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Alex,

Geoffrey's method would need a star catalogue but not necessarily an
almanac. Since the stars are fixed (at the level of precision of
instrument-free navigation), the declinations of the brighter ones can
be learnt with relatively little difficulty. (Equivalent knowledge,
though not expressed as "declinations", formed a key component of
traditional Polynesian navigation. It didn't need the support of
astronomers, telescopes and printed tables of numbers.)

Determination of the zenith without a sextant is said to be possible by
lying on deck, sighting up the mast, and visually averaging the position
of the masthead against the sky. However, I think I would rather
memorize the stars with declinations equal to my chosen co-latitude.
When on the intended parallel, those stars should of course be just
circumpolar, grazing the horizon at each inferior meridian passage --
something that could be observed with reasonable precision at tropical
and temperate latitudes.

Or you can make a kemal suited to the latitude you are planning to sail
down, particularly if it is in the northern hemisphere and Polaris is
available.


Trevor Kenchington


You wrote:

> Dear Geoffrey,
> I am not sure that I really understand the details of the method you
> describe but it seems to be based on the almanac.
> The people you mention just used the almanac BEFORE
> their trip to select the stars.
>
> In principle, you can memorize some data from the almanac,
> then leave the almanac at home and use the data from your
> memory:-)
>
> For example, the hero of Mark Twain novel "Yankee at the court of
> king Arthur" memorized the day and time of a solar eclipse:-)
>
> (He just happened to read an almanac before his travel.)
> I would not call this "navigation without almanac".
>
> Second, how do you detect without a sextant when a star it in zenith?
> I don't think this is easy to do.
>
> Alex.


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

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