I have been doing some calculations for an Hours To Sunset dial (that
is, an Italian Hours dial with the numbering running backwards). I
discovered that the maximum altitude for a given hour does not occur at
the summer solstice. I was a little surprised to discover this - not
amazed, but surprised enough to make me wonder if I have done my
calculations wrong.
The attached diagram is for the example case of 4 hours before sunset.
I'm getting a double maximum occurring a little after the vernal equinox
and a little before before the autumnal equinox. I get similarly shaped
curves for others hours, with less separation between the peaks when I
use higher (italian) hour numbers.
Assuming that I have in fact graphed the altitude correctly, it means is
that there is a period over the summer months when the altitude for any
given hour to sunset stays /approximately/ the same. In my case, at 49N,
it seems that over the summer months, the solar altitude for 1 hour to
sunset is approximately a little under 10 degrees, 2 hours to sunset is
approximately a little under 20 degrees, and so on at a little under 10
degrees per hour, at least for the last 5 to 6 hours of the day.
In this day and age, I think we would demand greater accuracy, but have
there ever been sundials or other devices that exploited this approximation?
Steve
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