The Earth is divided into 24 longitude lines, each representing an hour's difference from Greenwich since it takes the Earth 24 hours to rotate on its axis. There are 15 degrees between each longitude line since 24 divides into the 360 degrees of a circle 15 times. Early navigators realized they could figure their local noon by seeing when the sun's shadow was the shortest on a sundial. If they also knew the matching time at Greenwich, they could determine their longitude and follow a charted course. Remember, each hour east or west of Greenwich time represents 15 degrees of Longitude.
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