Hi All again,
Just to clarify John and Hendrik's suggestions:
- with a non-south-facing vertical mirror you can turn the axis of the date
scale and the dial ellipse to align with the path, so 12 noon will be due south
of the centre of the dial (requrement 1). But note that the mirror itself would
not be aligned with the path. The mirror can be placed just north of the date
scale which, by the way would have June/July at its southern end - the reverse
of normal. Or, the mirror could be moveable, sliding along the date scale. This
would allow a smaller mirror. Otherwise, it would have to be very big indeed.
- with a south-facing non-vertical mirror you can make the scale circular, not
elliptical, but 12 noon will not be due south of the centre of the dial
(requirement 2). The date scale is not needed - you always stand in the same
spot. The mirror can be smaller and non-moveable.
What you cannot do is both requirements together with a single mirror. You
could perhaps, in theory, do it with two vast mirrors, but the dial would be
hidden in between them, so the feature visitors would see first would be the
mirrors, not the dial, and the benefit of having the dial aligned with the path
would be lost.
You, or Mr Phillips, would have to decide whether you want the dial to be
circular or to be aligned with the path.
With both types, there's a small problem that should be mentioned - that there
will be two shadows! Let's say you stand with your back to a mirror and that
the sun is in front and to the right of you. The direct sunlight falling on
your right cheek will not reach the mirror, so will cause a shadow to appear on
the ground in front of you. Also, the sunlight that reflects off the mirror
onto the back of your head will cause a shadow. So, you will see two shadows.
Which is telling the time?
With a vertical mirror the shadows are parallel. One appears to touch your feet
directly - not via the mirror. That's the one to use.
With a reclining mirror, I think it should be placed right next to where you
stand. One of the shadows will similarly appear to touch your own feet. It's
also longer than the other. This is the one to use. But I have to confess that
having two shadows could be very confusing. By the way, rather than reading the
time on the physical dial, you could read it in the mirror. The hour marks
would have to be mirror images of normal numbers, of course. Even in the mirror
you can still see two shadows, unless you're a vampire.
To make the shadows easier to see I would suggest somehow shielding the hour
marks from direct sunlight. Put something just south of them.
My preference, for what it's worth, would be to use John Lynes' inclined
mirror. I think this would create a very striking semicircular feature. Indeed,
it may well be unique in the world, unless John has used it elsewhere. One
further feature it might be possible to incorporate would be to make the hour
marks moveable so that they could be turned 15 degrees in summer to show BST.
That's not possible with most sundials as the hour line angles aren't all 15
degrees, but with this one they are.
By the way, with an analemmatic sundial that uses a human shadow, one decision
is how big to make it. An noon in midsummer the sun's altitude is 62 degrees
and one's shadow is less than three feet. It's a mistake to make the dial too
big. But the reclining mirror effectively reduces the sun's altitude to just 23
degrees so the shadow is greatly elongated and the dial can be made much bigger.
Hope this helps
Chris Lusby Taylor
51.4N 1.3W
----- Original Message -----
From: Analemma Zonnewijzers
To: Chris Lusby Taylor ; John Carmichael ; [EMAIL PROTECTED] ; [EMAIL
PROTECTED] ; sundial@uni-koeln.de
Sent: Tuesday, June 10, 2008 8:27 AM
Subject: Re: Advice wanted, on 'Analemmatic' sundial orientation
Hi All,
very interesting issue, so after all:
with a verticle mirror with the right azimut you could turn the famous
analemmatic sundial so the the line with the dates is on a path which is not
north-south! and the ellips will be alligned with this path
looks to me that the issue can be solved after all.
One has to investigate if the dimensions of the mirror are realistic and you
will need 2 mirrors (or 2 sundials) to cover the whole 24 hours .
If the azimut of the path is alpha, the mirror should be at alpha/2,
kind regards,
Hendrik
---------------------------------
Analemma Sundials
H J Hollander
Fixed +31 20 6374383
Cell +31 616 462 879
[EMAIL PROTECTED]
www.analemma.biz
lat 52 23' long 4 57'
----- Original Message -----
From: Chris Lusby Taylor
To: John Carmichael ; [EMAIL PROTECTED] ; [EMAIL PROTECTED] ;
sundial@uni-koeln.de
Sent: Monday, June 09, 2008 7:38 PM
Subject: [SPAM]Re: Advice wanted, on 'Analemmatic' sundial orientation
Hi John et al,
I thought at first that John Lynes was making a joke, but you could
actually do this. If you have a large, fixed, plane mirror you can use the
sunlight reflected off it instead of the actual sun. By suitably angling the
mirror you can make the sun appear to rotate about any desired axis, not just
the earth's axis. So, you can make it appear to rotate about the local
vertical, just as it does at the Poles. So, just as at the Poles, an
analemmatic sundial using the reflected light will be circular and the
gnomon/person will not need to be moved for different seasons. But the numbers
will go round anticlockwise, which might cause further objections!
The actual angle you'd need is this, I think: take a vertical mirror,
facing south, then lean it towards the north at (90+latitude)/2 to the
horizontal. For 40 degrees North this makes 65 degrees. So, it's facing the
southern sky, as it were. If it were a dialface we'd say it was a direct south
reclining dial. Fix it there. Now, put a vertical gnomon close to the bottom
centre of the mirror and mark out a perfectly semicircular, equiangular dial
around it. It will work from 6am to 6pm throughout the summer. Or, if you want
the user to be the gnomon, just mark the spot where they must stand.
In the winter months, as John said, the sun's too low in the sky for the
reflection to shine on the ground.
A mirror of, say, four feet high and eight feet wide would work well. It
doesn't have to be 100% perfect optical quality. Users should back up to it to
avoid being dazzled, I suggest. I think this is an excellent solution to what
at first appeared an impossible brief.
Regards
Chris
----- Original Message -----
From: John Carmichael
To: [EMAIL PROTECTED] ; [EMAIL PROTECTED] ; sundial@uni-koeln.de
Sent: Monday, June 09, 2008 3:42 PM
Subject: RE: Advice wanted, on 'Analemmatic' sundial orientation
Hello John L.
I’ve never heard of such a sundial and I can’t imagine how it would
function. I’m not sure I understand the setup and positioning of the mirror.
Is the mirror permanently fastened to the ground or is the mirror moveable?
Does the mirror reflect sunlight onto the sundial face, or do you read the
sundial by looking at the face’s reflection in the mirror? I just don’t get
it. It sure would be helpful to see a drawing if possible.
Puzzled in Tucson
John C.
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL
PROTECTED]
Sent: Monday, June 09, 2008 4:08 AM
To: [EMAIL PROTECTED]; sundial@uni-koeln.de
Subject: RE: Advice wanted, on 'Analemmatic' sundial orientation
Dear Alison
One expedient, at least in theory, would be to fix a plane mirror,
suitably angled in plan to the main driveway and tilted to convert the
analemmatic sundial ellipse into a circle. The position of the human gnomon
would not vary with the season. The sundial would receive reflected sunlight
only on six months of the year, during the summer months, but this is when most
visitors would be around.
Unfortunately you would need either an enormous mirror or a very
modest analemmatic sundial.
John Lynes
-----Original Message-----From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Behalf Of Alison ShieldsSent: 03 June 2008 18:57To: [EMAIL PROTECTED]: Advice
wanted, on 'Analemmatic' sundial orientation Dear Sundial Experts, I have
recently joined this Mailing List, and hope that any memberswill be able to
give me some assistance on the following situation. Our local Stately Home
("Kentwell Hall", Long Melford, Suffolk) isconsidering installing an
Analemmatic sundial, as a new interactiveattraction for visitors - but we are
getting 'conflicting' advice,on whether this 'Human Sundial' will work in the
way we want it to. We have been in discussion with "Modern Sunclocks"
(apparently theacknowledged 'experts' for these features), who have told us
thatits central scale of dates must be aligned North/South - plus thathour
markers must be correctly positioned on an elliptical ring,and which would lie
on the Northern side of that scale of dates. Photographs on their website (
www.sunclocks.com ) confirm this. However, our 'Director of Operations' (Mr
Phillips) absolutelyINSISTS that he wants the scale to run exactly parallel
with ourmain driveway - on a compass bearing which is about 162 degreesfrom
North, with the hour points placed on its Southern side. He also wants the hour
points to form an exact semi-circle, andnot be elliptical in shape. Mr
Phillips refuses to accept thathe cannot arbitrarily position the Human Sundial
feature as hewishes, and says that it must be possible to create this so thatit
could then align with the existing layout of buildings/paths. Can anyone on
this Mailing List tell me whether it is possible toinstall a Human Sundial to
fit any existing orientations, (withappropriate re-calculation of its component
parts) - or, if not,just confirm that it must be as "Modern Sunclocks" have
told me. I can then show the 'weight of evidence' to Mr Phillips.
Because"Kentwell Hall" is a well-known Stately Home (open to the public),we
should not want to become a 'laughing stock' by installing afeature which does
not work - despite Mr Phillips assurance that"all types of sundial can be
adjusted to work, in any location". Looking forward to all comments (to this
List, or sent privately). Sincerely, Alison Shields.
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