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HOW COME? Mapping out latitude, longitude

How do latitude and longitude lines work? asks a reader.Think of wrapping the Earth in a planet-sized sheet of sticky graph paper. That's how it might look if we could actually see lines of latitude and longitude. What's the point of these imaginary lines? We are traveling across an enormous ball, 25,000 miles around, much of it covered in featureless water. No matter where we are -- even in the middle of an ocean -- the grid helps us pinpoint our location.

Around 2,200 years ago, Greek scientist Hipparchus first devised a grid to map the entire planet. Today's lines of latitude and longitude are not in exactly the same places as on the ancient maps, but the idea is the same: Imaginary latitude lines circle the globe from east to west, starting with the equator (the most famous line of all).

Longitude lines loop the planet from north to south, starting at each pole.

How does latitude work? Imagine a belt around our planet's 25,000-mile waistline, the equator. Now picture a plastic half-circle protractor. The flat bottom starts at 0 degrees; the top of the arc is 90 degrees. Likewise, the equator is at 0 degrees latitude; the North Pole is at 90 degrees. Turn the protractor upside down to represent the bottom half of the planet, and it's the South Pole at 90 degrees.

Girdling the Earth between the equator and the poles are the other lines of latitude, about 69 miles apart and perfectly parallel. Since our round planet curves, latitude lines get shorter and shorter as they trace smaller and smaller circles near the poles.

Longitude lines divide the Earth into slivers like orange sections. Unlike latitude, longitude lines aren't parallel. They start out together at the poles, spreading farther and farther apart as they approach the equator.

While latitude lines are all about distance, longitude is also about time. Twenty-four longitude lines, equally spaced around the planet where they intersect the equator, represent the hours in a 24-hour day.

Longitude's 0-degree starting point is a line drawn straight down through Greenwich, England. And as we travel from one longitude zone into the next, it's an hour later or earlier. (In real life, since longitude lines pass through countries and not just oceans, world time zones follow a somewhat zigzag path.)

Finally, at the international date line in the Pacific Ocean -- which roughly follows the 180-degree longitude line -- time shifts abruptly. Sailing across the date line from east to west? You must add 24 hours. In a head-spinning instant, it's tomorrow. Travel across from west to east, and subtract 24. Suddenly, it's yesterday all over again.

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