Calculating longitude is one of the most difficult things to do in maritime navigation. Today we use accurate clocks and sun-sightings to know our position, when we don't want to just read it off an electronic device.
I have to wonder just how accurately one could know the longitude from the positions of major planets and star-sightings, which actually give more information than daytime sun and moon sightings. If the machine worked as I think it worked, day- and night-time readings would be set on the machine, and the output would be a navigational position in both longitude and latitude.
With the appropriate map-discs set up to display, it would be like our current dashboard location devices. The only thing hampering navigation then would be fog and rain, and lack of wind. Hence the newly realized importance of the lighthouse at Rhodes.
Need I also point out that such a device could easily show one's position anywhere on a globe, and thus one could theoretically circumnavigate Antarctica, and map its perimeter? (See the controversy regarding the Piri Reis(Sp?) Map.)
Or, in an appropriate vessel, sail to the new world?