Base of Skull (if that is what Homer used as definition, this where all head motion on it own originates from)
Base of Neck (difference may be minor depending on the head orientaion which means it would only change the position on the ground by less than a few inches)
Interesting, this really coming down to an attempt to define a Fix Point in space that is sort of (words choosen carefully) independent of the body, but yet required percision to support proper and correct geometric alignments....
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