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Frank,

Good point.

In answer to various claims about the accuracy of lunars, it would seem
to me that the error in lunars should approach the precision of the
sextant, given enough measurements of decent quality and decent
reduction procedures.  That would be 0.1 to 0.2' of arc, or 12-24
seconds.

Fred

On Dec 23, 2003, at 5:40 PM, Frank Reed wrote:

> Fred you wrote:
> "I believe that to _rate_ a chronometer one needs at least three
> lunars spread over at least three days. "
>
> Just one lunar will do. When you leave port, you know your
> chronometer's error (assuming it's a port with a well-established
> longitude). Let's suppose it's sixty seconds slow as you depart. After
> five months at sea, you get some measure of your longitude. This could
> be from speaking another ship, from visiting a port, OR from shooting
> a lunar. Suppose your chronometer now appears to be 4 minutes fast.
> That means it's gaining 1 minute per month. That's the rate.
>
> On the other hand, there is some benefit in doing lunars for a few
> days in a row. Primarily, this would assure you that you didn't get an
> accidental close match from your first trial. You could also get the
> same result by having several navigators shoot lunars each working up
> an independent longitude. This was certainly the case on at least some
> American ships at mid-century.
>
> Frank E. Reed
> [X] Mystic, Connecticut
> [ ] Chicago, Illinois

   

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