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Brad Morris wrote-

"Sorry to disapoint all those who would ascribe my reference to Chauvnet.
It is to Captain William Thoms "A New Treatise on the Practice of Navigation
at Sea", 1856.  The title page explicitly states that it is published by the
author for use at his navigational academy in NYC (184 Cherry Street)."

and then described some unusual metods for deriving longitude, to be found
therein.

This is just to say that I've found a digitised copy of the relevant pages
at-
http://www.archive.org/stream/newtreatiseonpra00thomrich#page/n209/mode/2up

The complete volume is to be found on Open Library at-

http://openlibrary.org/works/OL12292744W/A_new_treatise_on_the_practice_of_navigation_at_sea

It seems a highly original work, but I've never come across any mention of
Thom, or that book. Perhaps he is better known in the USA.

Thanks for the pointer, Brad. My apologies for presuming that you were
referring to Chauvenet's book.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: 
To: 
Sent: Tuesday, January 05, 2010 11:12 PM
Subject: [NavList] Re: Longitude by lunar altitudes


Sorry to disapoint all those who would ascribe my reference to Chauvnet.  It
is to Captain William Thoms "A New Treatise on the Practice of Navigation at
Sea", 1856.  The title page explicitly states that it is published by the
author for use at his navigational academy in NYC (184 Cherry Street).

There are two interesting methods for finding the Longitude using the moon,
besides the standard lunar distance methods also described.

One method is "Finding the Longitude by observing the Moon's Declination".
The method states that when the Moon and a star are on the same meridian
together, that the distance be measured between them, from which one can
deduce the moon's declination.  The Greenwich time corresponding to this
declination, taken from the Nautical Almanac, compared with the mean time at
the ship is turned into Longitude.

Thoms states that "As the Moon changes her declination at the rate of about
14' in 1 hour of time, when near the equator, an error of 1" in the observed
declination will produce an error of 1' of longitude and an error of 1' of
observed declination will produce an error or 1 degree of longitude in the
most favorable case"

"This method, therefore, is not capable of much precision.  Besides, it can
only be used to advantage when the Moon's declination changes rapidly, that
is, when she is near the equator; but when the Moon had great North or South
declination, this method is not practicable.  It may, however, be found
useful om some cases, as the Observation (the objects being on the same
vertical line) is much easier to take than a regular Lunar Distance.

Thoms then shows the method of the method
1) Observe the Distance
2) Observe both altitudes "roughly" (Thoms words)
3) Correct for semi diameter and horizontal parallax
4) Correct for the Moon's parallax in altitude
5) Correct for refraction
6) Having the True distance between the moon and the star to find the
declination
7) Having the moons observed declination, to find the greenwich time and
longitude

Thoms also displays a method for finding the Longitude when observing the
meridian altitudes of the moon and a star.  This method is very similar to
the previous, only that we substitute measurement of each body's altitude on
the meridian for the distance, and from this, deduce the moon's declination.
Thoms states that the altitudes should be observed to the nearest second.
He states that this method has the same limits as the previous.

-----

Note the date of 1856 and that Thoms explicity uses the Nautical Almanac to
derive one's longitude from the observation.  Hence, to your question Frank,
apparently nothing is missing from the NA to derive one's longitude for
these methods.

I can scan in the full text, 4 pages, should you wish.

The same textbook, used at the same school in 1900, has nearly identical
words on this topic.  By this time, however, Captain Thoms had expired and
the school was run by his widow, as the title page states.

Best Regards
Brad Morris
(yup, that's my real name!)


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