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Cultures > Portugal

:: 1143 AD - PRESENT

Cultural Background | Surviving | Living in Community | Finding Meaning in the Cosmos


Because Portugal is so close to the ocean, water transportation is very important. The Portuguese turned to the skies to help guide them when they were on the water and landmarks are not available.

It was not possible for the Portuguese to determine longitude (distances east and west) at sea before the early 1500's. Determining longitude required a timepiece that would keep track of time back home. At noon they could compare the local time with the time back home and determine how far east or west they were. The timepieces, however, were not good enough to keep track of time for extended periods of time at sea. In addition, the maps of the day often published inaccurate longitudes, making sailor's challenge to reach the proper destination even greater.

It was quite easy, though, to measure the distance north or south of the equator, which is called latitude.

Prior to 1500 the Portuguese used the North Star (Polaris) to determine latitude. The North Star primarily stayed in the same position in the night sky; around 1500 it moved in a small circle with a 6 degree radius. Today the North Star does not move in the night sky. Using the North Star to determine latitude only worked as long as the mariners stayed well north of the equator because the North Star is not visible below the equator.

Tools to help mariners determine latitude were developed around the 1500's. In 1481 King John II ascended to the Portuguese throne, and consolidated Portuguese control over the Gulf of Guinea (on the African coast near the equator). In order to make navigation easier in the Gulf of Guinea and south of the equator, King John had people in the royal court produce new instructions and new tables that would make it easier to calculate latitude. These instructions required two pieces of data: the height of the sun above the horizon at mid-day, which the mariners could observe; and a number known as the solar declination, which varied from day to day and which was tabulated in the new tables.

Astrolabe and Quadrant:

To determine their latitude while at sea, they used tools called the mariner's astrolabe and the quadrant. Both instruments are used to determine the altitude of an object in the sky, or its angle above the horizon. For both of these instruments, the earliest records of their use at sea by people of any country come from Portugal in the 1500's.

The mariner's astrolabe was a brass ring used to observe the altitude of the North Star or the sun. This was a simple wooden or brass disk with degrees marked around its edge. It had a rotating arm with small holes at either end. The disk would be hung vertically from a ring. The user could move the arm until the sunlight shone through the hole at one end and fell on the hole on the other end. The arm would indicate the altitude by the degrees marked around the edge of the disk.

Once they had the altitude, the seamen next consulted a special almanac for a number known as the declination of the North Star (or sun) on that day. By plugging both numbers into a special equation, they were able to calculate their latitude.

Unfortunately, the mariner's astrolabe was not a very good instrument. Errors of four or five degrees were common. When the mariners miscalculated latitude, they sailed so far in the wrong direction that it was like driving your car to the wrong city today.

Navigators could also determine latitude by observing the North Star or the midday sun through a quadrant. The quadrant was a metal plate in the shape of a quarter-circle. From the center of the circle hung a weight on a string, which crossed the opposite edge of the circle. The navigator would sight the North Star or the sun along one edge, and the point where the string crossed the edge would show the star's or sun's altitude, or angle above the horizon. This gave navigators a very close approximation of their latitude.

When the navigators neared the equator, the North Star fell below the horizon. This forced navigators to rely on the sun, mariner's astrolabe and almanac to find their latitude.

Once they knew the latitude of their destination, they would first sail to that latitude and then sail east or west along the latitude line until they reached their destination.

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See an astrolabe. [enlarge]

Learn more about stars and constellations and how astronomy aided navigation.

Learn more about the role of navigation in Portugal During the Age of Discoveries from the Library of Congress.

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