普通高中课程标准实验教科书 英语选修9 Unit 2 Reading

　　SRILING THE OCERNS

　　We may well wonder how seamen explored the oceans before latitude and longitude made it possible to plot a ship's position on a map. The voyages of travellers before the 17th century show that they were not at the mercy of the sea even though they did not have modern navigational aids. So how did they navigate so well? Read these pages from an encyclopedia.

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　　Using nature to help Keeping alongside the coastline

　　This seems to have been the first and most useful form of exploration which carried the minimum amount of risk.

　　Using celestial bodies

　　North Star

　　At the North Pole the North Star is at its highest position in the sky, but at the equator it is along the horizon. So accomplished navigators were able to use it to plot their positions.

　　Sun

　　On a clear day especially during the summer the sailors could use the sun overhead at midday to navigate by. They can use the height of the sun to work out their latitude.

　　Clouds

　　Sea captains observed the clouds over islands. There is a special cloud formation which indicates there is land close by.

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