[time-nuts] Setting clocks 100 years ago

[Neville Michie]

The variation of solar time was obvious from the time from star  
observations.
Harrison used to line up a scratch on his window with a star  
disappearing behind a
distant roof top to get consistent time intervals when he was  
adjusting the
temperature compensation of his clocks and observing their rate.
Star time, known as sidereal time, is about 4 minutes a day different  
to solar time (365.25 / 366.25)
but is free from any perturbations. So it is relatively easy to  
observe the variations of solar
time by comparing solar noon with the time of transit of a star that  
night. It does not need a very good clock.
The difference is called the Equation Of Time.
cheers, Neville Michie

On 05/11/2010, at 9:17 PM, Hal Murray wrote:

>>> When did clocks get good enough to notice the analemma?
>
> lists at rtty.us said:
>> My guess would be the 1600's. They certainly had everything they  
>> would need
>> before 1620.
>
> What happened in 1620 to indicate that people could measure the  
> analemma?
>
>
> One way to "discover" the analemma is to have a good clock and take  
> a year
> long sequence of measurements of the location of the sun at noon.   
> So how
> accurate does that clock need to be?
>
> I found a web page that said:
>   For instance, on November 2nd the Sun is 16 minutes fast  
> according to
>   clock time and on February 11th it is 14 minutes slow. Mean solar  
> time
>   match on June 20th, April 14th, August 30th, and December 20th.
>
> If the clock was off by 1 second per day (number pulled out of the  
> air), that
> would be 5 minutes per year.  That wouldn't close the path cleanly,  
> but it
> would be good enough to show the idea.
>
> When did clocks become good to 1 second per day for a year?
>
> The wikipedia article says pendulum clocks were invented in 1656.   
> Harrison's
> H5 was good for 1/3 second per day in 1772.  That probably brackets  
> things.
>
>
> Another way would be to notice that the Earth's orbit wasn't a  
> circle and do
> the calculations to figure out the path of the analemma.  I'm not  
> sure when
> astronomers were good enough to do that.  It was probably a long  
> time ago.
> Some of those old-timers would be right at home with a time-nuts  
> discussion.
>  One of Harrison's competitors was using Jupiter's moons as a  
> clock, and then
> had worked out the speed of light correction for the Earth's orbit.
>
>
> Here is a recipe for constructing one:
>   http://www.analemma.org/constructanalemma.html
> I don't understand what's going on.
>
>
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ 
> time-nuts
> and follow the instructions there.