A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Fogg
Date: 2007 Feb 21, 14:42 +1100
I would suggest Letcher's Feb 1964 article in Yachting is the earliest
"modern" rediscovery of the longitude by lunar altitude methods.
As Frank points out, such methods were well known more than a hundred
of years ago. The Ashe discussion is from: Monthly Notices of the
Royal Asronomical Society, Vol XII, nov 1851-june 1852, page 179.
(Available from Google books.) Many of the astronomy and navigation
texts from the 1800's (and there are quite a large number on google
books) have references to longitude by lunar altitudes. Since this
was before LOP, most discuss only longitude assuming latitude by one
of a number of other methods. See for example, New Methods of finding
the Longitude at sea or on shore, by Yarrow and Lynn, 1826, also on
Below is an example using one of these methods with the data from
Bennett's recent "Longitude from Lunar Altitudes Simplified".
On 15 Feb (local time!) Dan Walden sent an example using "one
of these methods"
(which one? and why?)
to establish longitude / time via lunar altitude.
It seems quite a bit more complex than the method proposed by Bennett.
In Chichester's article, latitude was observed prior to a Sun/Moon observation. Position fixes were calculated at some convenient time interval before and after the Sun/Moon fix he chose half an hour. This required a plot to be made covering about fifteen degrees of longitude to find where the line joining the two fixes would intersect the previously determined latitude. Using the principles shown in the Bennett article in the Journal of Navigation, this process can be simplified by using a numerical technique. The following is an illustration of this:
Date: 4th March 1990. DR: Lat N40º, Lon W20º. H.E. 6 ft. Index Correction +3.5'.
Observed latitude: N39º 52.6'
Lat N40º. Watch Time 16h 02m 17.0s.
Body Watch Error Lon Altitude Lat Lon
Sun LL 0 W20º 31º 58.7' Fix N39º 48.8' W19º59.5'
Moon UL 0 W20º 40º 53.3'
Sun LL Slow 20m W25º 31º 58.7' Fix N39º 39.1' W24º46.7'
Moon UL Slow 20m W25º 40º 53.3' .
Diff. 9.7 4º47.2'
Let A = N39º 52.6' - N39º 48.8' = 3.8
Longitude: W19º59.5' A/9.7 x 4º47.2'= W18º 07.0'
Watch Correction: A/9.7x20m = Fast 7m 50s
While a simplification, this calculation technique is considered inferior to the method shown by Bennett in the Journal of the Institute of Navigation article.
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