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

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Re: Gyro sextant
Date: 2019 Jan 23, 19:29 -0500

The law of conservation of angular momentum decrees that undisturbed, a rotating body will continue to maintain the direction of its axis of rotation unless a force acts upon it.   The force which acts upon the gyro is friction.  The friction of the angular contact of the spherical tip against the side of the spherical cup causes precession. As it does so, gravity pulls the spherical tip downward into the spherical cup until at last, the spherical tip is rotating at the nadir of the spherical cup.  Friction along the wall ends, which ends precession.  But remaining friction of the side of the spherical tip as it spins at the nadir is minimized as the gyro stands up.

Bill, its kind of hard to explain this without understanding that: The force of friction creates a vector opposite to a tangent line, struck at the contact point between the fixed and rotating members.  This force pushes back against the precession and causes it to "stand up".

And excuse me for using the word nadir.  Its not everyday I get to use that in s sentence, so I felt compelled to do so!

On Wed, Jan 23, 2019, 7:03 PM Bill Morris <NoReply_Morris@fer3.com wrote:

I have just written an account of the German WW II See-Kreisel-Sextant here: https://sextantbook.com/category/gyro-sextant/

I have tried to give in one paragraph a non-mathematical account of why the axis of the gyro rotor becomes vertical as it spins. I am not happy with the result and should be glad to read how members might improve upon the account without adding too much to its length or using words that the general reader might not understand. I have a spare copy of John Perry's "Spinning Tops" to reward the best attempt, of which I will be the sole judge...

Bill Morris

Pukenui

New Zealand

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