How was it decided which hemisphere was North and which hemisphere was South on Mars? Clearly, the designations "North" and "South" apply only to the hemispheres which contain the axis of rotation at their center, but what determined whether a hemisphere received "North" or "South" as its designation?
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asked Jul 16, 2014 at 16:35
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2 Answers
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The research in other answers helped me come across the actual International Astronomical Union (IAU) standard:
The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system.
The north of the invariable plane is the side that Earth's North pole points to. In fact, one of our other users has observed:
The North Pole of any system being the pole on the same side as earth's north is indeed the correct definition. This applies to the Milky Way galaxy as well, in which case the North Galactic Pole is the pole towards the hemisphere of the galactic sphere which also contains the celestial north pole.
Dwarf planets, minor planets, their satellites, and comets do follow the right-hand rule mentioned in other answers (see Axial tilt on Wikipedia).
Sources:
answered Jul 17, 2014 at 13:34
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North and south are defined according to the planet's rotation about its axis. A "right hand" rule is used: If you make a fist with your right hand, and orient your curled fingers in the direction the planet rotates, your thumb points north. Alternatively: Standing on the equator, facing the direction the planet rotates, North is to your left.
Axial Tilt on wikipedia has this information (and more.)
update
hey! Don't up vote this answer; It's wrong... see the comments where the OP has actually researched and discovered the real answer. (I don't want to delete this answer because we'll lose the comments too!)
answered Jul 16, 2014 at 18:00
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2$\begingroup$ That's interesting; I wouldn't have thought to look for that information under "Axial Tilt". Thanks for sharing! $\endgroup$– called2voyage ♦Jul 16, 2014 at 18:12
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$\begingroup$ I also tried to find a better link on the IAU web site, but didn't find anything better. There's probably an official document that says exactly how they do it. $\endgroup$ Jul 16, 2014 at 18:15
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1$\begingroup$ Relevant portion: "The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system." The north of the invariable plane is the side that Earth's North pole points to. $\endgroup$– called2voyage ♦Jul 16, 2014 at 18:21
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1$\begingroup$ Now I've found it on Wikipedia as well: en.wikipedia.org/wiki/Poles_of_astronomical_bodies $\endgroup$– called2voyage ♦Jul 16, 2014 at 18:21
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1$\begingroup$ The North Pole of any system being the pole on the same side as earth's north is indeed the correct definition. This applies to the Milky Way galaxy as well, in which case the North Galactic Pole is the pole towards the hemisphere of the galactic sphere which also contains the celestial north pole. $\endgroup$– TakkuJul 16, 2014 at 20:43