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Old December 12th 17, 11:26 AM posted to sci.astro.amateur
Mikko
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Posts: 15
Default The Equation of Time(keeping)

In article ,
Martin Brown wrote:

On 09/12/2017 14:53, Mikko wrote:
In article ,
"Chris.B" wrote:

Accurate clocks have been running to sidereal time for far longer than you
seem to realise.


Have you got any evidence?


Ptolemy's Almagest Volume III shows conclusively that the variable speed
of the sun across the heavens was well known to the educated ancients.


It was well known that the number from equinox to solstice or solstice
to equinox was not the same for all seasons. No clocks were needed to
observe that. Whether lengths of days or lengths of seasons were
unequal could not be directly determined.

Although he explained it with circular epicycles rather than an
elliptical orbit he had computed pretty good tables for correcting solar
time. Epicycles were a crude predecessor of Fourier analysis.


Ptolemy did not use epicycles to explain the motion of Sun although he
did use them for planets. He explained the motion of Sun as uneven
motion along an eccentric circle.

https://en.wikipedia.org/wiki/Equati..._Babylon_and_E
gypt

I can't find anywhere online with free access though. Closest is:

https://www.sciencedirect.com/scienc...15086013000530

Full article is behind a paywall.

The accuracy of the best clocks before pendulum clocks was abut 15
minutes per day. Before about 1650 the best pendulum clocks'
accuracy was about 10 seconds per day. In order to determine the
equation of time with a clock even one second per day, achieved by
Harrison in about 1760, is very little. Only in 1870 was accuracy of
0.1 seconds per day achieved, and equation of time determined with
a clock.


To measure the day to day variation directly you need this sort of
precision and a transit instrument to go with it. But to notice the
variation of the suns motion against the constellations at different
times of the year he could watch the movement of the sun against the
fixed stars.


Comparison of motions of fixed stars and Sun shows that at least one of
them is not uniform.

At the time and until Huygen's pendulum clock the most accurate clocks
were water clocks. They were not sufficiently accurate to determine
whether Sun or fixed stars or both moved unevenly.

The suns motion was known to be uneven from the first
development of water clocks and calendars by the Babylonians.


Water clocks were not useful for the purpose but a calendar was.

The Chinese even had a mercury based liquid clock that was not subject
to the same viscosity problems as water. It kept better time year round.


But was it ever used to check uniformity of any celestial motion?

Mikko