I don’t think it is exactly zenith at exactly 12 noon, though now that you mentioned it, I remember that the apps I use shows it might shift as much as +/- 30 m

I don’t think it is exactly zenith at exactly 12 noon, though now that you mentioned it, I remember that the apps I use shows it might shift as much as +/- 30 mins where I am at. Playing with the app (Sun Seeker), I can see what you mean by different places and times of the year as it relates to clock time and zenith. On the note about the age of sail, I remember that accurate maritime clocks revolutionized and enabled sailing, and colonization. Though at the time, I was mystified by how the sextant and clock would get location. Two of my hobbies are permaculture and astrology, so I spent a lot of time observing the path of the sun as it relates to my location on earth.