A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Fogg
Date: 2007 Jan 31, 13:46 +1100
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:
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:
and then choosing the last option on the left:
Longitude from Lunar Altitudes Simplified
Date 19th January 2000: DR position N47º 45´, W123º 05´
Time Zone 8h W: Height of Eye 9 ft: Sextant Index Correction 0
Body Watch Time Obsd. Alt. Azimuth Int.
Mars 17h 50 m 00 s 25º 04.9´ 223.5º T18.2
Saturn 17 50 00 52 37.5 154.5 A3.3
Fix at 17h 50 m N47º 39.0´, W123º 35.1´
Moon (LL) Observation at 17h 50 m 00 s Altitude 19º 30.2´
Moon Observation and Intercept Calculations
at 17 50m 00.0s, Latitude N47º 39.0´,
Sextant Altitude 19º 30.2´.
Slow (+)10m(WS) W126º 05.1´(LS) T6.3(IS)
0 (WF) W123 35.1(L0) T2.1(IF)
Azimuth of the Moon not required
LS = W123º 35.1´+ 2º 30´ = W126º 05.1´
F = =
IS - IF 4.2
Required Watch Correction
WP = 10m - F x 10m = - 5m 00s (Fast)
LP = 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.
- 5m 12s (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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