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> effect navigation at sea in overcast daylight, modulo some basic idea of the time

What do you mean with "modulo some basic idea of the time"?

The sun moves. You can't sail a constant linear course using a device which is sun seeking, unless you include some sense of time. It describes an arc in the sky, you'd be steering in a curve.

Happy to be told otherwise.

At dawn and sun set you don't need to know the time. But then again that is only twice a day to adjust course.

I wonder if you can track mid day with such a stone.

Without wishing to be over pedantic, that's modulo time at the granularity of a day, and with set points.

With a wish to be overly pedantic, if you can keep a somewhat steady course during the day, or anchor, and pick the mid point between sun set and dawn you don't even have to know the day of the year, time of the day, or anything time related.

I was mostly confused by the wording. I didn't know 'modulo' could be used like that, but apparently it has a colloquial meaning of "except for the variation accounted for by" and now the phrase makes sense to me.

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