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From: Ken Muldrew (no email)
Date: Tue Feb 07 2006 - 11:40:58 EST
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. ;-)
http://www.ucalgary.ca/~kmuldrew/sundial.jpg
> 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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