Dear Donald,
You wrote...
> Brilliant idea Roderick!
>
> I put both animations on my website.
> Each one has a label under it stating
> which hemisphere it's for...
Whoa! Hold on a moment!
The fun has only just begun...
Have you thought what happens in the
tropics?
Someone living at 20 degrees north (well
into the northern hemisphere) will not
be impressed by your northern hemisphere
animation around the summer solstice.
This is what happens:
1. The sun rises somewhat to the
north of due east (no surprise
so far).
2. It heads south for a while and
therefore goes round clockwise
(still no surprise). Then...
3. Suddenly it reverses direction
and goes ANTI-clockwise, and it
stays running that way...
4. ...through noon and...
5. well into the afternoon. Then...
6. Suddenly it reverses direction
again and goes CLOCKwise until...
7. Sunset.
Phew! Quite a day, eh?
You get a hint of what's going on once
you draw out an analemmatic sundial for
20 deg. north. You will see that the
date line is LONGER than the minor axis
of the hour-point ellipse.
There are two times of day when the line
from the summer solstice point (say) makes
a tangent to the ellipse. These are the
times when the direction reverses.
I wonder how many readers think that
I am kidding :-))
Life can get tough when you start thinking
about special cases!
Frank King
Cambridge, U.K.
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