NavList:

L Martelly, you wrote:
"If the ecliptic on a earth-globe is marked off in 365 days, should the Julian date plus the 21 day off-set be at the latitude of the sun at high noon on that date? If so it would amount to a graphic solar table.
You would find the appropriate tic mark for the corrected Julian date scale to the latitude on the globe and site your sextant. The error would give your latitude."

I'm a little puzzled by your reference to Julian dates here, but otherwise, yes, that was, in fact, the original purpose of placing the ecliptic on globes. Just as you say, it is a "graphic solar table". It allowed the globe user to read off the Sun's declination on any given date. And by definition the Sun's declination is the latitude where the Sun is at the zenith (straight up). On most globes, the ecliptic is marked with the actual calendar date. You find September 1, for example, and you can directly read off the declination of the Sun.

Note that the analemma, which was printed on many old globes, was also a graphical chart of the Sun's declination as well as the equation of time. The equation of time represents the displacement of the Sun's longitude for a given instant of mean time. Often though, this was printed in a reversed fashion on globes so that the visual interpretation is lost. And bear in mind that both the analemma and the ecliptic on globes were there mostly for demonstration purposes. The actual accuracy of the data that can be obtained this way is very low.

By the way, you wrote:
"the error will give your latitude".

Yep. That's the essence of it. In slightly more detail, at local noon we find the Sun's zenith distance (how far away it is from straight up). The zenith distance is 90° - the observed altitude of the Sun (after the usual corrections for index error, dip of the horizon, refraction, and semi-diameter, which together amount to about 12'). If we are north of the Sun, we label the zenith distance a positive number. Otherwise negative. Then we add that zenith distance to the Sun's declination (the declination is counted positive if it's north or the equator, negative if it's south). The result is the observer's latitude (north if positive, otherwise south).
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