[time-nuts] Re Danjon Astrolabe

Tom Van Baak tvb at leapsecond.com
Thu Sep 28 13:58:45 EDT 2006

> 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


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