--- Canopus56 <canopus56@yahoo.com> wrote: <snip> --- Norm Hansen <***> wrote offline:
Hi Kurt, I've been waiting for someone to challenge the premise you state below; I do not see how that can be, except on the equator. At our northern latitude the shadow of the top of the pole would fall directly on the east-west line that passes through the base of the pole at sunrise and sunset (if you have a true horizon visible) but all the rest of the day the shadow would move north of that line, to well over 40 degrees at high noon.
Where am I going wrong in my thinking?
the pole would fall directly on the east-west line that passes through the base
Perhaps I mistated unclearly in my original post. The straight east-west line cast by the shadow of a gnomon on the equinox does not pass through the base, it passes north of the base, as do all shadows cast by gnomons north of the Tropic of Cancer. But the shadow is a straight line. It's counter-intitutive because we are conditioned to think about terresterial reality in terms of the physical parallax created by the distance between human eyes. That parallax is good to about the length of a football field. At 149.6Ã106 km distance, the Sun has a parallax of above 25", I believe, assuming that you stand on opposite ends of the Earth at the same time. This is below the ability of the human eye to detect. Because the light from the Sun is effectively parallel and is aligned with the Earth and celestial equator on the equinoxes - and this is true at all latitudes, all Earth-based gnomons cast a straight east-west shadow on those dates regardless of latitude. Otherwise the shadows are curved. See illustration at - http://www.scottdesignsundials.com/How_to_read/a_Graphic1.gif in - http://www.scottdesignsundials.com/How_to_read/body_how_to_read.htm and informal demo at 16 degree north latitude at website - http://www.sundial.thai-isan-lao.com/equinox_path.html The Pueblo Bonito Ruins in Chaco Canyon, New Mexico, are aligned along a similar east-west line that marks the equinoxes. http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16974 I would recommend downloading a crippleware program called "Shadows", that allows to you to rapidly design sundial plates for various latitudes, to experiment further. The "Shadows" program also allows you to view, the sundial that you design, an animation of the Sun travesing the sky and the resulting shadow cast by the dial. By my own informal test, yesterday at sundown, the Sun did set right down the middle of 300 South, an East-West running street in SLC. - Canopus56 (Kurt) P.S. Some more info - The ancient Greeks divided the world, in part, into three latitudinal bands based on the shadow cast by straight gnomons. (The triangular gnomon sundial was invented later by the Romans.) The word "Tropic" as in - Tropic of Cancer, at 23° 26' 22" N Tropic of Capricorn, at 23° 26' 22" S - means "to turn" in Latin, from the Greek trop, a turning. For all gnomons north the Tropic of Cancer, all shadows cast by the gnomon throughout the year fall north of the gnomon base. For all gnomons south the Tropic of Cancer and north of the Tropic of Capricorn, shadows cast by the gnomon throughout the year fall both north and south of the gnomon's base. For all gnomons south the Tropic of Capricorn, shadows cast by the gnomon throughout the year always fall south of the gnomon's base. The word "turn" in Tropic of Cancer and Tropic of Capricorn meant to the ancient greeks that the shadows of gnomons marked at noon each day would reverse direction at the solistices start going the other direction (towards or away from the gnomon's base). Because of 2500 years of precession, the shadows no longer turn when the Sun is in those constellations. By convention, we still use those terms on world maps and globes. Other good background on sundials can be found at: The Sundial Primer (website) by Carl Sabanski http://www.mysundial.ca/tsp/tsp.html __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com