NavList:

Lars,

Right you are.  I blundered in the final subtraction, and should have 
ended up with 4:27, not 5:27.

Fred

On May 15, 2006, at 8:16 AM, Lars Bergman ((HF/ESE)) wrote:

>
> 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?
>
> There was a reply from Fred Hebard with the answer 0:5:27. I do not
> agree. With the time sight you calculate your local apparent time 
> which
> is then converted to local mean time, LMT. Now
>
> LMT = GMT + easterly longitude
>
> Knowing longitude you can determine GMT. Comparing chronometer time 
> with
> the determined GMT gives the chronometer correction. The 
> chronometer was
> 3m39s slow on GMT when longitude 38'W was used. Using a longitude that
> is 12' further west, or 48s of time further west results in a GMT that
> must be 48s larger than in the first case (in order to keep the 
> observed
> LMT). If the chronometer is 3m39s slow of GMT in the first case, it 
> must
> thus be 48s more slow in the second case, that is 4m27s.
>
> In 1968 I made a voyage to the Shetlands in my farther's sailing 
> yacht,
> a gaff-rigged 25 tons displacement double ender, but have no memory of
> the actual longitude. Anyway, on our then brand new Admirality 
> chart of
> the Faeroes there was a small note at the bottom stating something 
> like
> "According to the latest Danish surveys the longitude of this chart
> should be moved 25 seconds of arc to the east" or maybe it was west, I
> do not remember. So, also in rather modern times (pre GPS at 
> least), the
> longitudes could be a little uncertain.
>
>    /Lars 59N 18E
>
> >


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