Hello Ken,
The examples using an analemma shaped gnomon that project the shadow the
equatorial line do what you want. An improvement could be a wire stretched
along the centerline of the gnomon in Willy's Spanish example. This would
provide solar time. The two values of the EQT at that declination would be the
shadow line of the aperture. Thus the two possible values of EQT could be read
directly without knowing the the date.
Regards,
Roger Bailey
Walking Shadow designs.
From: Ken Baldwin
Sent: Sunday, February 03, 2013 2:19 PM
To: peter.ma...@adelaide.edu.au
Cc: sundial
Subject: Re: equation of time sundial
Hi guys,
Thanks for your responses.
I'm not trying to read the EOT-adjusted time from the device, I want to read
the actual EOT value itself for that date. For example, on Aug 9th, at any
daylight hour, I'd like to be able to read, say, -5.5 min. Maybe I'm
misunderstanding, but do I get that from these analemmic gnomons or hour lines?
I'm suggesting something like Peter's design, but with the hour lines removed,
and the date lines labelled with the EOT value (-15 min -> 15 min).
I understand Simon's point that the altitude of the sun is ambiguous between
two dates, so perhaps it would have to be split into two plates (like
half-analemmas). Of course, this requires the user to know which 6 month period
of the year they are in, which partially defeats the purpose of not needing to
know the date :-)
Ken
On Sun, Feb 3, 2013 at 2:14 PM, Peter Mayer <peter.ma...@adelaide.edu.au> wrote:
Hi Ken,
Here's another example of a dial which may interest you. This is my
adaptation of a brilliant dial coded by Steve Lelievre. To fit the size limit
on this list, I've had to squeeze the image greatly, but you should be able to
grasp the principle of the dial: each month is a separate concentric circle.
Between month lines one must interpolate the date to read the civil time.
best wishes,
Peter
On 4/02/2013 6:32 AM, Ken Baldwin wrote:
Hello,
I'm a new list member, and have a beginner question:
Are there examples of sundials whose sole (or primary) purpose is to
compute the Equation of Time for the current date?
- I know that this information is often provided as a graph in the
furniture, but why should I have to know the date and perform the
look-up manually? Can't I use the position of the sun to do the
computation for me?
- I know that the EOT correction can be incorporated into the layout of
(some) hour lines, but I'm more interested in having dials which show
true solar time. I'd like a separate device dedicated to computing the EOT.
- I know that I can construct an analemmic noon mark to show the EOT for
that day, since it's simply the east-west component of the analemma, but
I'd like a design that can be read at any daylight hour.
It seems to me that it should be possible to build such a dial, since
the EOT is a function of date, and date lines can be read from many
sundials. In principle, I can just re-label the date lines with
corresponding EOT values and interpolate.
I hope that makes sense. But since I haven't seen anything like that in
introductory sundial books, I must be missing something... Is it that
the shadow length can't be read accurately enough to get a reasonably
precise EOT estimate? Or is it just too hard to make a readable layout,
given that solar altitude is ambiguous between two dates, and that the
component of the EOT due to the eccentricity of the earth's orbit is out
of phase with the equinoxes and solstices?
Thanks in advance,
Ken Baldwin
Corvallis, OR USA
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