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If a partial eclipse of the Sun was depicted and the date was known that would give good clues as to the location of the vessel
The solar observation idea would only work if the date of the year were known. The latitude/solar angle relationship depends critically on time of year. And the longitude/GMT relationship has a variability of about +/- 8 minutes of time (=32 minutes of arc = approx 50 km in tropical regions) because of the ellipticity of the Earth's orbit, and the variable length of the solar day. This correction always has to be made to sundial time. Once again the exact date would be needed.A very accurate latitude indication could be given by the position of the pole star (elevation above horizon = North latitude, always, with no correction necessary unless the elevation is less than about 10°). Moonrise of an exact half moon into the night sky would fix the local time at midnight, give or take the sundial correction. The difference between the geometric moonrise and the optical moonrise would mean that the moon fully visible but touching a nautical horizon would correspond fairly closely to geometric moonrise. However you look at it the longitude is more problematic than the latitude.Perhaps the best way for your artist to indicate the position would be to include a marked chart in the picture, casually tabled in the wheelhouse.