[Tango-L] Definition of Axis (was: On Axis, Off Axis)

[Evan Wallace]

Zoltan replied to Evan Wallace:
Actually, if you really want to be technical, each rigid body has at
least three mutually perpendicular principal axes, which *is* a
property of the object.  A rigid body will rotate freely around its
principal axes...


Yes, you are correct. I alluded to this in my posting when I mentioned that
you can always assign three mutually perpendicular axes about which you can
describe three independent rotations. I didn't go into the concept of
"principal axes" (which is indeed the correct term), because I was beginning
to bore even myself with the length and tediousness of my posting.

(I'll also concede your point that principal axes could be considered a
property of a rigid body, but if I were in a really cantankerous mood and
hadn't had my morning coffee, I might still argue that even these axes are
constructs of a special body-centered coordinate system riding around with
the object. I might also argue about whether or not it really makes sense to
talk about principal axes for a non-rigid body, like a dancer, and whether
or not rigid bodies rotate freely about these axes (the rotations are stable
about some of the axes, and unstable about others), but I have had my
coffee, so I won't.)

My point remains that the term "axis" is frequently misused in tango
discourse. I think most of the time the word axis is used, the terms balance
and/or posture would be more precise and more useful. 

Here is an example in which the terms axis, balance, and posture are used
fairly rigorously. If I were teaching ochos in an open embrace, I might say
to the lead: "You must keep the follow exactly on her balance the entire
time you are pivoting her, else you will pull her off her axis of rotation
and she will stumble, and perhaps glare at you." Or I might say to the
follow: "During the time you are pivoting, you must keep a consistent
posture and muscle tone, else you will not have a well-defined axis of
rotation to pivot about, and you will feel all wobbly." ("Wobbly" being the
precise scientific term.)

This is feedback that the student can actually use.

Evan Wallace 
Seattle, WA
www.tangoing.com