Before some of the greatest explorers in history were born, Vikings had already navigated (航海) their way around the world. But with no     1     (compass), satellites or radios, how did they manage     2     (map) the globe so impressively? The answer is     3     (simple) than you might expect — experience. Viking travelers studied the positions of the stars and the sun. Even the colour of the sea and movement of the waves would give them an     4     (indicate) of how close they were to land.