The Marine Compass
A tall ship is sailed on a straight course using a compass. A marine compass is made up of a magnet freely suspended and balanced on a fine point of metal that
has a light compass card marked with the points of the compass on it. The marine compass is also marked with degrees. The marine compass is encased in a sealed bowl of non-magnetic metal tha is both air tight and water tight.
At the forward edge of the compass bowl a line called the Lubber Line is marked representing the bow of the boat. That way the course that the tall ship is sailing can be read easily because the bow is in alignment with the lubber line on the marine compass. Of course, the compass card remains stationary, because it is the lubber line that moves along with the ship. The marine compass does not align with True North but the marine compass points instead to magnetic north, the north pole of the magnetic field of the earth. Allowance will be made in calculations for the variations between true north and magnetic north deviations. The corrections for magnetic variation is shown on a navigation chart. To calculate the true course you subtract the magnetic variation from a Western variation and add for an Eastern one.
Marine compasses for sailing ships have been used in China since the second century BC. The chinese called these marine compasses "South pointers". About 1000 AD Chinese trading junks were able to sail to places as far away as Saudi Arabia using these elementary marine compasses. In the fifteenth century Chinese sailing ships sailed from Korea and Japan to the East Coast of Africa. Western Sailing ships started using the marine compass in the Mediterranean Sea around the thirteenth or fourteenth century.