exactly to the celestial pole — it declines slightly to the east. The first documented observation of magnetic declination.

In his encyclopedic *Dream Pool Essays* (1088), the Song dynasty polymath Shen Kuo recorded a series of observations about the magnetic compass that were centuries ahead of European knowledge. He noted that a magnetised needle, when suspended by a silk thread or floated on water, points roughly but not exactly to the celestial north pole — it declines slightly to the east. This is the first documented observation of magnetic declination (the angle between magnetic north and true geographic north). Shen Kuo also described methods of magnetisation (rubbing with lodestone), compared suspension techniques (thread vs water vs fingernail-balanced vs bowl-edge), and observed that the best needles "always point slightly east of south, not directly south" (Chinese compasses traditionally indicated south). The observation of declination was practically important for navigation and theoretically important as evidence that the magnetic and rotational axes of the Earth do not coincide — a fact not understood until William Gilbert's *De Magnete* (1600) and not fully explained until the modern theory of the geodynamo.