Because Earth is spinning, that gives it two natural points that are distinguished from all other points, they are the North and South Poles, which are where the axis of rotation intersects the surface. So if you start at the Equator, you keep going north, or south, until you hit one of them, which takes 90 degrees. The point you start at on the Equator isn’t special, so it doesn’t make sense to rotate your latitude around it; instead, the latitudes are parallel to the Equator. Because the parallel line of each latitude goes around the Earth, you don’t keep going past the pole, because if you did, 89 degrees turns out to be the same line as 91 degrees. So there are only 180 degrees of latitude, because the other 180 to make 360 are shadows of the first 180 and there is no need to have two numbers meaning the same line. Meanwhile for longitude, it does make sense to rotate it around the poles because the poles are special, so all of the 360 degrees are unique lines that go straight from the equator north and south to each pole.
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