Re: Fix by equal altitude sights around localapparent noon
From: Hewitt Schlereth
Date: 2009 Oct 10, 18:56 -0400

Hey, Mal -

Thanks for sticking up for Equal Altitudes Around Noon. I used it for
years before I did the book. It has all the advantages you mentioned.
Its disadvantage is you often swelter under the sun for longer than
the St. Hillaireans.  :-)

Hewitt Schlereth
Latitude and Longitude by the Noon Sight
St. John, USVI

On 10/10/09, frankreed{at}historicalatlas.com
<frankreed{at}historicalatlas.com> wrote:
>
>  Since the previous message was a reply to a very old message (from 
1997), I am copying the original text here to save you all the trouble of 
digging it up:
>
>  ----BEGIN original message----
>  [Nml] Fix by equal altitude sights around local apparent noon
>  From: Malcolm Misuraca [celestial_mam---msn.com]
>  Date: November 13, 1997 11:02 AM
>
>  The objections to the use of equal altitudes of the sun around local
>  apparent noon are simple mischief, and they miss the most essential point
>  about celestial navigation in the age of GPS.  Most yacht sailors now resist
>  learning celestial at any level, because they consider that a GPS and backup
>  is more than they will ever need---and the backup can be purchased cheaply and
>  at less cost that a sextant and related materials and equipment.
>
>  The point is, therefore, that for all those who are fully invested in their
>  own mastery of sights not on the meridian, and who therefore deplore the local
>  apparent noon methods, and who use all sorts of claims and disclaimers to
>  discourage others from doing so, a terrible disservice is being done to the
>  effort to teach enough celestial to make yacht sailors safe even if their GPS
>  and backups fail.
>
>  The difference between teaching equal altitudes around local noon and the
>  use of sight reduction tables is a very significant difference.  In my class,
>  over four years now, I have taught both and found that the resistence to
>  learning how to reduce sights not on the meridian is increasing.  No one will
>  buy sight reduction tables anymore, and my friend at Armchair Sailor says it
>  is all he can do to persuade people about to sail into the Pacific to buy an
>  almanac.
>
>  For those who claim that if the equal altitudes method were valid there
>  would be references to it in respected elements of the literature, I would
>  simply say that you have not looked into the literature before giving out an
>  opinion.  Hewitt Schlereth in his book on navigation by the noon sight
>  explains the equal altitude method and corrections for movement of the vessel
>  for those who want to reduce the small error induced by the difference in
>  position of the boat between sights before and after noon.  And Volume 3,
>  pages 157-160 of The Admiralty Manual of Navigation (1954) explains the equal
>  altitudes method and gives a correction table to account for change in
>  position of the ship between pre- and post-LAN sights.
>
>  The principle of the equal altitudes method could not be simpler.  By
>  definition, the sun increases in altitude before local apparent noon at the
>  same rate as it declines in altitude after noon.  Local Apparent Noon is by
>  definition the moment when the sun reaches its highest altitude at your
>  position and is therefore on the meridian (either due south or north of your
>  position).  Everyone knows that the noon sight was used to determine latitude,
>  but the use of sights before and after noon is even more useful.
>
>  If you take a sight at exactly 50 degrees sextant altitude before noon and
>  later, as soon as you know that noon has been reached by the fact that the
>  altitude of the sun does not increase, reset your sextant to 50 degrees, you
>  then wait for the sun to descend in the sextant until it is tangent to the
>  horizon and note the time.  Exactly mid-way between the 50 degree sight before
>  noon and the 50 degree sight after noon, as determined by the time of each
>  sight, is local apparent noon.  A brief look in the almanac for the Greenwich
>  Hour Angle of the sun at that moment yields longitude, and the time of local
>  noon is a good check on your latitude sight as well.
>
>  By taking several such pairs of sights, a series of estimates of local noon
>  is made and the average of them yields an even more comforting check on the
>  time of local noon.  If, as someone said in a recent comment, the sun is being
>  obscured by clouds during the run-up to noon, start earlier and take as many
>  sights as possible.  (The same clouds that are conjured up by the nay-sayers
>  to get in the way of doing this are ostensibly clouding their sights for lines
>  of position, and once again they are doing a disservice to this simple and
>  precise procedure.)  This increases the odds of one or more pairs of sights,
>  even if the sun is being obscured after local noon, as before.  This procedure
>  can be used for sights taken relatively early in the  morning, well before
>  noon, if necessary, so long as they are matched by equal altitude sights after
>  noon.
>
>  No one claims this method had advantages over the use of sight reduction
>  tables, because the methods are complimentary, not competitive.  What makes
>  the objections to the LAN method tedious and sometimes misleading is the
>  uninformed claim that the equal altitudes method does not appear in the
>  literature and must therefore be voodoo.  It is the objections that are in
>  deep voodoo, not this method that has saved, and will do so again, the
>  relatively inexperienced and untutored navigator who has dropped a GPS
>  overboard and is trying to cope with anxiety and forgotten instruction to find
>  a way home.
>
>  For those, finally, who say this method has been tried and does not work,
>  remember that a large part of the success of any endeavor is attitude.  If you
>  don't want to use equal altitudes, because your private view of the world
>  might be threatened, by all means navigate as you please.  But stop trying to
>  discourage those whose hold on the theory, math, or practice of sight
>  reduction methods may be tenuous from saving themselves or making their
>  passage more comfortable by knowing this simple and effective way to sail to
>  any point on any ocean.
>
>  I repeat my offer to xerox the Admiralty Manual pages on this method, or
>  Schlereth, for that matter, for anyone who will send me an email with that
>  request.  Surprisingly, or maybe not so surprisingly, those who on these pages
>  have written that there is no scientific or academic support for the equal
>  altitude method don't seem to want the pages from the Admiralty Manual to
>  contend with.  For shame.
>
>  Mal Misuraca
>  "Celestial in a Day"
>  San Francisco
>
>  ----END original message----
>
>  Note that this message by Mal Misuraca was clearly part of some more 
involved discussion, however we do not seem to have any of the other original 
messages from this one. Also, in case anyone new is curious, "Nml" was the 
original name for this discussion group (before "Navigation-L" which was 
before "NavList").
>
>
>
>
>
>  >
>

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