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Douglas wrote, concerning coriolis acceleration:

"I have a theodolite made by British Aerospace with a North seeking gyro built 
in which 'feels' the acceleration right enough to provide North to within a 
few second of arc."

I have no knowledge about how his theodolite works but I wonder if it doesn't 
work like a gyro compass which doesn't use coriolis to seek north. A gyro 
compass seeks north while the common directional gyro in an airplane only 
maintains its orientation in inertial space much like the Foucault Pendulum. 
(I am in Paris now and saw the Foucault pendulum in the Pantheon the other 
day, in the same place where he demonstrated it more than a hundred and fifty 
years ago. They have a clock face on the floor under the swinging pendulum. 
You look at the time displayed when you go into the Pantheon and then look 
when you are leaving and you can see that the pendulum has changed its swing 
to now indicate the current time. Since it is obvious that nobody reached out 
and disturbed the swing of the pendulum it is clear to anybody that the earth 
has turned under the pendulum. The plaque says it turns 11 degrees per hour 
but doesn't point out that this is the sine of the latitude of Paris times 
15.042 degrees per hour, the rotation of the earth in inertial space, so it 
may leave people wondering, why not fifteen degrees per hour, 360 divided by 
a twenty four hour day? I guess I am expecting too much out of tourists.)

But a gyro compass does seek north and it is interesting to see how this 
works. Imagine you are at the equator and it is the equinox. You set up a 
gyroscope that is gymboled so it is free to move in all directions. At 
sunrise you point one end of the axis toward the sun, it is horizontal at 
this point. (Let's call this the pointing end and the end of the axis toward 
the observer, the observer's end. We will also ignore the change in 
declination of the sun and the earth's  movement around it's orbit. The 
gyroscope rotates clockwise as seen from the observer's end.) As the earth 
rotates the sun moves higher in the sky and the pointing end of the gyroscope 
follows it, pointing straight up at noon. Then the pointing end follows the 
sun, pointing straight west and horizontal at sunset, straight down at 
midnight and horizontally toward the east at the next sunrise. So far, no 
north seeking tendency.

Now we modify the set up by adding a pendulous mass under the gyroscope 
connected to the bearings holding the axis of the gyroscope. When the 
pointing end of the axis attempts to rise to follow the sun it is resisted by 
the weight being pulled down by gravity. This applies a torque to the 
gyroscope like a push to the east at the top of the gyroscope and a pull to 
the west at the bottom of the gyroscope. As a result, the gyroscope responds, 
like all good gyroscopes, as though the forces were applied 90� later in the 
rotation of the gyroscope resulting in a push toward the east at the south 
end of the gyroscope and a pull toward the west at the north end of the 
gyroscope. Continue this throughout the day and you end up with the axis of 
the gyroscope aligned with the axis of the earth at which point the pendulous 
weight swings parallel with the rotation of the gyroscope and no longer 
applies any torques to the gyroscope. Of course a real gyrocompass is more 
complicated with a power source for the gyroscope itself, dampening 
mechanisms, and a take off system to display the gyro heading on repeaters.

gl

douglas.denny@btopenworld.com wrote:

> Dear Gary,
>
> Coriolis is a very real acceleration. It is not fictional at all.
>
> The sextant (and person ) is being carried through space in a curved motion 
by the Earth itself, and the acceleration due to curved motion is real 
enough, just the same as if you are in the big Millenium Ferris Wheel in 
London, or in a fairground 'Skyboat' device whizzing around in circular 
motion.
> Coriolis just happens to be a small quantity - though relevant in 
calculation if you are in a jet moving at around 400 knots.
>
> I have a theodolite made by British Aerospace with a North seeking gyro 
built in which 'feels' the acceleration right enough to provide North to 
within a few second of arc.
>
> Coriolis can be allowed for because the motion of the Earth is 'constant' so 
the parameters can be calculated for the direction of travel and Latitude, 
the main components affecting the apparent local gravitational field due to 
Coriolis.
>
> Douglas Denny.
> Chichester. England.
>
>
> >
>
>   



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