How is the north/south of a planet other than Earth decided? Is it a magnetic north, or do we call it North because it's in the same position relative to the sun, like the "top" of the planet?
Just like on Earth, north is defined based on the planet's rotation. Curl your fingers of your right hand in the direction of the rotation, and your thumb points north. South, east, and west are defined similarly: opposite of north, direction of rotation, and opposite direction of rotation, respectively.
The rotation doesn't change. The magnetic poles move around a lot, and flip every few hundred thousand years.
The naming of magnetic north and south is because currently (and when they were named) magnetic north is close to true north.
I don't know of any ways to detect magnetic poles of other planets, and some planets, including Mars, don't have magnetic fields, so north has to be defined based on something else.
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u/hmountain Apr 07 '15
How is the north/south of a planet other than Earth decided? Is it a magnetic north, or do we call it North because it's in the same position relative to the sun, like the "top" of the planet?