NavList:

Francis Chichester described a method of finding longitude and watch correction from lunar altitudes ("Longitude Without Time"; Journal of the Institute of Navigation , Vol. 19, 1966) that he apparently devised independently, although it is said to have been earlier described by John Letcher, best known for his work on self-steering systems.

 

This method: longitude via lunar altitudes, has been discussed at least a couple of times on this and/or the earlier Nav List. It tends to be less accurate than the conventional method of lunar distances but has the advantage of using familiar sight reduction methods. It requires the moon's azimuth to be near to due east or west.  

 

The Starpath site:

http://www.starpath.com/catalog/accessories/starpilot/lun_alt.htm

describes the method, with an example, using the StarPilot calculator (based on a Texas Instruments TI-86) and successive reiterations to achieve a result.

 

George Bennett has devised what he claims to be: "a simple method of … calculating longitude from lunar altitudes that does not require a succession of approximations".

This proposed method, with a worked example, has been published as: "Longitude from Lunar Altitudes Simplified" in the Journal of the Institute of Navigation , Vol. 53, No. 2, 2006.

 

 This article can be accessed by going to:

http://gbennett.customer.netspace.net.au/

and then choosing the last option on the left:

Longitude from Lunar Altitudes Simplified

 

 

Date 19^th January 2000:     DR position N47º 45´, W123º 05´

Time Zone 8^h W:    Height of Eye 9 ft:    Sextant Index Correction 0

 

Body      Watch Time      Obsd. Alt.    Azimuth     Int.

Mars     17^h    50 ^m  00 ^s     25º 04.9´       223.5º      T18.2

Saturn   17     50      00       52  37.5         154.5         A3.3

Fix at    17^h     50 ^m        N47º 39.0´, W123º 35.1´

 

Moon (LL) Observation at   17^h     50 ^m   00 ^s     Altitude 19º 30.2´       

 

Moon Observation and Intercept Calculations

 at 17 50^m 00.0^s, Latitude N47º 39.0´,

 Sextant   Altitude   19º 30.2´.

 

           Azimuth of the Moon not required

            L_S  = W123º 35.1´+ 2º 30´ =  W126º 05.1´

 

                                    I_S                   6.3

                      F   =   ———   =  ——

                                I_S   - I_F               4.2   

 

Required Watch Correction

W_P   =   10^m - F x 10^m =   - 5^m 00^s (Fast)

                          

Required Longitude

       L_P    =  W126º  05.1´ - F x 2º 30´  =  W122º 20´

 

Check: When the above watch correction and longitude is used with the original data the intercept should be zero.

 

Starpath Values

- 5^m 12^s (Fast)     and       W122º 17´

 

In the context of lunar observations these differences are of no significance.

The Starpath values, while similar, involve five pages of graphics.

 

 

 

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