Dear Frank,
Surely Geoff Thurston is right? The width of the 6am shadow would be one
inch. (Assuming 6am and noon are local solar time, and ignoring penumbral
effects due to the sun not being a point source.)
If the hours are marked out as on Greg McDonagh's Pebble Dial (your article
in the December Bulletin, p 38), the result is clear from the positions of
the lines for 6am and 6pm.
Best wishes,
John
-----Original Message-----
From: Frank King
Sent: Wednesday, January 17, 2018 6:04 PM
To: sundial@uni-koeln.de
Subject: Hail to the Chief
Dear All,
I gather from the UK media that
the recent medical check-up
undertaken by the U.S. President
involved some "cognitive tests"
and he scored 30 out of 30.
One of these tests requires the
subject to mark in the hands of
a clock at a specified time.
It is, of course, reassuring to
know that he is familiar with
analog(ue) time measuring
instruments so I would like to
propose a test which is just a
whisker more ambitious for next
year...
I would present the subject
with a mock-up horizontal
sundial equipped with a rod
gnomon which has a circular
cross-section and is about
0.5" in diameter.
The dial would be properly
set up for a latitude of
30 deg N. so the gnomon would
slope at a 30 deg angle to the
horizontal.
I would then ask the subject to
draw the shadow of the gnomon as
it would be at 6am and 12 noon.
Since the mock up would include
a full set of labelled hour lines
this should not be too much of a
challenge BUT...
Part of the required answer is
that the shadow at 6am should be
wider than the shadow at 12 noon.
I would then ask by how much the
shadow is wider at 6am than at
12 noon and to explain why.
Can you help the President?
Frank
Frank H. King
Cambridge, U.K.
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