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From a round globe to a flat map
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True or false? On a world map, one centimeter on the map corresponds to the same distance in reality, no matter where on the map it is.
Here's a spotted beach ball. How would its surface look, if we showed it as a flat map? Maybe like this? Actually, it's not that simple. Let's count the number of spots going around the beach ball.
Across the middle of the ball, where its width is the largest, there are 12 spots. But here, closer to the top, five spots are enough to go round once. And at the very top, one trip around the beach ball is only "one spot long". This map doesn't match reality very well! How can we show the surface on a map, then?
Maybe we can cut the ball open, and try to smooth it out into a rectangle? Let's try that! The beach ball is made of plastic that you can pull and stretch. The surface can change its shape. Up here, and down here, you'll need to stretch the plastic a lot.
The spots on the beach ball that were circular to begin with, become oval. And the circle at the top - it's stretched until it covers the whole map! Along the middle of the ball, you don't need to stretch the plastic at all. Why is that? Why do we need to stretch the plastic different amounts, in order to make it flat?
Because, one lap around the ball, has different lengths depending on where you measure it. Here along the middle of the beach ball, the length of a lap is greatest. Closer to the top or the bottom, a lap is shorter. On the rectangular map however, the distance from left to right is the same, no matter if we measure it closer to the top, closer to the bottom or near the middle. The further we get from the middle, the more we need to stretch the plastic.
If you want to make a map of the surface of the Earth, you'll have the same problem as with the beach ball. One lap around the Earth has different lengths depending where we measure it, just as with the ball. Here in the middle, along the equator, the Earth is at its widest. Closer to the poles, the distance around the Earth is shorter. The map has the same length from left to right all over.
We'll have to stretch the surface more, closer to the poles, than at the equator. Then those areas get different shapes, compared to how they look on the globe. Look at Alaska for instance, in North America. It gets really long in the east-west direction, compared to its shape on the globe. There's another type of map, another projection, where we stretch the surface up and down the same amount as it's stretched left and right.
That makes the areas that were stretched out return almost to their actual shape. But the closer we get to the poles, the larger the magnification gets. On this map, Africa looks about the same size as it does on this globe. But look at Greenland. It looks much bigger on the map than on the globe.
It's because Greenland is so close to the North pole. There, we need to stretch the surface a lot more, to make it cover the width of the map. If we were to do the same thing with the beach ball, the result would look like this, despite the fact that the spots are of equal size! The best way to show the Earth's surface is to use a globe. Then all areas get the correct shape and size compared to each other.
But you can't view the entire surface at the same time, of course.