In a message dated 1/25/2002 10:30:31 AM Eastern Standard Time, [EMAIL PROTECTED] writes:
> Theoretically, the gnomon (or style) of a properly > oriented equatorial sundial will not cast a shadow on the dial plate during the > Equinoxes. I'm told (by a local sundial expert) that it should be > possible to observe the gnomon shadow move from the lower to the upper dial > face of the dial plate during the course of the day I have not witnessed this first hand, but I suspect this may be less impressive than it sounds. Because the sun is not a point source of light, and subtends a width in the sky of 1/2 degree, at the moment of equinox the upper dial plate should be illuminated by the upper half of the sun, and the lower dial plate by the lower half. I think the result is that both sides will cast shadows and be readable, though somewhat dimly. Furthermore, the sun's declination changes by 0.4 degrees on the equinox. This is less than its width in the sky, and so the change of light completely from one side to the other will take a full day at least. I do not think it will occur in a matter of minutes or even hours, unless the sundial is misalighned, and not in the plane of the equinox. This will be worth checking when the time comes. Bill Gottesman Burlington, VT 44.4674N, 73.2027W
