Chris,
Mechanical clocks that display local solar time have been built for over
a hundred fifty years. There are mechanical wristwatches that also do
the same thing and are currently available. They're extremely expensive
but are being constructed. The local solar time is usually presented on
the dial as a hand which displays the difference in time in minutes (+
or -) from that shown on the dial. It's a simple matter to subtract or
add the difference to the local time shown on the dial to get solar
time. I designed a clock to do this some 25 years ago and although a bit
painful to make, not really all that difficult. The clock that I
designed at the time used a differential to actually display the solar
time on the dial directly. The solar time is determined in a clock or
watch by means of a kidney shaped cam that is actually represents the
anelemma and a follower on the cam moves a hand showing the difference
in time from that shown on the local time dial. The difference in time
is known as the equation of time. One such modern watch showing the
equation of time can be seen here
<http://www.luxist.com/2010/03/09/girard-perregaux-1966-annual-calendar-and-equation-of-time-watch/>
My personal interest has been constructing clocks showing sidereal time
which is a bit complicated gearwise(if you want really good accuracy)n
and one of mine can be seen here <www.precisionclocks.com>. I do
remember seeing quite a few years ago an electric mains clock that had
on the dial display an equation of time hand showing the difference
between local time and solar time.
I guess the bottom line is that although not impossible, it is a bit
difficult.
Bill_S
On 1/19/2014 7:25 AM, Tom Van Baak wrote:
I wonder if you really need a special clock? Can't you adjust a normal
spring driven clock to run fast (or is it slow?) by about 1/3 of a percent
(one day per year)? This should be within the range of adjustment.
Chris,
When you mention 1/3 percent, you're thinking sidereal time, which is a
completely different concept, and much easier to implement than equation of
time. Sidereal time is simply a calendar-day independent, fixed (2730 ppm)
frequency offset. I already have PIC chips that do this; see PD28 under
www.leapsecond.com/pic/picdiv.htm or read the comments in the source code at:
http://www.leapsecond.com/pic/src/pd28.asm
Solar time, on the other hand, is continuously variable in rate (and phase)
throughout the whole year. A microprocessor implementation of solar time also
needs to know calendar date, time, and longitude. A 4800 baud GPS NMEA stream
input would be a convenient way to obtain this information. Without using
floating point or trig functions, a tiny PIC implementation would probably use
a 365 entry lookup table to adjust the output tick rate on a per-day basis. A
more capable Arduino or RPi might allow one to accurate calculate EOT directly
from planetary motion equations, avoiding hard-coded tables altogether.
/tvb
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