# NavList:

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

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Re: time divisions
From: J Cora
Date: 2006 May 11, 21:34 -0500

I remember reading that Harrison used a star and a vertical edge to measure a sideral day.  This should allow for a somewhat precise measure of time and the accuracy of ones timing device, probably good to a few seconds of accuracy.

Again referencing the book Newton's Clock, the author states that  Tycho Brahe's observations had errors of less than 2 or 3 minutes of arc.  He repeated his observations regulary, for instance the mars data covered a span of 30 years.
From Tycho's data , Kepler produced the Rudolphine tables, an ephemeris.   Although I havent looked at the historical document, I believe that the data measures the positions of the planets, etc with repect to the postions of the stars, rather than a time standard like UCT.

On 5/10/06, FrankReedCT@aol.com <FrankReedCT@aol.com> wrote:

Corallina, you wrote:
"By using such an instrument,  endevouring  to measure the time  interval of
24 hours or 1 day;  If we use the suns meridian passage, it is  discovered
that the time interval varies with the seasons, ie. the equation of  time. "

I think at the heart of the matter, you're asking when this  happened. That
is, when did people realize that the Sun's meridian passage is  not a perfect
clock, and thus when did they realize that "minutes" defined by  the Sun's
position in the sky are not quite the same length in different seasons  of the
year? The equation of time was known 2000 years ago. The equation of time  back
then could be understood then as the difference between local sidereal  time+4
minutes a day, and local apparent time or "sundial time", but which one  was
variable? There was no "scientific" answer to this question. Because the
celestial sphere (carrying the stars themselves) was assumed to rotate with
absolute constancy on aesthetic grounds, it was generally just assumed that the
variability was in the Sun's motion though only experts in astronomy would have
known about any of this. This inconstancy of the Sun was not "proved" until it
was shown that mechanical clocks can keep excellent sidereal time and mean
solar  time, which is proportional to sidereal time, but they can keep local
apparent  time only with complicated, variable rate designs known as "equation
clocks". It  is my understanding (someone correct me if I'm wrong!) that this
happened almost  immediately after the first good pendulum clocks were built by
Huygens in the  1650s. By the time John Harrison was building his first
clocks three-quarters of  a century later, a table for reading off the equation of
time was a commonplace  in good pendulum clocks, usually pasted somewhere
inside the case (there are  photos of a couple of these tables written in
Harrison's own hand in "The  Illustrated Longitude" by Sobel and Andrewes). By 1750,
it was  relatively "common knowledge" that minutes defined by the Sun were
inconstant while those defined by simple mechanical motions were in some sense
"true time".

-FER
42.0N 87.7W , or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

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