The Earth Wobbles

[knocker]

 

The earth wobbles about in its rotation.  This was predicted long before it was observed, which is a story itself that I'll tell later.  For now, consider the earth and its rotation.  The north pole of the earth points towards the north star, and rotates once per day.  Open your right hand.  Your thumb points north, and when you close your fingers, they are moving in the direction of the earth's rotation.  With your arm making a right angle at the elbow, hand aiming away from your torso, you have an x-y coordinate system.  When you rotate your forearm, that moves your thumb in the x direction (positive or negative), when swing your arm forward/backward, that's the y direction.

 

http://moregrumbinescience.blogspot.co.uk/2015/02/the-earth-wobbles.html