[time-nuts] Re Danjon Astrolabe

Brooke Clarke brooke at pacific.net
Thu Sep 28 15:21:45 EDT 2006

Hi Tom:

With an atomic quality clock it should be possible to measure the period 
of the earth every day using the sun and/or stars and see how far from 
86400 seconds it is.

I too have been thinking of how to use the sun in a precision way.  
Since UTC is close (UTC1 is better) to the sun's movement it should be 
possible to make a clock that's within 0.1 seconds based on the sun's 

If using the sun then just using 4 (East, South, West & Up) small solar 
panels mounted to the face of a cube might work.  It's easier if the 
cube is really square and sitting on a level surface and rotated so that 
one face is aligned with true North.  Although you could measure the 
current from each panel and calculate the sun's position, it might be 
enough to pre calculate ratios and note the time when those ratios 
occur.  But as  you know the Equation of Time complicates the relation 
between the sun's position and UTC.  For sundials the EOT is taken as 
fixed by day of year, but for precision work you need to compute it for 
the actual day in question, although taking into account where in the 
leap year cycle you are may be enough for 0.1 seconds.

Maybe it's about the same as calculating UTC based on sidereal time from 
a star sight?

Have Fun,

Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

Tom Van Baak wrote:

>>I'm interested in automatically measuring the earth's period by looking 
>>close to straight up with a fixed telescope.
>Here's a related idea for you; a modern digital sundial.
>Two different ways to implement it:
>1) Aim a webcam on a standard sundial and write some
>image processing software that at the pixel level monitors
>the amount of shadow in real-time compared to PC time
>(UTC). By the end of a day, you'd have enough samples
>to have nailed down solar time quite accurately. Collecting
>data over weeks or months would be even more revealing.
>Fun math and programming problem.
>2) Instead of a fixed base, gnomon, and slowly moving
>shadow like almost all sundials, you put a stepper or
>servo motor/encoder on the base. Then place matched
>photodiodes on either side of the gnomon and steer the
>whole sundial for constant *minimum* shadow. In real-time,
>a PC or microcontroller monitors the photodiodes, keeps
>the sundial in position, and logs continuous position data
>(as a function of UTC). At the end of the day your precise
>measure of solar time drops out of this data. At night it
>extrapolates where it should aim the sundial for sunrise.
>Again, collecting days or weeks of data gives you even
>greater precision.
>2b) Do the same using a PC-controlled telescope, where
>software constantly adjusts Az-El to maximize the
>measured brightness of the filtered sun through the
>time-nuts mailing list
>time-nuts at febo.com

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