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> My definition of local noon is:
>
> (1) When the centre of the sun crosses the observers meridian and
>
> (2) When the centre of the sun is due north of the observer i.e.
> its Zn is 360d true and
>
> (3) When the centre of the sun reaches its highest altitude

(1) and (2) are the same (at least south of the Tropic of Capricorn),
but (3) is not exactly the same.  Since the sun's declination is
usually changing slowly, it will reach its maximum altitude a bit
before noon in the summer and autumn, or a bit after noon in the
winter and spring, with the effect largest around the equinoxes,
when its declination changes about 1' per hour.  On July 20 it's
only about half as much, so it takes 12 minutes for the declination
to change 0.1'.

This effect is too small to have any practical effect on finding
longitude, as far as I know, but the similar effect that arises
when you observe from a vessel travling north or south needs to
be taken into account.
        -- Bill

   

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