The compass: how a needle on a silk thread showed sailors the way

On a cloudy night on the open sea, when neither the sun nor the stars are visible, how does a sailor find his direction? For thousands of years there was no sure answer. Then a small object solved the problem: an iron needle rubbed with a magnet, which, if left free, always comes to rest in the same direction. The first clear written description of the compass comes from China in the 11th century.

From lodestone to needle

Some stones naturally attract iron. They are called lodestone. In his 1925 book, the scholar Thomas Francis Carter wrote that the Chinese knew the properties of this stone from before the common era. Chinese writings of the first millennium AD contain several accounts of “south-pointing chariots”, but according to Carter their exact meaning remains unclear even today.

According to Carter, the earliest clear mention of a magnetic needle in the writings of any language was made by the Chinese scholar Shen Kuo, who lived from 1030 to 1093. He is the same Shen Kuo who described the printing of movable type in the same book, as mentioned in the previous article in this series.

Shen Kuo’s description

According to the American scientific organisation AAAS, Shen Kuo’s book “Dream Pool Essays” was written around 1088. An educational article from REACH, an educational project of the engineers’ organisation IEEE, quotes the book in English translation: when craftsmen rub the tip of a needle with lodestone, it begins to point south.

Shen Kuo tried several ways of using the needle. According to him, the needle can be floated on water, but it keeps wobbling a little. In his view, the best method is to fix a single thread of new silk to the middle of the needle with a small piece of wax and hang the needle in a place free from wind. It then always points south.

Why the needle does not point due south

The most interesting part of Shen Kuo’s account is a small detail. He wrote that the needle points south, but always leans slightly to the east, not exactly south.

Today we know that the Earth’s magnetic pole and geographic pole are not in the same place. This creates a small angle between the compass needle and the true north-south line. It is called magnetic declination. According to AAAS, Shen Kuo recognised this difference between magnetic and true direction.

First use at sea

According to Carter, the evidence suggests that in China the needle came to be known first and entered navigation later. He says the first mention of the compass at sea in Chinese writings dates from a little after 1100 and is found in a book by an author named Zhu Yu. The mention relates to the period 1086 to 1099, when sailors on ships plying between Canton (today’s Guangzhou) and Sumatra used it. Zhu Yu’s book was written in the early 12th century.

Arrival in Europe

In Europe, the magnetic needle is mentioned in the late 12th century. According to IEEE REACH, the English scholar Alexander Neckam, who lived from 1157 to 1217, studied and taught at the University of Paris in the 1180s. In his book “De naturis rerum”, he wrote that when the sun is not visible in cloudy weather or the darkness of night falls, sailors touch a needle with a magnet, and when the needle turns and stops, its tip points north. He also mentioned the same needle in another book, “De utensilibus”.

At about the same time, around the 1180s or 1190s, the poet Guyot de Provins mentioned in a poem a magnetic needle set on a straw, which shows sailors the right way on a dark night. Carter describes his poem as the first mention of the compass in Europe. According to IEEE REACH, the first known mention of the needle in Arabic writings dates from 1242 and in Persian from 1232. Historians do not yet have enough evidence to decide exactly by which route this knowledge travelled.

From floating needle to modern compass

According to IEEE REACH, by the early 14th century Italian merchant ships carried a compass in which a round card turned freely on a pivot, with the needle fixed beneath it and the directions written on top. In 1600, William Gilbert of England published “De Magnete” on the magnet, which IEEE REACH calls the first scientific book on magnetism and the Earth’s magnetism.

Today a tiny magnetic sensor in our mobile phones also works as a compass. But the principle is the one Shen Kuo saw a thousand years ago in a needle hanging from a silk thread: the Earth itself is a large magnet, and a small needle recognises its direction.