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Old 06-11-2019, 04:03 PM
brossa is offline
Join Date: Feb 2005
Posts: 1,141
Originally Posted by Chronos View Post
The reason nobody suggested that is because it only works at the equinox. At any other time of year, that line will indeed curve. If I were to do it today, for instance, at noon the shadow would be north of the gnomon, and at sunrise and sunset, the shadow would be south of the gnomon. It's tough to make a straight line do that.
You know, when I read that, it seemed wrong to me, so I did a quick Google search for shadow compasses and found a number of videos of people showing the shadows tracing out a straight line. This still seemed so wrong to me that I actually have a stick out in the yard right now, and it confirmed my belief that the shadow tip will trace an arc. So now the question is whether these videos were purposefully filmed on the equinox without mentioning it, OR whether they were filmed over a short period of time near sunup or sundown, when the arc is flattest and the curvature isn't easily detected. One guy did have a video where he compared his 'n-s' line to a magnetic compass and it was off by probably 20+ degrees, which he attributed to magnetic declination. Which made me wonder where in the world there could be such a severe mismatch (far southern New Zealand or far northern Russia, as it turns out, but he wasn't in either of those places). He probably wasn't trying to fool anybody by shooting on the equinox.

Noon is still the point at which the shadow is shortest - where the shadow arc is closest to the shadow-caster - and the angular height of the Sun at noon will let someone with the proper tables look up your latitude.

In the interest of full disclosure, I had a full post typed out about how you could tell if you were in the N or S hemisphere by whether the sun went around in the northern or southern half of the sky, forgetting about the whole band between the tropics. A few minutes on The Photographer's Ephemeris helped me out there.