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Wolfgang,

> As Geoffrey said there is a whole family of instruments
> basically representing the celestial and the horizon sphere on
> semi circles and circles.

I include to this family star globes and astrolabias (..ae?)
Not the Marine astrolabias, but of another type, which were
popular in the Muslim world...

> They were first proposed in the 16th century

I suppopse these astrolabias were proposed in antiquity, Hellenistic
times, Ptolemy mentions them, but probably none survived.

Theit function was exactly the same as of all those
analog calculators, to give you altitude and asimuth
(and rasing/setting times and points, and culminations)
from SHA, declination and time).
By solving a spherical triangle using the metal divided circles,
which rotate with respect to each other.

My globe does this very well to 1 degree.
Applying very fine pivots and arcs and verniers you can probably
approach sextant accuracy on the devices of the same size,
but this is much harder to make than a sextant.

Alex.

   

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