[time-nuts] Local Solar Time Clock

[Bill S]

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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