Dear Isabella,
As I expected, there are plenty of list members happy to offer advice. You
have to be a bit circumspect when reading some of it!
Willy Leenders is right to comment unfavourable on the drawing you find at:
http://www.sunclocks.com
The drawing and the wording round it are very misleading both as to how the
sundial is to be used and how it is set out. Importantly, the slabs on the
date scale are badly represented. I cannot think why Douglas Hunt hasn't
changed this page.
The good news is that the two rings of hour numbers on properly set out
Douglas Hunt Sundials correctly indicate local sun times on the relevant
reference meridians. This is usually within 15 minutes of clock time and
within 5 minutes during the summer. They do not indicate clock time
itself. The outer ring is for winter time and the inner ring is for summer
time.
As I mentioned, there are really two separate but superimposed analemmatic
sundials. Imagine the following steps:
1. Lay out a conventional analemmatic sundial where the single ring of
hour numbers is set out on an ellipse whose major axis is 5m across.
2. Now overlay this with an identical sundial scaled down 20% so the
major axis is only 4m across. Arrange for the mid-points of the two date
scales to be coincident. These are the two equinoctial points.
3. The two rings of hour numbers will be separate but there will be a
horrible jumble along the date line.
4. Here is the cunning bit: you discard the *summer half* (equinox to the
summer solstice) of the longer date line and you discard the *winter half*
of the shorter date line. *The equinoctial point is no longer in the
middle of the date line.* The summer half is short and uses the inner ring
(this is when shadows are short because the sun is high) and the winter
half is long and uses the outer ring.
With a single exception, everything is gnomonically sound. The one
slightly doubtful feature is that the change-overs from winter time to
summer time and vice versa are deemed to be at the equinoxes when, in
reality, the change-over dates are dictated by legislation rather than by
mathematics. I don't regard this as a serious problem. Blame legislators!!
The important thing for your daughter to note is that designing and
constructing this sundial is exactly equivalent to designing a conventional
single-ring analemmatic sundial, scaling it, putting both dials together
and then selectively throwing away the bits you don't need.
Since you live in North America, I would use Roger Bailey as your principal
source of advice!! He is very good at practicalities.
A Happy New Year
Frank
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