# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Coriolis and gyros
From: Frank Reed
Date: 2009 Aug 20, 06:02 -0700

As Douglas Denny stated originally (later withdrawn), the Coriolis
acceleration is most definitely responsible for the north-seeking capability
of a gyro-compass (IF you work in coordinates rotating with the Earth --the
natural coordinates for these problems). Just make sure you use the full
three-dimensional vector cross-product when thinking about the strength of
the Coriolis acceleration.

The Coriolis acceleration is frame-dependent --it depends on the choice of a
rotating frame of reference. It's intriguing to note that there is also an
extremely tiny physical, non-frame-dependent, version of the Coriolis
acceleration which is created by spinning masses. This is known as
"frame-dragging" or "gravito-magnetism" and was measured (barely due to
various problems with the ultra-sensitive gyroscopes) by a spacecraft known
as "Gravity Probe B": http://einstein.stanford.edu/highlights/status1.html.

There is a deep, non-accidental analogy between the Coriolis force and the
common magnetic force. And just as a spinning current loop experiences a
torque which causes it to precess in a constant magnetic field, a spinning
"mass loop" (a.k.a. a gyroscope) experiences a torque which causes it to
precess in a rotating frame of reference.

Finally, for Gary, there is nothing "fictional" about the so-called
"fictitious forces" in rotating frames of reference. They are every bit as
real as the common acceleration of gravity, or, if you like, they are every
bit as un-real. The complicated explanations for various phenomena which you
have learned that avoid using the expression Coriolis are un-necessary,
though they can serve as useful reminders that physics can be done in
different frames of reference and the results have to be the same no matter
what.

-FER

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