## Wednesday, June 24, 2009

### Metaphor and Mapping

If you look at the longitude and latitude lines on a globe, you may notice (if you haven't in the past) something interesting: we don't handle north-south and east-west the same way.The longitude lines are great circles that meet at the north and south poles, while the latitude lines are just horizontal (if you will) circles of varying size, like cross-sections of the planet.

I commented to Ed recently that we should try doing east-west like we do north-south, and he said, "But then, if you went east for a while, you'd eventually be going west."

"Well, now, if you go north far enough, then eventually you start going south, so if we handled east-west the same way..."

And that was a very weird thought. At first I couldn't make any sense of the idea that if you go east for a while you'll find yourself heading west (because it's not true), and then I started wondering why it does seem to make sense for north-south. Why am I comfortable with the idea that once you're at the north pole, if you keep going, you're heading south?

Is it purely because I was educated about how latitude/longitude lines work?

Then it occurred to me that it's because I think of the north pole as "up" (the way that north is up on most maps). And so I associate it metaphorically with the direction that heads away from gravity. And clearly if you were to climb up a sphere for long enough (using your sticky gecko feet), you'd get to the top, and if you kept going, you'd be headed back down. While if you just geckoed your way sideways around the sphere, there wouldn't be any reversals.

Is it natural that we think of north as up, or is it just a convention? Clearly it's a matter of convention whether north or south is up, but could east or west be up just as easily? We would then picture the earth as rolling around through its orbit rather than spinning, and we would view the solar system as being a flat vertical plane rather than a horizontal platter.

What do you think?

Sally said...

The east-becomes-west thing was not quite as weird to me as it seems that it was to you, but I agree that north-becomes-south is easier to understand, and that may be mostly convention but also that the I do view the north pole as being like the top of a mountain.

rvman said...

It's the spin. The earth spins on its North-South axis, which makes it an obvious pair of focal points. Both are also far enough from any normal traffic that the 'weirdness' of north-south near the poles doesn't affect much. Similarly, compasses point along the curves, toward the magnetic north pole, and the magnetic poles are 1000 miles from nowhere as well.

There isn't a comparable axis for east/west. Any axis you choose would be arbitrary (it would be on the equator, but where?) and near traffic, so people would actually have to deal with the weirdness.

Another item is that with curved north-south lines and straight east-west lines, east-west and north-south are perpendicular. With curved east-west, they wouldn't be. In fact, as you approach the poles, they can become very nearly parallel, which isn't helpful for navigation, and means you need different compass faces based on actual position. We only have this problem with very long distances along the 'great circle' routes.

rvman said...

I think Uranus works more or less like an east-west pole, since its axis of rotation is angled 98 degrees from the celestial plane. Most of the planet is either 'above' or 'below' its arctic and antarctic circles - only the 8 degrees N/S nearest the equator see 'days' and 'nights' during every axis rotation.

Mosch said...

(Mosch actually emailed me this, but I have his permission to put it here. -Tam)

Geez! No! It's not arbitrary. North vs. south is arbitrary, yes. But up/down orientation is not the issue with regard to the use of latitude lines vs. longitude lines. The key thing is that the earth is spinning. The poles, are the only 2 points on the surface of the sphere that don't move, they don't revolve around anything, they just rotate. They're the part of the straight line that goes through the center of the earth that only rotates with regard to the motion of the globe. On the other hand, all the other points in and on the globe revolve around that line.

As a result, among regular sky phenomena, nothing rises over the north or south poles. The North Star just stays where it is with regard to our perception. Undoubtedly there's some equivalent, if not quite as precise, marker over the south pole.

Furthermore, if you move in either direction such that your distance from either pole doesn't change, then your relationship to the sun and stars doesn't change either. The sun rises at precisely the same time on that day. Hence, the latitude lines run parallels directly around that center line that goes from pole to pole.

But the poles are actual terminal points. As you approach the pole on any given day, the sunrise time changes. It's later and later in the winter and earlier and earlier in the summer. Of course, once you cross a certain latitude, the sun never rises for half the year and never sets the other half of year. Then, if you continue traveling past the terminal point known as the pole, your relationship to the sun starts to reverse. That's why longitude lines terminate there.