Willy, thanks.
I'm travelling away from home now, so can't respond in full - but your
chart helps me and gives me an idea that I will combine with Gian's rule
of thumb and post on the list later.
Steve
On 2017-11-08 10:10 AM, Willy Leenders wrote:
Hello Steve,
Based on your discovery and to prove that from the vernal equinox to
the autumnal equinox for a certain time to sunset the sun altitude is
*/approximately/* equal, I made calculations and a graph showing that
this is the case for 1, 2, 3 and 4 hours to sunset.
Willy Leenders
Hasselt in Flanders (Belgium)
Visit my website about the sundials in the province of Limburg
(Flanders) with a section 'worth knowing about sundials' (mostly in
Dutch): http://www.wijzerweb.be
Op 6-nov-2017, om 02:15 heeft Steve Lelievre het volgende geschreven:
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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