[time-nuts] Re Danjon Astrolabe

Bill Hawkins bill at iaxs.net
Thu Sep 28 16:00:15 EDT 2006

Tom Van Baak wrote,

"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
Again, collecting days or weeks of data gives you even greater

Now there's a fine idea, with some good mechanical challenges.
I may give up on the Shortt pendulum project (we live near a
railroad track with slow freight trains).

My programming skills have degraded since all of what I create
has been done with MS Word and Outlook. Can anyone recommend a
pretty good starter kit for this kind of work? I used Unix and
C twenty years ago. I do have precision frequency standards.

Bill Hawkins

The scheme probably needs three photocells to be sure that the one
in the middle is darker than the others. Might be able to mask it
with a slit and use a fine wire gnomon, in a coarse/fine servo.
Could use a variable frequency motor and precision reduction, like
a phonograph turntable only much slower.

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