# East and West

1. Jun 19, 2009

### cepheid

Staff Emeritus
Simple (embarrasing) question. If Earth were not rotating, would there be any sensible definition of the terms "east" and "west?" I've been trying to come up with a consistent definition for these two terms in my mind, and the only ones that seem to work are, "in the direction of Earth's rotation," and, "in the direction opposite Earth's rotation," respectively.

2. Jun 19, 2009

### Danger

It seems to me that you could pick any point and call it a pole. The directions would just be a convenience for navigating that globe. Magnetic or spin axis poles give a bit more 'validity' to the choice. Note that magnetic north and true north very rarely coincide.

3. Jun 19, 2009

### denisv

Everything left from any great circle from N to S is East, and everything right is West.

This would a consistent definition wouldn't it?

4. Jun 19, 2009

### cepheid

Staff Emeritus
EDIT: This was in response to Danger's post. I guess I was too wordy.

Right, okay. I understand that if Earth weren't rotating, I could still set up a spherical polar coordinate system to describe points on its surface. So let's say I pick an arbitrary point and call it the north pole. Then I can find the south pole by drawing a straight line that passes through the north pole and the centre of the earth. I guess you could fix the origin of the coordinate system. You already have a zero latitude great circle (because defining the poles defines an equatorial plane). You can pick the zero longitude great circle arbitrarily.

So in this coordinate system we have constructed, what do east and west mean? I guess there are only two possible directions in which you can go around your latitude circle. Is moving straight eastward just, "going around your latitude circle in such a way that the right hand rule says your angular momentum points north?" Is moving westward just, "going around your latitude circle in such a way that the right hand rule says your angular momentum points south?" Are there simpler ways of stating the definitions?

If anyone answers that, then I'll go ahead and state why these questions came to my mind in the first place.

5. Jun 19, 2009

### cepheid

Staff Emeritus
Don't 'left' and 'right' depend upon what direction I'm facing in the first place? Also, don't 'left' and 'right' only correspond to west and east, respectively, if I am facing north?

6. Jun 19, 2009

### D H

Staff Emeritus
Any definition with a non-rotating Earth would of course be completely arbitrary. Our choice of the zero longitude line is also completely arbitrary. The choice of what demarcates the equator and the poles is a bit arbitrary, as well.

North points along the the direction (right hand rule) of the Earth's rotation back in 1800 (within the accuracy limits of such measurements 200 years ago). The Earth's rotation axis is not constant. The polar motion exhibits multiple cyclical behaviors but also has a secular term. The mean pole is moving at about 1 degree per million years. Triple or quadruple the current rate and sustain it for 10-20 million years and Siberia will be downright tropical. This apparent secular trend might of course be some larger (unknown) periodic motion. On the other hand, the Earth apparently has undergone true polar wander events in the distant past.

7. Jun 19, 2009

### TVP45

It's only a convention. But, it has some cultural and geopolitical meaning...

OH, East is East, and West is West, and never the twain shall meet,
Till Earth and Sky stand presently at God’s great Judgment Seat;
But there is neither East nor West, Border, nor Breed, nor Birth,
When two strong men stand face to face, tho’ they come from the ends of the earth!

So, you'd need other names if not East and West.

8. Jun 19, 2009

### Danger

What if one of them is an alien?

9. Jun 19, 2009

### Cantab Morgan

But, the Earth is always rotating with respect to something. If it were not rotating with respect to the stars, then you could still define, say, East to be where the Sun rises (once per year). Somebody check me, but I think that would reverse our current notion of East and West.

On the other hand, if the Earth rotated so the Sun was held fixed in the sky, then you could define East as where the Moon rises, or based on how the stars (slowly) rotate across the sky.

10. Jun 19, 2009

### DaveC426913

If a non-rotating Earth were to make East and West meaningless, why would it not also make North and South just as meaningless?