NavList:

Hello Brad,

Little time left for me tonight to reply in depth...

First of all, I think that a lot of your comments make sense.

Maybe just one early clarification here in reply to your specific lines herafter :

QUOTE

In a previous communication, you provided values with a 29 arc minute variability in latitude.

UNQUOTE

I would think that - if no typo on my behalf - you refer to the various POSSIBLE values of Latitude which I derived as a result of various conjectures on possible realistic values of HOE (Height of Eye):

If we assume that - everything else being equal - the height of eye is 0 FT (i.e. published heights already corrected for Dip ), then Latitude derived from all other data would be S 28°28'18",

For HOE = 12 FT, under the very same remaining assumptions, get S 20°49'29", and

For HOE = 17 FT, get S 20°53'50" and

For HOE = 23 FT, get S 20°57'36".

It actually amounts to a "latitude swing" of 27', but it is not really any "observed" swing, since this 27' difference between "computed" values is only a consequence of guessing for different HOE's.

Why would I tend giving some favor to "computed latitudes" more or less matching "published" latitudes ? Simply because I have observed many times - a few times on this page, a quite a few on other examples - that the published latitudes are - and sometimes by far - the most reliable information they could achieve then. And it simply makes a lot of sense.

Even if all heights were given to the utmost accuracy, given both the Almanacs and sextant (in)accuracies - we should never forget that final fix position - whatever the computation method actually used - could not then be consitently better than some 15'/20' in Longitude (see a recent interesting note from George yesterday). On the contrary, Latitudes are much more reliable "by nature" and, time permitting I would not be overly surprised to observed that their latitudes of known spots (another recent post from George) where accurate to some 1' in Latitude.

So, do not be afraid of such "latitude swing" which is by no means any kind of "observed" swing.

We are all aiming at getting a thorough understanding of what is behind the published altitudes : are they corrected for dip - and if so, for which HOE ? - or not, are they corrected or not for Refraction ?

Given the much higher reliability achieved then on Latitude determinations, we should favor assumptions on HOE which close us from the published Latitudes rather than from the published Longitudes.

Hope these few lines might help you, unless I have totally mis-understood your point here.

*******

Aqs regards the SUN declination, that is very strange to have a 3' mintue difference, since by these Times, the Sun's Declination was consistently known to better than 6".

I have quoted the DE406 values for a time TT equal to 15h48m. You need to first tranform this time into UT (TT-UT = 16.4 s) then into Greenwich Apparent time through the correct proprotion rule + adding/removing 12 hours, and then compute results from the Almanac published data (which I have not looked up yet).

As regards the use of Apparent Time versus Mean Time ... well, I have had a lot of experience with such Apparent Time computations which were an UNESCAPABLE NUISANCE then.

Computations with Mean Times are so much easier now, since we have so many reliable Time Keepers.

*******

hope it helps.

Meanwhile, if you have additional comments / requests, I will be happy to help you to-morrow. Almost no time left for proof reading (any difference any way in my own case ?)

Best Regards

Antoine M. "Kermit" Couëtte
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------