Archeomagnetic and paleomagnetic dating
It was not until sometime in the late 14th Century that compasses were used for sea-going navigation in China.
According to Needham (1962), changes in magnetic declination were discovered in China around 720 CE when the astronomer Yi-Xing measured magnetic declination (see Figure 14.2b).
The strength of the radial component of the magnetic field inferred for the core mantle boundary at two time intervals in the GUFM1 model is shown in Figure 14.4.
Compare Figure 14.4b with Figure 2.3a in Chapter 2 which is the strength of the magnetic field observed at the surface.
Magnets and compasses were discussed in a letter (Epistola) by Petrus Peregrinus written in 1269 (finally printed in 1558).Compare for example the “line of no variation” in Figure 14.3 with the line of zero declination from the IGRF of 2005.It has moved significantly to the west in the equatorial and southern Atlantic realms.The magnetic properties of lodestone were already well known by the early Greeks.Aristotle (384-332 BCE) wrote of the work of Greek philosopher Thales of Miletos (624-546 BCE) in his book on the soul ( But the earliest compass appears to date from the first century in China.