How is speed of rotation of Earth's Inner Core calculated?

[RJLiberator]

I am interested in this.

I'm looking for a simple answer and a more in depth answer (if possible).

Any links regarding the math behind it would be great.

Thank you.

 

[Bystander]

http://www.columbia.edu/cu/record/archives/vol22/vol22_iss1/Core_Spin.html

 

[RJLiberator]

Thank you kindly for this helpful link.