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Hi Richard:

I think you can get 15 seconds accuracy at any time the Sun is shining.  There are a number of heliochronometers that do it.  But it's not a static sundial, that's to say you need to turn dials, etc.  In the extreame case you could use a surveyous transit/theodolite to shoot the az and el of the Sun and using data for the current year know exactly the time to about a second.  See:
http://www.cadastral.com/

I did not file down the tacks.  The heads are a little less than 1/8" diameter and were seated so only a tiny fraction of an inch was sticking up.  You can feel the head with you fingers.  No problem to walk on them bare foot.

Have Fun,

Brooke Clarke, N6GCE

Richard M. Pisko wrote:

On Sat, 9 Aug 2003 13:37:34 -0700, Brooke Clarke wrote:

  

I made one of these by placing a metal plate at the lower end of a South
facing skylight and using a 3x5 card with a number of concentric
ellipses and a small hole at their center.  I just tracked the Sun's
image with the card and at exactly noon stopped and drive a small brad
nail into the hardwood floor.  After many years there would be more than
one nail for the same date because as we go through the 4 years of the
leap year cycle the timing is a little different.

    

Thank you.  Got it now ... not all the details, but the general idea
... and will visit your referenced sites.  I read too much into your
comment, and was thinking of 15 second accuracy at any daylight hour.
A bit difficult, I thought.   :-)

Did you sand the (copper or steel?) brads flush?


--
Richard ...