“Prince Henry’s Captains Learn About the World”
SOURCE: Age of Europe

In the year 1400 Africa was almost an unknown continent. The Mediterranean coast was well known, but just beyond the coastal lands lay the Sahara, the greatest desert in the world. Sometimes Arab traders brought goods across it, but no European really knew what lay beyond. Soon after 1400 a young man in Portugal became interested in the great unknown land of Africa. He was Prince Henry (see below), son of the king of Portugal. Henry’s country was small and had little wealth. He wished to make it rich by building up its trade. There were a number of reasons that Prince Henry began his voyages of exploration. Azurara, one of Prince Henry's contemporaries and chroniclers, summarized them as:
1. The desire to know the country beyond Cape Bojador.
2. To establish trade relations advantageous to Portugal.
3. To determine the strength of his enemies in the region.
4. To seek allies to help wage battles against the enemies of Christianity.
5. To spread Christianity.
Henry could not hope to send ships to trade on the Mediterranean Sea. The Italian merchants had the Mediterranean trade in their own hands. Henry decided to send his ships southward to explore the coast of Africa. At first, perhaps, Henry thought only of trade with the people of Africa south of the Sahara. Then he had a bolder idea. If his captains could find their way around Africa, they could go to the Indies by sea. No longer would the Italian merchants have all the valuable trade in goods from Asia.
It took courage and imagination to think of going to the Indies by sea. In the time of Prince Henry, the Atlantic was called “The Sea of Darkness”. It was not a narrow, sunny sea like the Mediterranean, with the shores always comfortably close. It was often dark and stormy, and it seemed to stretch away endlessly to the west and south. No one knew what lay beyond, and people have always imagined that the unknown is full of danger. In Henry’s time many people thought that great sea serpents and other strange animals lived in the ocean. Many would not believe that the earth is round. If the earth were flat, what would there be to keep a ship from falling off the edge? They knew that the weather grew warmer toward the south. If it kept on getting warmer and warmer, would not a ship sailing to the south come at last to a place where the water was boiling and no one could live? These were imaginary dangers. Even in Henry’s time educated people knew better than to believe in them. There were real dangers, however, and no one understood them better than Prince Henry. Ships were very small. They were tossed about in storms and often blown far from the course their captains meant to follow. They could easily be thrown against rocky shores and wrecked. Ships depended upon the wind to push them through the water. If the wind did not blow, they could not move. If it blew from the wring direction, they had to travel in zigzags that added many, many miles to their course. A ship’s captain had no good way of telling the ship’s directions or finding out where he was. How could tell directions by the sun and stars, but sometimes on the stormy Atlantic the sky was covered by clouds for days at a time. Until about the time of Henry, European captains had no instruments to help them find the way. Now two new instruments were coming into use, but as yet few captains had them or knew how to use them. These instruments were the [magnetic] compass (see below left) and the astrolabe (see below right). You have probably seen compasses. The important part of a compass is a needle that points toward the north.

An astrolabe is an instrument that measures the exact height of the sun. You know that the noonday sun is directly overhead at the equator in March and September. Between March and September it swings north as far as the Tropic of Cancer. In the other half-year, it swings