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How are the directions on a planet determined? Is it based on sunrise, if the sun rises in the east? Or is it based on magnetic north?

I would like to know about directions for planets which may have horizontal axis rotation, or two suns or something else.

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    $\begingroup$ How can this have answer different from "whatever way fits your story best"? $\endgroup$ – Mołot Sep 8 '17 at 11:00
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    $\begingroup$ Short answer: yes, all of those things are used to define directions. Long answer: ...and that makes everything extremely confusing. Just the fact that we have several different north poles should give you some indication of how confusing this is. So how is it all determined? That usually depends on the situation at hand. Depending on what your needs are, you can pick any of several methods. $\endgroup$ – MichaelK Sep 8 '17 at 11:02
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That greatly depends on the respective culture and available technology, as all your examples have been present and actively used at various times in history within various cultures.

1) With 1 or 2 suns, you can determine the direction of the sunrise and continuous movement of the sun(s) in the same way human scouts have done so for millenia, as long as each sun reliably will rise the same location and there is no 'spiralling', rotationary or simply randomised effect which changes this direction within a short time (e.g. a few days or months).

2) For measuring a magnetic north, you require knowledge about magnetism or at least practical experience how to use certain materials which will always align according to a specific direction. This certainly requires far more cultural knowledge than option 1) which is available to any creature with high enough intelligence to help with orientiation. A scenario, where this orientation would fail, is when the magnetic poles are switching/rotating at far more frequent intervals than they do so on earth.

3) Orientation by the stars which remain fairly fixed for long times has been a valid option for millenia and is possibly the most reliable, as long as the suns do not produce too much 'background' light such that the stars are not visible any more.

4) Orientation by triangulation or landmarks (e.g. mountain ranges) which are unmistakable and possible to see from a very large distance while at the same time they remain stationary and allow orientation relative to the landmark. Here, the problem is of course that the landmarks have to be visible and should not be obscured by large obstacles (forests, chasms,...) A variant of this is similar to GPS, if your native species has the ability to fly high enough to choose fixed points with respect to which they get oriented.

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Directions are always in the direction of something (fairly obviously), although what that something is is not necessarily reachable: there is no such place as an east or west pole, for example.

The only real requirement for directions, for the purpose of communication and navigation, is that they are unambiguous and meaningful in context. If the context is relatively small, for example a people living within a valley and rarely venturing outside, meaningful directions would be towards landmarks, ideally clearly visible, on the outskirts of the valley. These would be adequate, and possibly far more useful than N,S,E,W for such a society and probably not evenly placed.

A global civilization, however, assuming a globe and not something else like the discworld, is more likely to adopt something like the compass points, even without a magnetic north - with rotation, against rotation, perpendicular to rotation (two directions). Of course, a bird society is likely to also include Up and Down as meaningful 'compass' points.

A civilization living upon the face of a cliff, or underground in a network of tunnels may well have something more exotic.

I doubt that the number of suns would make much difference. Even with one sun, the direction of the sun itself cannot be used since it moves in the sky, although the direction of where it rises, and where it sets, often is. With two suns, sunrise and sunset becomes less noticeable, individually, and rather more confusing if taken together.

Basically, for your purposes, choose whatever works for you and your people.

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