Eli5 why there are more lines of Longitude then Latitude
In: 35
I’m not an expert on this, but I’ll go with basic intuition here.
There is a defined “top” and “bottom” to the earth, as most people understand it. The further north you go, the higher value latitude you find yourself in, ranging from -90 at “true south” to +90 at “true north”. It wouldn’t make sense for you to keep going past north and climb to +180 latitude as you once again approach the equator.
Longitude, however, does not have the same definitive concept of “this is as east as I can get”. It wouldn’t make sense for you to travel east up to 90 degrees and then start counting back down again as you go further east.
That would also cause there to be two points of longitude for each possible degree on a map. The -90,90 latitude system paired with the -180,180 longitude system allows for a simple, relatively intuitive coordinate reference system that will only ever have one location for a corresponding coordinate.
I probably did a bad job of explaining, so someone with the actual history could probably help.
Latitude is based on fixed real-world geography: the equator is the equator and the poles are the poles, and they would be there no matter what species was figuring out latitude. The only thing we contributed is how many degrees of latitude we stuffed between them (instead of 90 we could have went with 10 or 519).
Longitude is arbitrary; while it makes sense to make lines that run between the poles, the actual positions of the lines are something we invented. Another species could have put the prime meridian five feet to the left. Longitude circles are only half-circles and stop at the poles where they “start over” as the other half (W versus E); it wouldn’t make any sense to have one circle that goes all the way around, because then you’d have two “15N latitude 30 longitude”s.
Independent of history or whatever, it’s unnecessary to have 360 degrees in both axes. You would end up just wrapping around and having duplicate coordinates for the same place.
Imagine you’ve got a sphere you want to wrap in a blanket. The blanket has to be the circumference of the sphere in one axis (to go all the way around), but only half the circumference in the other axis (since it only has to reach the top and the bottom, at which point it will meet the blanket from the opposite side). The orientation doesn’t matter, anyway you’d do it you would need a rectangular blanket that’s twice as long in one dimension as the other.
Each line of latitude describes a circle, but each line of longitude only describes a semicircle. If you walk along an entire latitude line, you end up where you started, but to walk along an entire longitude line, you have to start one of the poles and end at the other. In terms of describing a plane intersecting the Earth, some longitude lines are actually the same as one another.
For instance, if you start at New Orleans on the 90th meridian W, walk due north, you’ll hit the north pole. If you keep going in a straight line, you’ll now be travelling south along the 90th meridian E and go through Dhaka. Keep going south, past the south pole and now you’ll be going north through New Orleans again.
But even though these two lines are actually two halves of one circle, it makes more sense to us to think of them as different because they describe areas on opposite sides of the world.
What do you mean by more? Both are just arbitrary ways of dividing up a globe. You can have as many or few of either of them as you want!
In theory, one could have decided that the the South Pole was 0° with the equator being 90° and the North Pole being 180° (or vice versa). But that didn’t make sense for several reasons: there would be no logical delineation of which hemisphere one was in while the equator and poles could easily be determined mathematically ( even though the poles had not yet been discovered at the time these conventions were decided) and most of the time for navigation one is working in one hemisphere so it’s simpler to start at 0° at the equator and go 90° North or South.
Similarly, there could have been 360° of longitude instead of 180° east and west but again, it was more practical to assign lines of longitude as East or West starting at the Prime Meridian; which could have been anywhere such as Paris or Washington but Greenwich was settled upon because of the primacy of British sea power at the time and the Naval Observatory’s advances in determining longitude at sea.
The historical reason is that way back in the day the math & science heavy civilizations in the Middle East decided a circle would have 360 degrees.
If you hold out your hand, palm facing you, you count with your thumb against each segment of each long finger and (if you have all four fingers in their normal arrangement) you end up counting 12. Raise a finger on the opposite hand and start another 12. By the time you count all five fingers on the opposite hand, you have 60.
For reasons that are probably more mystical than scientific, they took six 60s to be special (eg. days in a year are nearly that)…anyway, long story short there are 60 minutes in an hour, and 60 seconds to a minute.
We have 360 each of longitude and latitude, but North/South they are broken up into 90s and East/West into 180s. But of the former four quarters add to 360 and in the latter to hemis add to 360. The *label* changes but the *number* does not.
As the Earth rotates, local noon follows the Sun at the rate of fifteen degrees of arc per hour (24×15=360). The relationship from there to longitude gets more complicated, has linguistic and real-world relationships but not 1:1, and is NOT something I have an ELI5 ready for but they are related. If you have a good enough clock, a way to determine noon (eg. a shadow you can trace) and a timetable of true local noon for a bunch of locations, you can work out how many circular degrees you are offset from any of those locations with just a little math, and by little I mean some high school geometery and trigonometery.
The sun appears to move by one degree of arc (of the 360) every four minutes, or 15 degrees of arc per hour (see above). That is, if you and I are separated by one degree of longitude, you will see shadows strike local noon four minutes earlier than I do. If we are separated by 105 degrees of arc, the difference is 105 degrees / four-sixtieths (also known as 15). 105/15 = 7. My time zone is GMT -7; when I get home at midnight local time I can listen to London breakfast news live because it is 7am in London (105 degrees or 15 degrees x 7 hours) to my east. And, indeed, the 105th Meridian runs through my city and is marked by a metal line crossing the plaza through our train station, [like this](https://ducdn.denverurbanism.com/wp-content/uploads/2015/12/18145643/2015-12-29_105th-meridian-plaza-400×600.jpg) which is exactly that 105 degrees of arc (420 minutes of time) from [this similar 0 line](https://th.bing.com/th/id/R.e11758ade66a1d870d6162946802e7f3?rik=iZAFilnMDMbgTQ&riu=http%3a%2f%2flh3.ggpht.com%2f-HAsRiY6z9qE%2fVJL1PBGstoI%2fAAAAAAAA934%2fI1lLPuZdUEA%2fprime-meridian-3%2525255B6%2525255D.jpg%3fimgmax%3d800&ehk=jxq9fPEBxrG52XeVAOZ0rC8XU3SnzT7qzG1tA4kgRTU%3d&risl=&pid=ImgRaw&r=0) in the Royal Observatory in Greenwhich, England.
Anyway, I digress, point is there are direct relationships between minutes of arc and minutes of time but this is not a geometery ELI5, I just thought it was a neat tie-in to history from civilizations that occupied Earth so long ago that the mammoth were around and the pyramids were not a thing…yet we still bear homage to their ingenuity and math/science breakthroughs in our modern efforts to measure the Earth and understand the universe.
Because the Equator goes all the way around the Earth and the Prime Meridian doesn’t.
We decided long ago that there would be 360 degrees in a circle for reasons mostly to do with how easy that number is to divide. When the planet was being mapped, we started from the equator and went a quarter-circle to each pole, and that was 90 degrees north and 90 degrees south, for a total of 180. Each of these lines was a circle that goes all the way around the Earth.
When we did longitude, however, We drew a line from pole to pole through England, then started measuring from there. Because the line was only a half-circle, it took 180 of them to get to the other side, and then another 180 back.
TLDR:
Latitude measures angles away from the equator. You can’t get any further away than the north or south pole, which are 90 degrees from the equator. So Latitude stops at 90 degrees.
Longitude measures the angle away from the line connecting the North and South pole that runs through Greenwich England. The furthest you can get from this line is on the opposite side of the earth at 180 degrees from Greenwich. So Longitude stops at 180 degrees.
So it only takes a total of 90 degrees North and 90 degrees south of the equator to complete the grid of Latitude (180 total). Where as it takes 180 degrees East and 180 degrees west to complete the grid of Longitude (360 degrees total).
The earth spins around an imaginary line through the center of the earth and out to what we call North and South poles. These poles make dividing the earth into coordinates pretty easy.
Latitude tells us how close we are to the poles. Longitude tells us where we are around the earth.
Latitude measures the angle between two lines joined together at the center of the earth. The first line goes to the surface of the earth at the equator. ( The equator was picked because it is the furthest you can get from the poles.) We set this as ground 0. The second line from the center of the earth goes to where you are on the surface. If you were at one of the poles, you are as far away from the equator as you can get. If you took out your protractor and put it at the center of the earth and measured those two lines, you’d get 90 degrees. So Latitude maxes out at 90. We add North or South to tell us which side of the equator we are measuring on.
At this point, all we know now is that we are somewhere on a line that circles around the earth at a particular Latitude.
Longitude tells us where we are on that circle. If we use the poles as the center of our Latitude circle, we can measure the angle between two lines radiating out like spokes on a bicycle. These lines start at one pole, follow the curve of the earth and end at the other pole. The first line runs through Greenwich England, and is our starting point and we call angle 0. The second line runs through where we are along that circle of Latitude. We could potentially be up to 360 degrees away from Greenwich if you keep measuring around in one direction, but because 180 degrees is as far from Greenwich line as we can get (on the opposite side of the earth from Greenwich) we stop there and say whether we measure going East, or measure going West. So Longitude maxes out at 180 degrees east or west.
The reason why Latitude has less lines is because there isnt as big of an angle away from our start point compared to Longitude.
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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