|
Sounds like it might work. We'll try it on
Thursday.
Thanks
John
John L. Carmichael Jr.
Sundial Sculptures
925 E. Foothills
Dr.
Tucson Arizona 85718
USA
----- Original Message -----
Sent: Monday, June 17, 2002 2:36 PM
Subject: Re: Image Problem Solved?
In a message dated
06/13/2002 11:58:46 PM Eastern Daylight Time, [EMAIL PROTECTED] writes:
A tilted card *above* the mark probably won't work, as I'm sure
you've
decided by now. You could use a tilted card, whose edge is in
contact with
the ground. Move it until the image of the Sun and shadow
edge are split
over the grounded edge, 50% on the card, 50% (elongated)
on the pavement.
At least you get the good imaging surface, and can tilt
it to normal to
the Sun/shadow line.
John,
I have been away for a few days.
Wow, has the mail been thick! Congratulations on the acceptance of
the proposal. You did a grand job!
I too was sucked in on thinking that a
circular hole held parallel to the ground would make a circular spot of light.
It will, if the hole is large enough so that it is simply cutting off
the light that does not pass through the hole (say, a one inch hole held 36
inches above the ground), but if the hole is small enough to make an image of
the sun it will make a circular image only if the card is held perpendicular
to the rays of light. Otherwise it will make an elliptical image.
We all know that now.
Now
to the problem of marking the shadow. I like Dave Bell's solution, but
before I saw his I developed a different one that you might try. I
punched a 2 mm hole in each of two 3"x 5" cards. I then thumbtacked
(drawing pinned, I believe in the UK) the cards to each end of a thick yard
stick (3/4" square cross section) so I had a kind of "sighting" device.
Don't try to look through it! Now I held the stick in the sunlight
so that the card near the Sun made an image of the Sun on the lower card.
If the lower end of the stick rests on the ground it is not hard to
steady it and cause the image to surround the lower hole. The light
passing through the second hole marks the position of the Sun's image on the
ground. It is necessary to have a light colored surface on the ground on
which to see this spot of light. Now one person can move this apparatus
into the penumbra of the shadow, adju! st it until the circular image on the
card is over the hole and divided in half by the image of the edge of the
gnomon. A second person can then mark the spot of light coming through
the "almost center" of the image, and at the appropriate time drive in the
tack. Try it out if you have time.
Bill Walton
Plymouth, MA, USA
42 N 71 W
|
