Sundials

[Hank Harwell <cleireac@juno.com>]

Poster: Hank Harwell <cleireac@juno.com>

Below is a copy of correspondence between myself and Matheus de Troyes
from the Arts list regarding the Westminster Abbey "clock" I was
inquiring about:

From: Brian Songy <bxs3829@usl.edu>
To: "'sca-arts@raven.cc.ukans.edu'" <sca-arts@raven.cc.ukans.edu>
Date: Thu, 27 May 1999 11:42:27 -0500
Subject: Making Sundials

On Thursday, 27 May, 1999 10:56, Hank Harwell [SMTP:cleireac@juno.com]
wrote:
> About a year ago I was leafing through a book merchant's wares and came
> across a book on constructing sundials.  In this book, the author
showed
> a picture of a handheld sundial purportedly found in the cloisters of
> Westminster Abbey.  It was long and narrow, with holes drilled in the
> surface along two parallel lines.  In these holes, one would stick pins
> to mark the month/day, and the sun's shadow would fall across other
holes
> marking the time.  has anyone else seen this, and could possibly point
me
> to a much better description, where I might possibly be able to
construct
> one?
>
> Brother Cleireac of Inisliath


Brother Cleireac - - -

I have a copy of a book very much like the one you describe (perhaps the
same one), but it is still packed after a recent move.  I'll see if I can
dig it out tonight.

IIRC, the book provides instructions for producing the sundial you
describe.  If memory serves, it operates like this:

<all of the following is from memory, and therefore, may contain errors>
The sundial is a small wood tablet, long and narrow, as you describe.  To
use it, It is laid upon a flat, horizontal surface, with the long axis
oriented East-West. The sundial itself can be used to determine this
orientation.  A pin is stuck into one of a series of holes, to serve as
the gnomon (?spelling), that is, the portion of the sundial that casts
the shadow.  The proper hole was selected based upon the current date and
location - the different holes are used to correct for the seasonal
change in the tilt of the Earth in respect to the Sun, changes in
Latitude, and/or the seasonal differences between Solar and other time
keeping standards (e.g. Sidereal).  Moving the pin along the long axis
(i.e.. East-West) adds or subtracts a certain amount from the time shown.
 Moving the pin along the short axis (i.e. North-South) roughly adjusts
the "speed" of the clock, that is, lengthen or shortens the day.  The
tablet can be marked in a variety of ways (regular linear markings,
logarithmically spaced markings for equal length hours, sets of parabolic
curves so that sunrise and sunset fell on a certain hour everyday, etc,.)
The actual markings were often determined empirically, for example, the
sundial was set up and when the sun was at it's highest point in the sky,
wherever the tip of the shadow fell was marked as noon.  Sunrise, sunset
and the various hours of the day could also be derived empirically, or by
subdividing the tablet (e.g. dividing the portion between one end of the
tablet and noon into equal portions).  The holes could also be determined
empirically.  If a sundial worked in one location, but was off in
another, a new hole could be drilled to correct the time displayed.

Another type of small wooden tablet was also produced.  It hung
vertically from a string with the pin extending vertically.  My
recollection on that type is hazier.


I've made several sundials, and have enjoyed the process.  If nothing
else, I recommend you to make a simple sundial by mounting a vertical pin
into a board.  Start deriving your markings empirically, and as time goes
by you can start adding more sophisticated embellishments to your sundial
(i.e. corrections for 
date, latitude, etc.).The process is a great learning experience.

If I find the book, I will post the ISBN, etc. tomorrow.

I hope that helps.  If there is anything else I can assist with, please
do not hesitate to ask.

- - -	Matheus de Troyes  (mka Brian Songy)
	Trollfen, in Meridies
++++++
He later followed up on a question I asked as a result of his post...

From: Brian Songy <bxs3829@usl.edu>
To: "'sca-arts@raven.cc.ukans.edu'" <sca-arts@raven.cc.ukans.edu>
Date: Thu, 27 May 1999 14:49:59 -0500
Subject: Making Sundials

On Thursday, 27 May, 1999 13:09, Hank Harwell [SMTP:cleireac@juno.com]
wrote:
> Milord 	Matheus,
>
> Thank you for your response.  I do have a few questions I'd like to
> ask...
>
> First, you mentioned a vertical sundial consisting of the plate
suspended
> on a string with the gnomon extending horizontally out.  Is this
similar
> to sundials found at Irish monastic sites?
<scratches head>  I have to admit that my feeble memory is insufficient
to 
provide any historical data on these sundials.  Perhaps if I find the
book I 
mentioned it will provide the information you're looking for.

>
> Second, I have a question regarding your construction advice for the
> "Westminster Abbey" version:
>
> >If nothing
> >else, I
> >recommend you to make a simple sundial by mounting a vertical pin into
> >a board.
> > Start deriving your markings empirically, and as time goes by you can
> >start
> >adding more sophisticated embellishments to your sundial (i.e.
> >corrections for
> >date, latitude, etc.).
>
> So, it really doesn't matter *where* I drill my initial hole for the
pin,
> I just line it up correctly and then mark where the shadow falls for
each
> hour?

No and Yes.  :)   If you want to recreate a particular sundial, achieve a

certain degree of accuracy, etc. then the location of the holes is
critical. 
 The book I have has mathematical formulas for calculating size, shape & 
position of the various pieces of the sundial.

If you want to create a sundial in a manner that, plausibly, was used to
create 
early medieval sundials, then the location is not critical.  Remember,
though 
that the marks you make are good only for one orientation of the sundial
(the 
sundial must be in the same position relative to North every time), one
time of 
the year and one Latitude.

Note that you can use this nature of sundials to generate an even more
useful 
device.

For example, make observations several times in a day, once a week for a
year. 
 Now connect all of the 7am's by a (curved) line.  Then connect all of
the 8am. 
 Then connect 9am's etc, etc.  When you are finished, you have a sundial
that 
will keep accurate time throughout the year.

...Or.....

Make the same observations as above, but connect the marks that belong to
the 
same week.  That is, draw a curve connecting 7am, 8am, 9am, etc. from
week one. 
 Then connect all of the marks from week two.  Now you have a device that
will 
tell you the date!

...Or....

Throughout the year, make a mark at the same time every day.  Connect
these 
marks with a line.  Now from a different location (i.e. a different
Latitude), 
make marks the same time every day.  Connect these marks with a line. 
Now you 
have a device for finding your Latitude relative to these location!

And there's a lot of other things they can do.  People are usually only 
familiar with the simple "garden sundials", which are often not even set 
correctly, and are not aware of all of the things sundials can do.  Find
true 
north. Measure the precession of the Earth.  Find the proper date of
Easter or 
Ramadan. etc. etc. etc.


It's to get the "feel" (i.e.. internalize their method of operation) of 
sundials that  I recommend you try building your own empirically out of
an 
arbitrarily placed pin.  It can be a pain to make all of the
observations, but 
it can be great fun too.  It's  good to know, though, that if you need to

generate a sundial quickly, there are formula's to do so.
>
> >If I find the book, I will post the ISBN, etc. tomorrow.
>
> Please do!
>
Remember though, It's in one of several dozen boxes.  It may take me a
bit to 
find it.  I'll do my best.

> >I hope that helps.  If there is anything else I can assist with,
> >please do not
> >hesitate to ask.
>
> Thank you again for your response!  I, too am preparing to move soon. 
So
> I will certainly try out your hints at my first opportunity!
>
Good luck with your move.  If you happen to have a post, flag pole or the

corner of a roof that cast a shadow on a good spot, you can try to create
your 
sundial on the ground.  Very medieval - - I saw several like this in
public 
squares in Northern Italy.


Brother Cleireac of Inisliath

The Welsh pray on their knees, and on their neighbors. The Scots keep the
Sabbath, and anything else they can get their hands on. The Irish don't
know what they want but are willing to fight to the death for it. 
Whereas the English consider themselves a race of self made men, thus
relieving the almighty of an awesome responsibility. (_As Others See Us_,
anon.)

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