Not that hard as it seems - here are some ideas ...

Not that hard as it seems - here are some ideas from the post at PMWF:

- the name ASTRODEA comes from something like ASTROnomical Diagram of the sky from the EArth

- while I never liked big wristwatches this might be one of the incredibly few instances where I must say that bigger is better Rolling on the floor laughing my a.. off since things are incredibly small on the dial and you really need the 10x magnifier that comes with the watch;

- there are a LOT of models - (mine is the 2006 titanium one) the watches seem to be made just in limited editions (like 500 ?) and each year they seem to be changing something - there is a quick image with the history here;

- the 'dial' with the sky map is rotating (clockwise in 4x8y calibers, counterclockwise in 4x9x calibers), on top of it there is a fixed transparent (AR-treated) part with certain markings for the zenith, horizon and other measuring stuff (and on top of that normal hands and sapphire+AR crystal); on my model the visible part of the sky is the small oval from the bottom half delimited by the white circle with cardinal points (which is the horizon) and in the middle of which you have the zenith; where the normal hands are you have the celestial North Pole with Polaris;

- there are 'whole sky' models and 'sky facing south' models; most models are with the view from 35 degrees latitude North (Japan) but there were some models from 50 North and there are also 2 models from 35 South! it is still perfectly possible to use the watch 10-15 degrees of latitude away - mine is 'whole sky from 35 N but still works on 45 N Laughing)

- the central point of the watch is that it keeps 2 times - the normal 24 hours time (from the solar cycle) and the sidereal time with a cycle of 23h:56m:04s - that local sidereal time is at the periphery of the rotating dial (when correctly set, and on my model you read the value from the very bottom of the dial where it intersects the meridian); also that sidereal time is the most natural way to express the Right Ascension - a kind of longitude for stars, which together with the declination (a kind of latitude for stars) will describe the position of the star; the nice thing about such a measurement system is that Right Ascension is also the moment expressed in sidereal time when that star is at the meridian - for instance Sirius will always be at the meridian when the local sidereal time will be 06h 45m 08.9173s - and knowing that you can easily find it even on my watch photo!

- you can calculate a lot of things like sunrise/sunset or positions of the stars at given moments (usually losing either sidereal time with the crown at first click, or both that and the normal time with the crown at the second click.