It occurred to me that latitude and longitude are not homologous in that the lines of latitude lie on parallel planes or slices through Earth, while lines of longitude are not parallel and meet at ... More on gis.stackexchange.com

What is the difference between Latitude and Longitude More on vedantu.com

I would say both descriptions are lacking, but the second description of latitude and longitude (i.e., the "new" way) is maybe slightly better than the first (i.e., the "old" way), but the introduction of "horizontal" and "vertical" are problematic and imprecise. Neither description is wrong, but both are missing key elements. The way I would describe them are that latitude and longitude are a spherical coordinate system (i.e., a way of slicing up a spherical shell) allowing you to uniquely define a position on the surface of the sphere with a set of two coordinates. In terms of the orientation of the lines of latitude or longitude, it's more correct (and maybe easier) to consider them with respect to the rotational axis and the equator of the planet. The equator is a line that marks the intersection of the surface of the planet with a plane perpendicular to the rotational axis that goes through the center of the planet. Lines of latitude (other than 0 degrees, i.e., the equator) are the intersection with the surface of the planet of planes that do not go through the center of the planet, but are parallel to the equator. For this reason, lines of latitude are sometimes referred to as "parallels". Line of longitude, or meridians, are the intersection with the surface of the planet of a series of planes that go through both the center of the planet and the axis of rotation. So if you were to cut the sphere along any line of longitude, you would divide the sphere into two equal hemispheres, but if you were to cut the sphere along any line of latitude but the equator, you would divide the sphere in two unequal halves. Then, in relating the coordinates to the lines of latitude and longitude, because we are on a sphere, we measure these in degrees. The angles between lines of longitude are a little easier to visualize since these all intersect at the poles. The angle between lines of latitude are a little more difficult to picture since by definition lines of latitude to not intersect. Here, it's useful to consider a hemispherical slice along a meridian, i.e., we slice the sphere in half perpendicular to the equatorial plane, bisecting the poles. Then, on this hemispherical slice if we draw two radii between any two lines of latitude and the center of the sphere, the angle between those two radii is the angle between the two lines of latitude. The final piece of the puzzle is the origin of the coordinate system. For latitude, this is not arbitrary and it's fixed to the equator, i.e., to define the degree of latitude we measure the angle between the equator and that line of latitude per above. For longitude, it is arbitrary and we have to pick some meridian from which to measure our angles. Once we have a set of lines, ways of measuring angular distance between those lines, and origins for the two sets of lines, we now have all the pieces to define a unique coordinate for any place on the surface of the sphere. More on reddit.com

Longitudes go all the way around the earth, but latitudes all meet at the poles. If your latitudes continued past them, you'd have a latitude that increased moving towards the poles, then increased moving away from the poles on the other side. We could do this, but then we would be double-counting locations on the surface of the earth (e.g., 120 degrees W and 120 degrees N would be the same place as 60 degrees W and 60 degrees N) unless we made the longitudes only go from -90 to +90 degrees instead. Another way to think about it: make a complete, 360-degree circle out of wire and hang it from a piece of string. If you turn the wire circle around by twisting the string, you only need to turn it around half a turn before the wire will have swept out a complete sphere. If you keep turning it, then one side of the circle will overlap where the other side was. So you need to have 1 360-degree circle and 1 180 degree circle to make a sphere. We choose to write this as -180 to +180 degrees (or rather N/S instead of +/-) and -90 to +90 degrees (E/W) instead of 0 to 360 and 0 to 180, but that's just a choice we have made. More on reddit.com