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
