Dear Chris,
I am a bit behind with my reading and I have only
just read your comments, and corrected comments,
on the umbra discussion.
Subject to your corrections, I concur with almost
all you say but I feel a little amplification of
one of your follow-up remarks is needed. You
say...
If the
Dear Frank,
Try as I might, I can find no published information on where people judge
the edge of a shadow to be. It would certainly be an interesting experiment.
As John Davis observed, the human eye is very non-linear. Experience with
photography seems to suggest that, like the ear, it is
Dear Frank, Chris and the List,
An experiment to determine the position of the shadow edge was reported by
Allan Mills in his article Sunlight and Shadows - or, what's the point of big
sundials?, British Sundial Society Bulletin 96(1) pp.22-27 (February 1996).
Allan's key result for the
I remember John Carmichael did some experiments on a truly monumental
sundial, The McMath-Pierce Solar Observatory, Kitt Peak Arizona. I think he
put the judgment point for the transition well towards the dark side at
about 85% dark.
That was long ago and far away so I would appreciate any
Greetings all,
With their usual impersonal and rather scary efficiency, Amazon have just
delivered to me a copy of Jim Morrison's book The Astrolabe. Ordered on-line
11 Jan, had to wait for stock (as Jim warned us), delivered on 18 Feb in
Australia. Not bad.
If you haven't bought the book,
