NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude by Lunar Distance
From: Henry Halboth
Date: 2006 Nov 13, 00:40 -0500
From: Henry Halboth
Date: 2006 Nov 13, 00:40 -0500
Frank,
I certainly agree that the averaging of multiple observations of a
single body appears to have been frequently used in the past - it is, as a
matter of fact advocated in Norie's 1839 and 1889 editions, as well as in
numerous more modern texts, but is generally dealt with in the context of single
sights or Lunar observations, not in that of multiple sights such as may be
taken for a star or Sun-Venus-Moon fix. It was my custom, as before stated, to
observe five stars (if available) to establish a star fix - it is difficult to
imagine the amount of record keeping which would result if four observations of
each star were to be taken, not to mention the additional probability of running
out of optimal twilight time in the North Atlantic before completion of the
round. By like token, if only two stars were to be available, I would be
inclined to take a number of observations of each body to rule out inadvertent
error - assuming that other circumstances permitted.
I guess what I am basically saying is that if only one object is being
observed for Latitude or for Longitude, as in a noon night or time sight,
averaging seems entirely appropriate, whereas in LOP navigation where multiple
bodies will be observed to establish a fix, it becomes inappropriate. Again, as
I have so often said, the navigator must evaluate every situation as it occurs
and take the appropriate action; at sea, changing conditions of passing cloud
cover, fast approaching darkness, horizon obscuring squalls etc., are likely to
dictate the amount of time to be spent in observing each body.
Regards,
Henry
Henry
----- Original Message -----From: FrankReedCT@aol.comTo: NavList@fer3.comSent: Sunday, November 12, 2006 7:22 PMSubject: [NavList 1694] Re: Latitude by Lunar DistanceGeorge, you wrote:"That assumption, of "no underlying bias", is a big one, and an
unjustified one. You can halve the error by taking four observations,
but only when those observations are all statistically independent."It is no "assumption", George. It works in practice, again and again. Or I should say, it works 'for me' again and again, and that's why I recommend it. Also, in the log books of vessels from the early 19th century, you frequently find references to averaging four or more observations. This was standard historical practice.-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
