sea / nautical / marine chart
Nautical charts are maps of the depth of the sea and the currents of the oceans. They provide modern sailors with routes through the seas, rather like roads through the countryside. The information began to be collected in the nineteenth century and has continued to be updated ever since. These charts were not available to Captain Bligh or Zheng He.
He (1728-1779) was the greatest Britain navigator. He enlisted in the Royal Navy in 1755.
In 1768 he was promoted and sent to the Pacific where he surveyed Tahiti, New Zealand, and Australia. On his famous 2nd expedition (1772-75) he explored Antarctica. In 1776 in his third and final voyage, he explored the West coast of North America and discovered the Hawaiian Islands.
The navigator can then see two images, the horizon and the sun side by side. He can then measure the angle between them on a scale at the bottom of the instrument. The scale goes from 0 to 120 degrees. An earlier instrument, called a quadrant, measured in angle in the same way but it could only measure angles from 0 to 90 degrees.
