[time-nuts] Re Danjon Astrolabe

Bill Hawkins bill at iaxs.net
Thu Sep 28 17:14:12 EDT 2006

I wrote,
"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."

We are talking about a clock, here. Build a precision clock
movement run from a precision frequency, with no regard for
leap seconds. Insert differential gearing between a 24 hour
period of rotation and the turntable carrying the sun sensor.
Use a stepper and position sensor to operate the differential
to trim the turntable to the equation of time for your GPS
latitude. Now we have a turntable that should follow the sun,
whether you can see it or not.

Insert another differential, or use other means, to center
the shadow of a fine wire on a slit with a photocell behind
it. This gives the difference between actual and predicted
sun position. Use another photocell to see if the sun is
visible. When it is, record the actual difference.

Oh, and you'll need a servo to drive a mirror to correct for
solar elevation, to keep a bright image on the slit. The
longer the path from the gnomon to the slit, the better.

Come back in a year and see what happened. Good science takes
time and lots of consistent readings from the apparatus.

Wish I hadn't sold my HP-113BR clocks. They had the differential
gearing to set seconds and time of day.

Bill Hawkins

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