NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: one second of time
From: J Cora
Date: 2008 May 18, 08:50 -0700
From: J Cora
Date: 2008 May 18, 08:50 -0700
Using sidereal time, meridian transit of a star.
I am going to investigate this method as it intrigues
me as a solution for a totally non electric culture,
Although they would have to be able to make
mechanical clocks.
With a telescope having crosshairs in the eyepiece, to
what accuracy can the meridian transit timing be made?
Assuming accuracy of a few seconds, then I would
guess that next requirement would be tables converting
sidereal time to mean solar time for each day of the year?
After which the primary clock could be adjusted based
on a timer which started running at the transit.
If I am missing something here I would appreciate
clarification.
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I am going to investigate this method as it intrigues
me as a solution for a totally non electric culture,
Although they would have to be able to make
mechanical clocks.
With a telescope having crosshairs in the eyepiece, to
what accuracy can the meridian transit timing be made?
Assuming accuracy of a few seconds, then I would
guess that next requirement would be tables converting
sidereal time to mean solar time for each day of the year?
After which the primary clock could be adjusted based
on a timer which started running at the transit.
If I am missing something here I would appreciate
clarification.
On Sun, May 18, 2008 at 1:34 AM, George Huxtable <george@huxtable.u-net.com> wrote:
Commenting on Coralline Algae's statement-
Bill Wells wrote-
|" In thinking about Galileo's experiments with pendulums and the
length of the| string, he must have had some standard to decide what
the length of the| string ought to be for accuracy to a second. Going
to look into this| further as this seems to have set a new standard
for clockmaking in general."
================
"I found it interesting that the period of a pendulum of length one
meter is almost exactly two seconds."
Yes, very nearly, it is. But accidentally so. It nearly wasn't accidental,
however...
When the revolutionary French were casting about for a new, logical, unit of
length, which would be applicable worldwide, a favourite proposal was to
base it on the length of a one-second pendulum. However, an expedition had
established what had been suspected, that the length of a one-second
pendulum varied somewhat over the Earth's surface, because of The Earth's
ellipsoidal shape and its rotation (to name but a few factors) altered the
effective strength of gravity. So that idea was dropped, and instead the
metre was chosen, to be one ten-millionth of the distance around the Earth's
surface, between the Equator and the poles. This was based on a careful
survey, right across France, North to South. You can read about the problems
involved in making that survey, in revolutionary turmoil, in "The measure of
all things", by Ken Alder.
Then it was discovered that such a measurement, confined to France, was not
truly representative of an average value for the whole Earth, but by then it
was too late. The metre had been adopted, based upon it. They did their
best.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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