Concept behind longitudes

Question

Why longitudes are not parallel and why do they meet at the poles? What problem would have happened if like latitudes they were parallel circles running from west to east? I would request the explainer to use a layman's language with no technical words.

Answer 1

Latitude and longitude: theoretical constructs, based on natural phenomenon

Latitude is an empirical measurement — the distance you are between the pole and the equator — and has been measured for millennia by the stars. Longitude, on the other hand, is a human construct. https://www.directionsmag.com/article/11881

The above quote is a good starting point to understand the basic facts. However, I disagree in the absoluteness it states the "natural" vs. "constructivist" character. "Pole" and "equator" and thus the resulting concept of "latitude" are constructs, as well. On the other side, longitude has a real world ("natural") correspondence as well. So latitude and longitude are theoretical constructs for measurement purpose, but constructs anchored in some natural phenomenon.

Meridians: longitude

The meridians correspond to the natural fact that the sun reaches the zenith once a day simultaneously for a set of points that form a straight line from one pole to the other, cutting the equator in a right angle. So meridians are not just artificial lines, but correspond somehow to a natural aspect, something that can be measured. Even with your bare eye, without any instrument, you can observe what time of the day it is right now. If you would collect this information for a given moment from people at diferent places all over the world, you could roughly say how much to the east or west of your point of observation every reporting observer is placed.

Longitude corresponds to time zones

Compare with time-zones, they are slices from north to south pole, indicating how much to the east or west of a conventionally definned line they are located. If longitude were parallel circles, you would have different points on the same longitude with different day-time (somewhere midday, somewhere else morning). Such circles have no corresponding phenomenon in nature and thus are pure conventional lines, very difficult to measure (at least in pre-modern times).

The problem to measure longitude

However, historically it was very difficult to find adequate ways to measure longitude, even with the meridian system. Latitude was easy to measure, even on sea (e.g. azimuth of the sun; stars). To measure longitude, however, exact time measurement was critical to find longitude. Until the 18th century, on ships on the ocean, it was almost impossible to determine exact longitude. There were no clocks that ran exact enough.

Longitude is defined as the (angular) horizonatal (east-west) distance (in degrees or daytime) form a pre-defined meridian: if you depart from Lisbon towards America: how many degrees west of Lisbon are you after three days of navigation? To determine this, you must know exactly how much time has passed from the sun being in the zenith in the departure port vs. the sun's zenith on the current day.

It took quite an effort (and many sunken ships) to be able to determine longitude accordingly (latitude in contrast was relatively easy to measure on a ship with a sextant and other tools).

Longitude vs. (hypothetical) parallel circles

So think again: if it was almost impossible to measure longitude from natural phenomenon (like sun and stars), it would have been even more difficult to measure conventional, parallel circles (that you propose) that do not represent any natural phenomenon. So there is some reason for the tradition. Parallel lines for measuring longitude instead would have no real advantage.

For more information, do an internet search for "finding longitude" or "history of longitude", there are also some good videos out there that explain everything in detail with vivid animations.

Between many others, see e.g. this site, with a nice video included: https://www.sea.museum/2016/06/08/where-on-earth-are-you-a-beginners-guide-to-longitude/

Answer 2

Trying to put it really simply, the way I see it is this.

If I go out with my compass (or gps, or some other kind of navigation tool) and head north I will eventually get to a point where I can't head north anymore.

But if I go out with my compass and head east I will never get to a point where I can't head east, I can just keep going.

If I head North, from any point on the planet, I will eventually end up at the same place (the north pole). If I head East, this won't happen. My easterly travel is along that line of latitude. My northerly travel matches the lines of longitude, which converge at the poles. If we had parallel lines of longitude this couldn't happen, and would make it much less useful for navigation or for map reading, etc.