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    Re: chronometer question.
    From: Fred Hebard
    Date: 2006 May 13, 09:05 -0500

    On May 13, 2006, at 8:42 AM, George Huxtable wrote:

    > Departing from Shetland, with its Northerly point in sight, at a
    > certain time-by-chronometer, an alt. of the Sun (supposing the N. end
    > of Shetland to lie in long. 38'W) gave the Chronometer slow of
    > Greenwich 3 minutes 39 seconds ...
    > However, even in British home waters, the less-frequented parts had
    > not all been well surveyed by 1818, and the charts carried may have
    > been somewhat defective. Anyway, a modern atlas puts the N end of
    > Shetland at about 0 deg 50' W, rather than 0 deg 38' W as had been
    > assumed; quite a difference.
    > In the light of that information, what should the chronometer error
    > have really been?


    I'm not sure what slow of Greenwich means; I assume here that if the 
    old position were correct, a chronometer 0:3:39 slow of Greenwich 
    would have been reading 11:56:21 at noon in Greenwich (also assuming 
    the day was reckoned from midnight).

    Let's say time sight gives LAN as 12 noon (yes, noon is a bad time 
    for time sights, but it's a convenient time for us).  Let's also 
    assume the equation of time yields a correction of zero for this date.

    Old position of 38'W would put you 38/15 minutes after noon in 
    Greenwich, or 12:02:32, observed Greenwich time.

    New position of 50'W would put you 50/15 after noon in Greenwich, or 

    Chronometer is slow of Greenwich in old position by 0:3:39.  That 
    means it was reading 12:02:32 minus 0:03:39, or 11:58:53.

    The chronometer is actually slow of Greenwich by 12:03:20 minus 
    11:58:53 or 0:5:27.

    The key here is to find out what the chronometer was actually 
    reading, assuming a LAN of 12:00:00.

    As an aside, these types of questions always baffle me, but not my 
    wife.  My daughter goes to Purdue in Indiana, where Alex and Bill 
    are.  Until this year, most of Indiana did not observe daylight 
    savings time.  They were on Central Daylight Time in the summer, and 
    Eastern Standard Time in the winter, perhaps; I still can barely 
    phrase the question, let alone figure out what time it would be in 
    Indiana if they were not observing daylight savings.


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