Solar navigation

13 June, 2010

Earth's magnetic field.
Earth's magnetic field.

Question: I would like to know if there is any way I can take a compass reading of a building's facing direction using the reference of the sun (to detect a true south direction)? I am having problems taking accurate compass readings due to interference from a man-made magnetic field (like underground wiring and light poles) – can you help?

Answer: A tricky problem… compasses indicate magnetic north or south as they react to the Earth’s magnetic field but that’s offset with respect to our planet’s rotational axis (what most call geographic or true north/south). The direction of magnetic north or south is therefore slightly different.

But putting that small difference aside, it is possible to find south using the position of the Sun in the sky but it will be hard to be really accurate. However, a reasonable result good for most purposes can be got by observing the motion of the Sun across the sky.

Wait for local noon by using a clock on standard time (simpler than having to allow an hour for daylight savings). At that time the Sun will be at its highest point above the horizon, so face the Sun and south will be behind you if you are in the Southern Hemisphere (or north will be behind you if in the Northern Hemisphere).

Better still, use a sundial or a tall pole that will throw a shadow towards south at local noon (or towards north if in the Northern Hemisphere). You can then compare compass readings at the building and some well away from it with your noon observing of the Sun. Overall you should get a pretty good facing direction.

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