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 15, 19:04 -0700
I forget the name of the book but it covers the history
of sun clocks that were built in european churches
using a slit to pass a beam of light and marks upon
the floor or a wall to measure the passage of time.
I suppose it was possible over a long period of time
to determine the average length of a solar day.
I read further online about Galileo and apparently he used
water clocks to time some of his experiments as
they were more accurate than using a pendulum
as he had not worked out all the details.
I agree with using sidereal time as the standard for
clockmakers as they had to have some accurate
standard which was easily accessible to base their
clocks upon and to adjust the rate. I have no clue as
to how they knew by what amount to adjust the rate so
that the clocks would run in time with a solar day,
that being 24 x 60 x 60 seconds.
Whether you can accurately time a sidereal day by
carefully measuring the transit of a star, doesn't seem to
imply that you know that relative to a solar day it is
about 4 minutes less in time.
It seems that for some time mechanical clocks were adjusted relatively often to match the varying solar day.
But at some point the 86400 second day became the standard for the clockworks. Perhaps the long history of measuring the length of the solar year provided the time
difference relative to sidereal time.
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From: J Cora
Date: 2008 May 15, 19:04 -0700
I forget the name of the book but it covers the history
of sun clocks that were built in european churches
using a slit to pass a beam of light and marks upon
the floor or a wall to measure the passage of time.
I suppose it was possible over a long period of time
to determine the average length of a solar day.
I read further online about Galileo and apparently he used
water clocks to time some of his experiments as
they were more accurate than using a pendulum
as he had not worked out all the details.
I agree with using sidereal time as the standard for
clockmakers as they had to have some accurate
standard which was easily accessible to base their
clocks upon and to adjust the rate. I have no clue as
to how they knew by what amount to adjust the rate so
that the clocks would run in time with a solar day,
that being 24 x 60 x 60 seconds.
Whether you can accurately time a sidereal day by
carefully measuring the transit of a star, doesn't seem to
imply that you know that relative to a solar day it is
about 4 minutes less in time.
It seems that for some time mechanical clocks were adjusted relatively often to match the varying solar day.
But at some point the 86400 second day became the standard for the clockworks. Perhaps the long history of measuring the length of the solar year provided the time
difference relative to sidereal time.
On Thu, May 15, 2008 at 10:28 AM, Herbert Prinz <ml-2@hprinz.us> wrote:
In fact, measuring the sidereal day is the *only* option. The mean solar
coralline algae wrote:
> The thing I can't quite understand though, is what standard the
> clockmakers used to decide on the length of a second, in building
> their timepieces.
> Here is my first guess, the solar day length on the equinoxes along
> with the suns meridian transit set the day length standard.
> Then build a gear train dividing the day into 24 by 60 by 60 parts.
> Recalling that Harrison used sidereal time to calibrate his clocks, it
> must have been well known by his time ( or much earlier ?) The "exact"
> difference in time
> between the solar day and the sidereal day so the clock needed to run
> slower by about 4 minutes. Wikipedia says 86164.1 seconds. If the
> exact difference in time was well known and transit times could be
> measured to tight accuracy this would seem to be the best option.
day is a computational fiction that is not directly measurable. It
results from the combination of earth rotation and annual solar motion,
the latter being non-uniform. The true solar day varies throughout the
year by a few seconds. At the equinoxes, the difference between true and
mean solar day is particularly large because the effect of the
inclination of the ecliptic is at its maximum.
Herbert Prinz
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