North and south celestial poles

Homework Statement

The earth's angular momentum is directed toward which celestial pole(north or south)?

none

The Attempt at a Solution

I just assume that it is the south celestial pole considering that the earth is spinning from left to right, or west to east.

Is this correct?

cepheid
Staff Emeritus
Gold Member
What is the relationship between the direction of the angular momentum of a body and its actual rotation direction? Is there a rule that governs it?

EDIT: Also, can you see that "left to right" is not very meaningful?

hmmmm....i cant find the rule but the example in my book shows a bowling ball spinning from what we will call west to east and the angular momentum is directed upward, and if it was spinning opposite this it would be spinning downward. From this i could say that the answer would be the north celestial pole. Also, i was saying left to right when looking at the earth where the equator has a plane that extends from left to right. But what rule are you referring to?

cepheid
Staff Emeritus
Gold Member
I was referring to the Right Hand Rule. Do you know what the definition of angular momentum is? Do you know what a vector cross product is? It's part of that definition, and follows the Right Hand Rule.

EDIT: You have the right answer, but I'm saying all of this to make sure that you understand why. The Earth rotates counterclockwise as viewed from above the North Pole. The Right Hand Rule says that if you curl the fingers of your right hand in the direction of rotation, the thumb points in the direction of the angular momentum (in this case). Mathematically, the reason why ang. mom. follows the Right Hand Rule is because it has a vector cross product in its definition. You should look that up if you do not know what it is.

Also, i was saying left to right when looking at the earth where the equator has a plane that extends from left to right.

That still doesn't make sense. We're talking about rotation here. Even if you specify how you are oriented, the best you could do would be to specify clockwise or counterclockwise.

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thanks, i do know what both of those rules are, he just didnt explain it in class yet, guess I was supposed to have picked that up in my book in a chapter we havent done yet.

ideasrule
Homework Helper
Of course, the cross product's association of "clockwise" with "down" is entirely arbitrary; it isn't any more justified in terms of physics or mathematics than associating clockwise with "up". An alien might as well say Earth's angular momentum is directed towards its north pole.