Shadow near Tropic
Date: Winter 2012-2013
I have a vertical pole in my back yard here in Cairns Australia. Today, 31 Jan, the sun is still casting a slight southern shadow. Also, a table I read says it should face north/south at 12.27 pm. But it does not get there untill 12.33. I have been watching the shadow for about 2 months now, and at first it was in the north at 12.27. Why is it so?
Wikipedia ( http://en.wikipedia.org/wiki/Cairns) tells me that Cairns Australia is at 16ª 55' 32" South Latitude.
The tropic of Capricorn, at 23ª 26' 16", is the southernmost latitude over which the Sun oscillates on the surface of the Earth as the seasons change.
Since the Winter Solstice occurs around the 21 of December, the Sun is a little North of 23ª S but I think the Sun is still running South of 16ª, so your shadow should point North at solar noon. Solar noon may be different from your local noon because you are bundled into a 15ª wide time zone.
First you may have a verticality problem. Make sure your pole is vertical by using a plumb line to make sure your pole is vertical, or just simply use the plumb line in place of the pole. The pole should really point north at your true solar time.
Second, you may have a solar noon time problem. You may not be in the exact center of the time zone from which you get your Solar noon times (12:27 vice 12:33) or the table you use is not adjusted to the exact Longitude of Cairns Australia.
Third, you may have North sensing problem, especially in Australia. Magnetic compasses suffer from all sorts of local and global influences and do not point to True North at the North Pole. Magnetic compasses point to some place near the Hudson Bay in Canada and it is moving. I am not saying you cannot tell North from South, but if you are measuring time for Solar noon, you have to have a good bearing on where the North Pole truly is.
Believe me, at Solar noon your time, a perfectly vertical pole would point to true North Cairns Australia.
Good question. The shadow would probably be straight N_S if you were on the precise centre of the time zone, which almost nobody is. That probably explains the discrepancy, though I am not certain of this.
David H. Levy
I do not have a detailed picture of your experimental setup; however, many factors can enter into giving a high precision/accuracy of your measurement.
Wind, atmospheric pressure, temperature can all have small but cumulative effects. Even the moisture in the soil can change the orientation slightly. The experiment you are doing is a classic one used to measure the diameter of the Earth. I think by the ancient Greeks. It sounds simple in context, but actually carrying out the experiment accurately is a difficult geographic / geometric problem. It is hard to isolate the size of these effects (and others).
Thanks for the question. It is reasonable to expect some discrepancies between the observed time and the published time. Some reasons for the discrepancies are the local elevation and tilt of the ground you are on. Also, the pole may not be completely vertical. I have personally found "straight and level" fence posts to tilt some after a few years. So, I would check the vertical orientation using a level.
I hope this helps. Please let me know if you have more questions.
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