Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: Lunars: Jupiter's BIG.
    From: Fred Hebard
    Date: 2003 Dec 24, 11:51 -0500

    I also must thank George for his expansion of his point that lunars
    would be inadequate for rating chronometers.  I agree with his
    On Dec 24, 2003, at 11:14 AM, George Huxtable wrote:
    > Fred Hebarf wrote-
    >> 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.
    > That may be what Fred can reliably achieve on land, with a stable
    > platform
    > beneath his feet. It's rather better than could be expected at sea, in
    > the
    > size of vessel that was used in the heyday of lunars.
    > Replying to Fred's comment that-
    >>> "I believe that to _rate_ a chronometer one needs at least three
    >>> lunars spread over at least three days. "
    > Frank Reed replied-
    >>> 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.
    > Finding the current rate isn't quite as simple as that. Let's say that
    > at
    > departure the rate had been determined to be zero; neither gaining nor
    > losing, on the basis of, say, time-gun signals over a day or perhaps a
    > few
    > days. And the error in the chronometer time, at departure, happened to
    > be
    > 60 seconds behind Greenwich. And after five months, as Frank presumes,
    > the
    > time error is found to be 4 minutes ahead of Greenwich. We can agree,
    > then,
    > that the AVERAGE rate over that 5-month period has been 1 minute per
    > month,
    > gaining. But that's no more than history, water under the bridge. What
    > we
    > need to know is what is the rate NOW, in order to extrapolate
    > chronometer
    > error into the future.
    > If the rate had been changing smoothly and steadily, from zero at
    > departure, and the average rate over the period was 1 minute per month
    > (gaining), then its current rate at the end of the period would be 2
    > minutes per month (gaining). It's a big assumption, of course, that the
    > rate would change smoothly and steadily.
    > The point I am trying to make is that in order to RATE a chronometer,
    > rather than simply establish its error, one needs to find the rate AT
    > OR
    > NEAR THAT MOMENT, not what it has been averaging over a previous
    > passage.
    > For this to be done, one needs to be back in harbour, with a
    > time-source
    > that's precise to the second or better, and the longitude held
    > constant. It
    > doesn't even require a harbour with known longitude:just any old
    > anchorage
    > will do, using successive time-sights over a day or days, to determine
    > the
    > RATE, though to find the clock-error then requires a spot with known
    > longitude.
    > The trouble with using a lunar to determine rate is that because each
    > measurement is so inaccurate (to a minute or two of time) then any
    > determination of rate over a short interval is hopelessly imprecise.
    > George
    > ================================================================
    > contact George Huxtable by email at george---.u-net.com, by
    > phone at
    > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ================================================================

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site