Next message: Robert Gainer: "Re: suggestion for a satisfactory celnav narrative"
Bill,
An appendix of things that a naive navigator might suppose are good,
but really are not, would be helpful. I wouldn't put this in the main
section, except maybe to say that noon sun shots are lousy for
longitude and to refer to the appendix.
A fix gives both longitude and latitude without knowing either first,
where the fix is the intersection of two or more lines of position. A
pure latitude shot, such as a noon sun shot, also gives a line of
position, one that runs smack dab east-west, giving no longitude info.
Likewise, a line of position from a body with a due east or west
azimuth gives only longitude and no latitude. You have to know GMT to
use any shot other than a meridian passage shot, an example of which is
a noon sun shot.
You learn GMT from a watch, the radio, lunars, occultations, Jovian
moons, etc.
Fred
On Jun 3, 2005, at 12:26 PM, Noyce, Bill wrote:
> Perhaps the fact that "longitude by noon sun" comes up so often
> is a good reason that there *should* be a discussion of this
> method, pointing out why it is a bad idea...
>
> Is there any other observation that gives longitude without
> knowing latitude first? Maybe this is part of its appeal.
>
> (By the same argument, the advanced version should discuss lunar
> altitudes for GMT, and what's wrong with them. I think George's
> "parts 1-4" do include such a discussion.)
>
> -- Bill
>
> -----Original Message-----
> From: Navigation Mailing List
> [mailto:] On Behalf Of George
> Huxtable
> Sent: Friday, June 03, 2005 8:59 AM
> To:
> Subject: Re: suggestion for a satisfactory celnav narrative
>
>
> Andrew Corl wrote-
>
>> For Courtney, I am raising my hand to help with this. Don't know what
> I can
>> do but here is a list of techniques I feel should be in the manual:
>> Dead Reckoning
>> Latitude by Noon Sun
>> Longitude using a shortwave radio and the noon sun
>> Sextant operation and how to determine the elevation above the horizon
> of
>> the sun, moon, star, and planet
>> Sight reduction using H.O. 249 - method I am presently learning
>> Sight reduction doing all the math (the "apex of celestial navigation"
>> according to Frank)
>
> As a world-weary old cynic, my opinion is that any offer, saying "this
> is
> how I think it should be done, and I'm prepared to contribute" is worth
> at
> least 10 proposals of "this is how I think it should be done, but by
> someone else, not by me". So Andrew's suggestions are indeed useful.
>
> No doubt, he expected a bit of nitpicking about his choice of topics,
> and I
> am happy to provide it. It relates to his suggested topic-
> "Longitude using a shortwave radio and the noon sun"
>
> Although proposals for timing the moment of noon, by observing the Sun
> around noon, reappear on this list at regrettably regular intervals,
> the
> fact remains that noon is the worst possible moment for doing that job.
> Although it's possible to make a crude assessment of the moment-of-noon
> by
> making an extended set of observations, before noon and after, the only
> way
> to get a PRECISE value of time-by-the-Sun (and hence longitude) is to
> do
> so
> at a time WELL AWAY from noon, so that the Sun is rising and falling at
> a
> measurable rate. If Andrew were to rephrase his suggestion to read
> instead-
> "Longitude using a shortwave radio and a time-sight of the Sun",
> then I, for one, would be happy.
>
> George.
>
> ================================================================
> contact George Huxtable by email at , by phone
> at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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