Dave, Edley, and others,
Thanks for the excellent suggestions!
Edley's idea is simple and elegant. The only slight drawback I see is that the
base of the sundial has to be made thick enough to accommodate the hinge.
Dave's idea is also nice, and it avoids the need for a thick base. Adrien
Poncet liked to refer to his invention as a "no axis" telescope mount (meaning
that the polar axis was not a physical thing but was implied by the 3-D
geometry and degrees of freedom of the parts). That is helpful in a
telescope/camera mount because it means the equatorial table can have a very
low profile. But a sundial of this type has a solid gnomon anyway, so why not
make use of it, as Dave's idea does!
In a separate e-mail, Patrick Powers commented that the usual garden sundial
has a thick gnomon (in which you read the shadow from one edge of the gnomon in
the morning and the other edge in the afternoon). So the gnomon theoretically
needs to tilt *with* the base, rather than being independently fixed while the
base tilts under it.
I'm trying to come up with the mathematical shape of a suitable cam for
controlling the base tilt in a sundial of this general type, during the course
of a year. So far, no luck. It may be easier to go ahead and make one
empirically.
-- Roger
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>From Edley McKnight:
A true equatorial mount is truly fine if you have one, A simple hinge mounted
in parallel with the earth's axis at some point below the horizontal surface,
allowing the dial to tip a little one way or the other will allow adjusting for
both Longitude and the Equation of Time. One could consider the sundial to be
a slightly slow or fast clock and set it daily for the correct time, like any
other clock. (even daylight saving's time if the tilt is large enough) Mounting
an east west hinge below that would allow adjusting for errors in Latitude as
well. Setting up such a table is a fun thing to test various designs or
sundials found. I call it an "Latitude/Longitude table".
BTW, Even Pillar or Shepherd's dials made for other places seem to work just
fine locally when rotated on such a table, set for the correct Lat/Long rather
than dangled by their cords. Has anyone else observed this?
I still like Hendrik Hollander's cone gnomon design the best currently.
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>From Dave Bell:
A variant on that would be to use a long, straight shaft for the polar
axis and gnomon, letting the horizontal dial hang from the axis.
The adjustment would be very slight, barely noticeable as off-level...
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