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    Re: Latitude by Lunar Distance
    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 -----
    Sent: Sunday, November 12, 2006 7:22 PM
    Subject: [NavList 1694] Re: Latitude by Lunar Distance

    George, 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


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