NavList:

Ken,

of course the purpose of the astrocompass is to give true north when you know your latitude and longitude and have exact time (which gives you LHA and dec of the sun) which then gives true north. But you can do it also the other way around: with exact time you get and set dec (and keep GHA in mind), then set your latitude and point to true north. You then adjust the sight and read off LHA. From GHA (derived from time) and LHA you get your longitude. It is - as I found - only accurate to about a degree so it only tells you where about in the ocean you are, you can't really navigate with it. But it shows the principle.

Alex,

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). 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 lat and lon.

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

Wolfgang

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