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?
[email protected] 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.
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