I've often wondered if there is a best time of day to set a sundial using
the "Time method" (as opposed to the compass, the Polaris, the GPS and
plumbob shadow methods).
I'm thinking that the best time of day to precisely set and/or read a
horizontal sundial would be at mid-morning and mid-afternoon (those times
that are halfway between sunrise and apparent noon and apparent noon and
sunset).
These setting & reading times would avoid the early morning and late
afternoon affects of maximum atmospheric refraction and would also avoid the
compressed hourline markings that are close together at noon which make time
estimation more difficult.
Does anybody agree with my theory? Is there a best time for setting
sundials?
You are right. Making a bit of mathematics it can be shown that the
highest insensibility to errors is
reached when the Sun crosses the prime vertical (ie, the one that
contains the East and West points).
You can ask a topographer or consult a good astronomy book for more
information about this, but
your reasoning is much crisper and essentially correct.
Best regards,
Anselmo Perez Serrada
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