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I had written

> "Frank made the  interesting suggestion that a lunar occultation might
> have
> been employed to  get the "timing" error of the chronometer, and indeed,
> so
> it could.  "

and he replied-

> I agree with you about the relative impracticality of using  occultations,
> but actually, I mentioned "moon culminations" which were very  popular for
> land-based determinations of longitude in the first half of the 19th
> century.
> These are apparently more practical than occultations.

So he did, and I had carelessly misread his earlier posting.

But I'm not so convinced about the practicality, for a ship's navigator, of
timing Moon culminations, the moment when the Moon exactly crosses the
North-South line of an observer; near, but not at, the moment of maximum
altitude of the Moon..

Frank's original contribution on this topic read-

>If we don't trust the tabulated longitude, you have other options  for
>determining longitude ashore like moon culminations that are generally more
>accurate than lunars. These depend on a careful determination of true
>azimuth.

Indeed they do, and that's not an easy matter using a navigator's toolbox,
even if he goes ashore. Timing Moon culminations (with respect to nearby
stars) was a technique that normally needed some sort of transit instrument,
carefully aligned to swing in the North-zenith-South plane. That was
something a geographer or surveyor might carry, but not a navigator.

With the Sun, it was practical to use equal-altitudes to determine the
moment the Sun passed the meridian at noon. But to try to do the same thing
for the Moon was far more difficult, because of the rapid shifts of
declination, and the irregularities in the motion. Hence, as Frank says,
lunar culminations depend on "a careful determination of true azimuth". And
how is a navigator to ensure that, to sufficient precision, with the
instruments he has available, even if he goes ashore?

It seems to me, then, that the lunar occultation of a star, which needs
nothing more than a telescope firmly planted on the ground, and some
communication with chronometer time, is a far more practical proposition for
a ship-navigator, even though it might call for a lot of difficult
computation, and a few days wait for a convenient star to pass behind the
Moon.

George..

   

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