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Christian Scheele wrote, in [9389],

...let me try to answer your questions:

"1. Is it true that:

on land, the culmination of a heavenly body, including the moon, can be
established rather accurately in reference to altitude and in reference to
time as indicated by a stable local clock? "

Yes, and not just on land but anywhere on earth. The almanacs publish this
data, tabulating time against declination and GHA of the sun and Aries for
all major celestial bodies used for navigation pruposes. Of these celestial
bodies, the moon's declination changes most rapidly. The altitude of a
celestial body at culmination will be a function of your latitude and the
declination of that body at that time extracted from the almanac directly
against the relevant GMT or found by interpolation. The function of the
sun's declination can be approximated to a high degree of accuracy using a
simple equation, a slightly more complex equation does so for  the equation
of time, the effects of precession and nutation can also be estimated (it
would interesting to hear from other members how this is done exactly, I do
not know), but you will need an almanac for navigation involving the planet,
moon and stars unless you are an expert. Inaccuracies may occur as the
result of anomalies in the surface of the earth and the usual practical
problems involved in taking sights.

===================

Comment from George.


Well, you can accurately calculate, from an almanac, the GMT moment of
culmination of an object, from any position, land or sea, IF you know the
longitude of that position but not otherwise.

And you can accurately observe the moment of culmination of a body from on
land, once you have set up a North-South direction line and can time the
moment when its azimuth crosses that line. But not, by that method, at sea,
in which case you have to use the imprecise technique of timing the highest
altitude of that body, or else, more precisely, by bisecting the times of
two equal altitudes taken well apart. Not a "one-shot method".

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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