The artificially magnetized compass needle was used in ocean sailing, and in overcast weather ships no longer had to depend on observation of the sky.
Zhu Yu's Pingzhou Table Talks of the Northern Song records that the pilots of Guangzhou "watch the south-pointing needle when it is overcast".
A few years later, Xu Jing's Illustrated Account of the Xuanhe Embassy to Goryeo of the Northern Song records the envoy's fleet steering by a "floating south-pointing needle" in overcast weather.
In the Ming, Zheng He's fleets navigated by the compass bearings called needle routes and sailed far into the Western Ocean, and navigation by compass reached its height.
Compass Navigation is the class in the chapter on Sciences on the use of the artificially magnetized compass in ocean sailing. In overcast weather a ship cannot see the sky and steers by the compass needle, and in the Ming the fleets of Zheng He sailed far into the Western Ocean along needle routes.
