# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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?

George,

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
12:03:20.

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.

Fred

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