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Wolfgang,

> I agree that the globes use the same principle; again: the other way around.
> You use your known position (lat and lon) on the equatorial system to
> find the position of the star on the horizon system (height and
> azimuth).

That's exactly what you do when finding a fix with the Sumner
or St Hilaire method.
Using you "known" (DR) position you calculate the asimuth and altitude.

Only the globe does it with smaller accuracy.

> The calculating devices find the position on earth starting from time
> (GHA and dec of the celestial body) and height and azimuth to find your

As I understand, there are several types of devices.
(Though I've never seen a real one except in the photos.
Some of them seem to calculate the elements of the Sumner position
line: the altitude and asimuth for the given DR place.
(Hagner position finder?) That is solving one triangle.

Others like Zerbee do a complete fix from 2 stars for you,
that is they solve several triangles simultaneously.
I read the patent of the Zerbee device, and I remember someone on the list
mentioned long ago that there was one for sale (on e-bay or some other
place).

I don't now how many were actualy produced, and how they performed.
But from the patent I conclude that the thing was probably of very
poor accuracy: it is too complicated, there are no provisions for any
adjustments, etc. Too many pivots, circles inclined at
various angles and micrometers. Every pivot will certainly have
some play, every micrometer will have backlash, and every piece of
the device will be non-rigid. (In a normal sextant, we have only one worm
one arm and one pivot, and all these problems are already present,
and it took almost 200 years of development to fix them).

I suspect that this invention was a complete faillure, even if few
of them were made.

> Astrolabes are astronomical calculating devices but are
> basically different as they use a stereographic projection of the
> celestial sphere. The ancestors of the calculating devices therefore
> aren't the astrolabes but the armillary sphere and probably the
> torquetum. Astrolabes are the ancestors of methods using stereographic
> projections like the ARG1

You are trying to narrow the meaning of the word "calculating devices".
The astrolabes, globes, armillary spheres, star finders, Bygrave rule etc.
are analog devices that solve the same problem: solve a spherical
triangle, or several of them.

They do it differently. Some use stereographic or some other projection
and work in the plane. Some use an arrangement of circles in 3 dimensions.
Some really calculate with digits but in an analog way
(Bygrave rule). But they are all calculating devices, and all
solve essentially the same problem.

Alex.
P.S. I am reading an interesting book, M. Wright, Most probable position.
A history of aerial navigation to 1941. Having a lot of fun.

(And yes: star globes were also used to find a fix:-)
It is really amazing to learn what they were trying to use on the
early stages of air navigation.

   

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