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I am not sure if this is the right place to post at, but I tried to find the best place to do so.

To my point, I wonder if it is posssible, to determine where north is, just by one shadow, that means no Sun, no time, just a shadow in a picture or video...

Do you get my point? Like this picture, can I tell where north is?

Like this image:

Shadow

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2 Answers 2

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With only one shadow in a picture, no, you can not.

In order to get North's position from a shadow you need to know where you are and which solar time and date is it there. These are two extra data and you can not disentangle them, even if the picture is perfect.

e.g. suppose a picture in which you see exact measurements of shadow and people, and they are both exactly of the same size. This means Sun is 45º above Horizon. But that may be local noon at Bourdeaux on March 20th (thus shadow pointing North), or local noon at Christchurch on same date (thus shadow pointing South), or local 3PM solar on Quito on March 20th again (thus shadow pointing East).

I chose March 20th because it is easier to visualize where the Sun would be (exactly over the Equator), and these cities due to their latitudes:

  • Bourdeaux: 44º 50' N (almost 45ºN)
  • Christchurch: 43º 31' S (almost 45ºS)
  • Quito: 0º 13' N (almost on Equator)
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With only the shadow in the picture, I do not think it would be possible. Consider a stick on the equator with a shadow. In the winter, at noon, since the sun would be "south" of the equator, it would point the opposite direction as in the summer (when the sun is "north" of the equator). If we assume it is summer, the calculations would show north is the opposite direction of if we assume it is winter.

However, the "fuzziness" of the shadow could possibly be used to establish a time (at least help determine the season) and aid in calculations. When we are closer to the sun, the shadow should be more fuzzy. The length of the shadow could be used to determine the time if the location were known. Still, I believe there are too many unknowns to determine the direction of North.

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  • $\begingroup$ Hi! Thanks! That was exactly what i thought about, i was discussing with someone, and now you've proved my point. $\endgroup$
    – Kilise
    May 14, 2014 at 19:01
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    $\begingroup$ You could find the time roughly by approximating the height of the people and comparing that to the length of the shadow. $\endgroup$
    – Mitch Goshorn
    May 14, 2014 at 19:40
  • $\begingroup$ @MitchGoshorn Yes, i'd guess that it the time is 12 at this picture , what would you say? $\endgroup$
    – Kilise
    May 14, 2014 at 20:24
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    $\begingroup$ @Mitch Goshorn - One thing that would confuse this calculation is their lattitude (which is unknown in this question). For example, shadows would be longer at noon if you are far north of the equator, than if you were at the equator. During an equinox, at the equator at noon, the shadow would be minimal if the person is standing straight up. $\endgroup$
    – Jonathan
    May 15, 2014 at 14:54
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    $\begingroup$ determining the season by the fuzziness of the shadow would be difficult - as the distance to the sun only varies by around 3% - and the seasons are, therefore, determined by the inclination of the earths axis relative to the sun. $\endgroup$
    – db9dreamer
    May 16, 2014 at 19:26

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