Thanks, Gian, for validating my calculations, and for that story. It
must be based on the rule of thumb that the thickness of a finger (or
maybe a thumb?) is 2 degrees. For my latitude, that would mean that each
fist is about one hour to sunset. I must remember to try it next summer.
Cheers,
Steve
On 2017-11-06 12:28 AM, Gian Casalegno wrote:
A similar result can be obtained by means of Orologi Solari.
The attached pdf shows a dial at 49.2 N where hours to sunset lines
and sun height lines are drawn.
The asymptotes of the sun height lines are parallel to the hour lines
in the afternoon i.e. sun height is nearly constant for a fixed time
to sunset value.
The attached image shows a shepherd dial with the last seven time to
sunset lines drawn.
Hour lines 1 to 5 are nearly horizontal in the right side of the graph
(summer).
The result is so again confirmed.
Therefore time to sunset can be deduced in summer from the height of
the sun on the horizon.
This fact reminds me about a trick that someone told me a long time ago.
It was said that sailors used to estimate the time by measuring with
their fingers the height of the sun above the line of the horizon.
I always thought it was a fake but now I see it can work, at least in
the evening in summer time.
Of course the relation between height and time depends on the latitude.
Ciao.
Gian
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2017-11-06 2:15 GMT+01:00 Steve Lelievre
<steve.lelievre.can...@gmail.com
<mailto:steve.lelievre.can...@gmail.com>>:
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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