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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Longitude by lunar altitudes
Date: 2010 Jan 5, 15:12 -0800

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.

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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
(yup, that's my real name!)

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