As you travel horizontally, in an East - West direction, each longitudinal line, perpendicular to your path, is a mile marker (expressed in degrees of the planets circumference). Where any two Latitudinal lines make a round disc, any two longitudinal lines make a slice of planet cake, from pole to pole. Longitudinal lines are also Meridians - markers of the sun's path, time zones, and more generally regions, rather than locations. On a carpentry bench with a ruler attached, the length of a work piece will be determined by a line on that ruler, a measuring line that is perpendicular to the length being measured. 'Long'itudinal lines run North -South because they are a measure of East-West distance at all Latitudes. Longitudinal planes are massive on a global scale; but as your East-West path crosses each Longitude, it's imaginary presence might be just a small sign - A sign whose flat face, turned toward you, is in a perpendicular North-South plane that marks distance along all East-West lines. Likewise, Latitudinal lines running East-West, are measures of distance and location along all the perpendicular lines running North-South. I hope this helps. More on reddit.com

It's worth starting with the problem that lines of longitude - defined as great circles that pass through the rotational axis of the planet - solved. Specifically, in early navigation (primarily at sea), measuring latitude was relatively simple because either through observation of the sun position or star positions you could work out how far north or south you were of the equator. This is in part because that the equator is not an arbitrary line as it's defined as the great circle (the only line of latitude that is a great circle) 90 degrees from the rotational axis of the planet (and this is why it makes sense to define lines of latitude that represent fixed angular distances from the equator as what you were measuring was the angular distance from the equator and/or from the pole). In contrast, figuring out how far east or west you were while at sea was more challenging. The idea behind longitude was to define great circles where positions along a singe line of longitude reflect locations where the sun will be at its zenith at precisely the same time. Then, with precision time keeping, a reference point (the prime meridian), and the difference between your local zenith time and the zenith time at the prime meridian, you could work out how far west or east you were in angular distance. This would not work with lines of longitude that were parallel. If we imagine that now with more advanced ways of measuring positions there's nothing necessarily tying us to lines of longitude as originally defined, there are a few problems that pop up. One is again related to time. Specifically, from above, one property of a line of longitude is that it represents points along the surface of the Earth at the same time with respect to the position of the sun in the sky. A system with parallel lines of longitude would not maintain this property (except at whatever arbitrary position you defined the equator equivalent in this hypothetical longitude system). More importantly, if we imagine a system where both lines of longitude and latitude were internally parallel (and orthogonal to each other), we've introduced a non-uniqueness to coordinates. At present, latitude spans -90 to 90 degrees and longitude spans from -180 to 180 (or 0 to 360) so each point on the surface has a unique coordinate pair. However, if we had a system where longitude spans from -90 to 90, any coordinate would represent two possible locations. I.e. a coordinate of 0 latitude, 0 longitude could represent either of the two possible intersections (on opposite sides of the globe) of the latitude equator and longitude equator, and the same would be true for every intersection of lines of longitude and latitude. It probably goes without saying, non-uniqueness is a really bad property for any proposed coordinate system. The arbitrary nature of the position of the "equator" and "poles" of this hypothetical version of longitude is also a bit of a problem, or at least, it introduces a pretty annoying set of properties to a coordinate system designed in this way. More on reddit.com

Ever peel an orange 🍊. North pole and south pole. There's no east west poles More on reddit.com

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