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    Re: Sumner's Line (Navigation question)
    From: Ken Muldrew
    Date: 2006 Feb 7, 09:40 -0700

    On 7 Feb 2006 at 1:45, Frank Reed wrote:
    > After we've done our time sight with our  sextant (or read our sundial),
    > and we have learned that the local apparent time  is, say, 3pm exactly,
    > what next? As we know, the difference between local time  and absolute
    > time, typically GMT, is exactly the same thing as the difference in
    > longitude between the observer's location and the absolute time location,
    > typically Greenwich. But there's a catch. Local apparent time runs a little
    > fast  or slow during the year compared with accurate clocks. That
    > difference is the  so-called "equation of time". So if you use a sundial
    > and you want to know if  it's accurate, you need to have a table of the
    > equation of time handy. Many  sundials in public settings have tables or
    > graphs affixed to them or posted  nearby (unfortunately, for most people,
    > these tables usually create the  impression that the sundial is "broken").
    Here's a picture of a sundial that I made with the equation of time "built-
    in" by rotating the plate so that the date matches against a sliding scale
    along the top. As long as you know the date to the nearest week (or so),
    the sundial will read clock time. People still think there's something
    wrong with the sundial.  ;-)
    > Time, for better comparison with chronometers.  There were also "novelty"
    > chronometers designed to read apparent time instead of  mean time. This was
    > really pointless, so they didn't last.
    Something to keep in mind when reading accounts of 18th century navigation
    is that the error in their watches (which often appears considerable when
    they do a time sight) was not only mechanical, but also due to the effects
    of the change in equation of time since their last time sight and
    (obviously) any East/West movement.
    Ken Muldrew.

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