Finding the Way at Sea
1Picture a wooden sailing ship far out on the ocean, with only water and sky in every direction. There are no roads or signs. For thousands of years, sailors faced this challenge and had to answer two questions: Which way are we going, and how far have we come?
2The earliest sailors often stayed close to land. They kept the shore in sight and watched for landmarks, such as a tall hill, a rocky point, or the mouth of a river. Hugging the coast made it easier to know where they were. However, following every curve of the shore made trips longer, and it could not help anyone cross a wide ocean.
3To sail beyond the sight of land, sailors turned to the sky. By day, the Sun rises in the east and sets in the west, which gives a rough sense of direction. At night, sailors in the northern half of the world looked for the North Star. Other stars seem to circle slowly across the night sky, but the North Star stays in almost the same spot. Facing it means facing north. Sailors also noticed that the farther north they sailed, the higher the North Star appeared above the horizon, the line where the sea seems to meet the sky. By measuring the star's height, they could estimate how far north they were.
4The sky had one big weakness. On cloudy nights, the stars were hidden. On foggy days, even the Sun disappeared. The magnetic compass solved this problem. A compass holds a magnetized needle that swings until it points north. Many historians believe that people in China were the first to use compasses to guide ships, roughly a thousand years ago. Over time, the idea spread to sailors in other lands. Unlike the stars, a compass works in any weather, day or night.
5Knowing the direction was only half the puzzle. Sailors also needed to know how fast the ship was moving. They used a log line: a long rope with knots tied at evenly spaced spots and a piece of wood at one end. A sailor tossed the wood overboard, where it floated almost in place while the moving ship pulled the rope out behind it. At the same moment, another sailor turned over a small sandglass. When the sand ran out, the sailors counted how many knots had slipped over the side. The more knots, the faster the ship. That is why a ship's speed is still measured in knots today. Like a compass, a log line did not depend on a clear sky.
6With the ship's direction, its speed, and the time that had passed, a navigator could estimate how far the ship had traveled and mark its position on a chart, a map made for sailors. This method is called dead reckoning. It was useful, but small mistakes could add up over many days at sea. So careful navigators checked their work against the stars whenever the sky was clear.
7Today, most ships use GPS receivers, which pick up signals from satellites circling Earth and show a ship's position within moments. Even so, many sailors still learn to use a compass and read the stars. If the electronics ever fail, these old skills can still guide them home.