Greetings, fellow dialists,
(This message was earlier blocked by the sundial regulator.)
Frank is quite correct, as usual.
If I may expand a little, the only information found from a solar altitude is a position
circle. If a true latitude is known at the time this may be crossed with the position
circle to find the longitude, the navigator's well-known "longitude by
chronometer". But an error of one minute in longitude is an error of four seconds in
time. That is, any dial correction would be 4 seconds wrong for each inaccurate arc
minute of longitude (regardless of latitude).
My proposal, though, is not for an altitude dial (see above) but for an azimuth
dial using the east or west sun limbs. I cannot think it would be more than a
paper exercise, though.
Frank's story of the Rome noon line is typically impressive. We must treasure
him.
Frank 55N 1W
On 24 May 2013, at 18:33, Frank King<frank.k...@cl.cam.ac.uk> wrote:
Dear Frank
As ever, you prompt several interesting
trains of thought...
In fact any run-of-the-mill astro-navigator
with a merely passable sextant could at
least manage an accuracy of a minute of
arc...
I have never used a sextant but I have
heard this arc-minute figure so many
times I am willing to believe it!
...or four seconds of time.
Careful here! Most of the year the sun
goes round in a small circle so an
arc-minute change in position takes
longer than four seconds.
Also, I am not sure what angle you
are measuring? You can't mean the
solar altitude. That doesn't change
anything like an arc-minute in 4s
where I live!
For a really accurate dial could the
east or west limb of the sun be
projected and utilised?
Well you can project the whole solar
disc using a continental camera
obscura sundial and thereby see
both east and west limbs of the
sun.
I have observed transit at the
Basilica di S. Maria degli Angeli
in Rome many times, sometimes with
enthusiasic Italian diallists
adding to the fun!
Each diallist shouts "Ora" [Now]
at what he perceives to be the
critical moment and the chorus
lasts two or three seconds, so
that's an observational error
for a start.
The very first time I did this,
I noted the time of the middle
"Ora" on my radio-controlled
watch and did the reductions.
This includes allowing for the
difference between UTC and UT1.
It was pretty good, the error
was about 3.5 seconds.
Alas, I have done this many
times since and noted errors
of up to 10 seconds.
There are too many things to
go wrong...
Mario Catamo says that you get
a build-up of muck on one side
of the hole which shifts the
apparent centre. There would
need to be a lot of muck to
account for 10 seconds though!
I believe the line isn't quite
north-south but since I always
go at much the same time of
year the error should be
consistent!
That said, I think 10 seconds
is pretty good.
Frank
Frank King
Cambridge, U.K.
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