Form the basis of paleomagnetic dating
Thus Europeans discovered secular variation of the magnetic field in 1634, nearly a millenium after the Chinese.
Captain Edmond Halley carried out scientific exploration at sea with the expeditions of the Pink Paramore (1698-1701).
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 compass arrived in Europe some time in the 12 century.
The magnetic field is one component of the highly complex Earth system.
The geomagnetic field changes on frequencies of 10s of microseconds (radio waves) to millions and perhaps billions of years.
Direct observations contribute to our knowledge of field behavior for the last few centuries, but on longer times scales we need to use paleomagnetic and archaeomagnetic techniques.
Gauss provided the mathematical framework we use today for dealing with geomagnetic data when he derived the spherical harmonic expression for the geomagnetic potential field (see Chapter 2). Westward drift would imply that these correlated features would “rise” to the right.
The first such analysis (done in 1835) was based on 84 data points evaluated on an evenly spaced grid from isomagnetic charts of the magnetic field elements available at the time. Fastforwarding to the current millenium, we find researchers still poring over these centuries old measurements.