The
epact link didn't work for me. I was "not authorized to view this page". The
main link did work. http://www.mhs.ox.ac.uk I then went to the collections
database link and searched for 54054. There are six pictures of the Kratzer dial
available. Pictures 1 and 2 are from the south west and show the top
horizontal, superior polar, south vertical, inferior equatorial and west
vertical dials. Pictures 3, 5, and 6 are from the north east showing the top
horizontal, superior equatorial, north vertical, inferior polar and east
vertical.
Picture 5 shows the details of the north facing inferior polar dial from
the north west. The engraved lines are not too clear and no numbers are visible.
I could see only three lines, one for midnight and one one each side, perhaps
showing sunrise and sunset as discussed in my earlier note.
Thanks
for the question David. It got me to this website and the wonderful collection
of antique dials. As search of their data base for " sundial" brings up 83
listings.
Cheers,
Roger
Bailey
N
48.6 W 123.4
Ah, so that's what the enquiry was about!
In the picture in the catalogue, which is the only one
I've seen of this dial, this inferior polar dial isn't visible.
But I have to agree that 4,6,8 sounds
unlikely.
How much do we know about Kratzer's ability to make
accurate dials? The direct west-facing dial visible in the picture seems to be
for 45 degrees, not 52 degrees latitude. And what do you make of
the gnomon of the reclining south-facing dial?
When studying the Kratzer-like polyhedral dial in
Holbein's Ambassadors I found a surprisingly simple equation that allows you
to convert angles and dimensions in a photograph or perspective painting
into the original object's dimensions. I'll email it to anyone interested. It
allowed me to calculate, for instance, that the apparently square dial on the
end of the dial is actually very inaccurate. Which could have been Holbein's
error in painting it, not Kratzer's.
Anyway, we'll be in Oxford in a couple of months so I'm
looking forward to the chance to see this dial for myself.
Best wishes
Chris Lusby Taylor
51.4N, 1.3W (so only about 25 miles from Oxford. Lucky
me)
----- Original Message -----
Sent: Friday, January 30, 2004 8:56
AM
Subject: Inferior Polar Dial
On 25th Jan I posted the following
query:
What hour lines should be inscribed, and in what places, on an
inferior polar dial for latitude 52N? The centrally-placed gnomon is
13 units high, the overall plate width (east-west) is 37 units and the
north-south plate height is 26 units.
Thank you for the various replies
sent in (4).
I clarified the term 'inferior' later on that day
thus:
"Inferior"..........Think first of a 'normal' polar dial, parallel
to the polar axis, aligned E-W but face open to the south i.e. that part of
the celestial equator above the horizon. Now consider the underneath
of this polar dial, face open to the north, facing that part of the
celestial equator which is (mostly) below the northern horizon. You've now
got an 'inferior' polar dial. The term is used in a document I came across
recently (on the internet). I suppose that an equivalent description would
be 'a polar dial facing north' or a direct north dial inclining at an angle
to the horizon equal to the latitude of the place (52 deg in this case). Yes
- it will be latitude independent, but I included that detail so that it
would pin down the discussion a bit.
The BSS Glossary also defines
'inferior' as "refers to an event on the celestial sphere below the horizon.
Opposite of superior."
Let me now tell you the reason for my
enquiry:
I was looking at the website of the Museum of the History of
Science, Oxford (epact.mhs.ox.ac.uk/catalogue), and brought up the details
of Nicolaus Kratzer's portable polyhedral dial (1525) made of gilt brass
which he presented to Cardinal Wolsey (Inventory No. 54054). It appears to
have been made for latitude 52 deg. N
There are dials on nine faces. One
is described as an 'inferior polar dial, with lines marked 4,6,8. This
puzzled me - what is the sense of only hours 4,6,8 on any dial?
The
illustration given on the museum's web page is not clear enough to get any
detail other than to see that the north face is not visible. However, a
postcard I bought at the museum some years ago does show the north face and
knowing the overall height of the dial (100mm) I estimate that the height of
the gnomon on the inferior polar dial is 13 mm. It seems that the lines for
4 am ( to the west and 8 pm to the east) are wrongly positioned - they
should be about 22.5 mm away from the centre of the dial, and are nowhere
near this distance.
A further puzzle - why did Kratzer mark the line on
which the gnomon lies with a 6? It should be 12 (midnight!) if it's going to
be anything - or was it because 6 comes exactly between 8 and 4? A blunder,
perhaps?
More measurement pending..........
David Brown
Somerton,
Somerset, UK
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