If a new planet was discovered, where the magnetic north pole was at the opposite end of the planet from the geographic north pole, which end would we consider the north pole.
Restated: Which defines the north pole? The magnetic orientation, or the rotation of the planet?
Codicil: What if the Earth’s magnetic poles were to switch again? Would the north pole be where your compass points, or 90º to your left when facing sunrise at the equinox?
Generally speaking there are 3 Norths
True North
Magnetic North
Grid North
Magnetic North is the magnetic North pole, currently somewhere in Canada or Alaska or one of those cold frosty places up north. Most survey instruments such as needle compases or tri axial magnetometers all measure azimuth relative to magnetic north. Knowing where you are pointing relative to magnetic north is not much use as the magnetic north pole moves around a fair bit. So long as we know where it is relative to true north, we are all hunky dory as can apply a correction , known as magnetic declination, to get us pointing back at true north.
Magnetic declination changes depending on where you are on the earths surface and the time of the year. Really the magnetic declination can be anything, if the earths magnetic field does a big jump, we just apply a big correction, no sweat.
True Noth is, well the real deal, its´ name being ´true North and all. This is the axis of the earths rotation. It should stay the same barring catastropic events such as giant meteors, giant alien invasion or ameboic dysentry in a giant blue whale causing a catclysmic bout of flatuence that cause the earths oceans to off balance the worlds rotation. Some survey instruments such as north seaking gyros measure the earths rotation, and we can get a measurements to true north without all that messing around with magnetic declinations. Incidentaly looking at the stars is a measurment of the earths rotation , and thus is a measurement relative to true north.
True north is the most constant north and is the most suitable reference point for most earth bound day to day measurements.
Now given that most useful applications for knowing north relate to maps and putting stuff in a precise location relative to other stuff, we have to deal with maps. Maps are a projection of a sphere onto a flat bit of paper. There are a myriad number of projections and ways to do this. The upshot is for any given map using any given projection, there will be a correction to get from grid north to true north. If you are on the meridian of the map, this is a zero correction. As you move closer to the edges of the map you have to apply an increasingly large correction.
Typically construction projects, showering your opponent with artillary shells or drilling an oil wells will be planned off of a map and a certain prjection system will be used. The survey measurments will be taken to magnetic north. These need to be changed to true north with the appropriate magnetic declination (which is location and date dependent). These will then in turn need to be translated to a grid north (by applying the appropriate grid correction angle) so everything on the map is pointing in the right direction.
So if we go to a new planet, the true north will be the axis of rotation. There would be nothing to stop someone saying, the south is north and vice versa. However I suspect they would try and keep the north pole being the one that is pointing closest to the direction as the earths north pole. The Magnetic North will be off set by where ever the magnetic pole is and you just apply the magnetic declination. If the magnetic pole is in the south, it is just a big correction. Grid north will depend on whatever projection systems you set up.
If the planet does not rotate and has no magnetci field then good luck finding yor way around. I gues most things would be done with inertial measurements or relative to some fixed point, such as the stars. What you say is North is really pretty arbitray, just get everyone to agree.
ETA ’ have a look here to see what the earths magnetic declinations are depending on your current location.
Actually, the magnetic north pole actually is a south magnetic pole. On your standard bar magnet, the north pole is the one that will point north if the magnet’s allowed to rotate freely. But since the opposite poles attract, and the bar magnet’s north pole is being pulled northwards, the North Pole is actually a south pole.
Ow, my brain.
You have defined the north pole of the magnet to be the one that points to the earths magnetic north pole. The bar magnets North pole is really its’ south pole but with a big letter N stuck on it.
Remind me not to visit your world without a very good gas mask.
Thanks for the info. I knew some of that but hadn’t seen it all laid out in one place quite so nicely.
I’d actually originally been pondering if humans are more attached to the idea that the sun always rises in the east, or to the direction that our compasses point. In a world where the 2 don’t match, which one would we be more likely to give up. My guess is the general population would never notice if you reversed which end of their magnet had the “N” on it, but would rebel at the idea of the sun rising in the west. On the other had, most scientist/engineers I know would happily watch an eastern sunset in order to keep their physical (magnetic) properties constant.
I think we’d stick with directions being the same as they are now. There are too many places with compass directions inherent in their names.
The rotation of the planet. Astronomers define the north pole of a planet to be the pole above which the planet rotates counter-clockwise. That’s why Venus is considered to be “upside down” relative to Earth, with its north pole on the same side of the ecliptic as Earth’s south pole.
Or, to put the same thing another way, what is familiarly called the “north pole” of a magnet is actually the “north-seeking” pole. But since all magnets are labelled up the same way, you might just as well say that the Earth’s magnetic field can be loosely modelled by a big bar magnet inside the planet, with its “south” pole (actually, its “south-seeking” pole) in the north.
This is essentially correct, but keep in mind that a bar magnet really has no idea where magnetic north is located. A bar magnetic (that pivots freely) simply aligns itself with the magnetic filed that is passing through it. The azimuth component of this field usually points to the location of “magnetic north,” but not always.
Here’s something else I learned recently: for most locations on earth, the direction of the magnetic field is not parallel with the surface of the earth. In the northern hemisphere, the north end of a bar magnetic that is hovering in free space would point to magnetic north and also point down 20 or 30 degrees. This is called the “dip of the field.” When you build a compass that is to be used in the northern hemisphere, you must affix a small weight to the south end of the needle as a counterbalance in order to make the needle approximately horizontal with the surface of the earth. Likewise, when you build a compass that is to be used in the southern hemisphere, you must affix a small weight to the north end of the needle. Hence a compass that is designed for the northern hemisphere will not work in the southern hemisphere and vice versa.
Sure, and not to disagree, but also keep in mind where magnetic north is and how it is defined is a different kettle of fish to how accuratly you can measure where you are pointing relative to magnetic north. As you say a bar magnet is (or any magnetometer device for that matter) is susceptible to local field distortions and variations in the declination and field strength. This is a measurement accuracy issue and does not materially affect what you define as magnetic north or true north. If you stand inside a big steel building you will have no idea where magnetic north is , but that doesn´t affect where magnetic north is.
You excellent point on magnetic dip also has a significant impact on the measurement accuracy. When we measure our azimuth with a magentic tool, it is measuring the horizontal component of the earths magnetic field. If we stand on the magenetic north pole, the horizontal component is zero, so we can measure nothing as the magnetic field is comming straight down through our heads and the compass would just point in a random direction. If you move off until the dip is say 80 degrees and the field stength is say 75K, then 75Kcos(10) is the horizontal component and 75ksin(10) is the vertical component. Now this horizontal component may be measureable, but if there is a small local disturbance in the earths field (say we are next to some power lines of a big chunk of steel) that field distortion is a big component of what we are measuring and so we get an inaccurate measurement.
If we move around to the equator where the dip is zero, pretty much the entire magnetic field id in th horizontal plane, so now if our field strength is 50K but the dip is zero, our horizontal componet s 50Kcos(0) , which is 50K. Now if we get close to the same external distortion as above the impact on our measuremnt would be significantly less so we can get a better measurment relative to magnetic north with the same interference and being further away from te magnetic north pole in a weaker field.
Fixed title-changed “in” to “is”.
Guys, guys, guys, you’re over thinking this. Everyone knows north is that way ^
It can change seasonally? How does that work?
(I note that the Wiki article on declination doesn’t mention this.)
Exactly. The easiest way to find north is simply to follow a river upstream.
The earths magnetic field is not as symmetrical as is often portrayed and the magnetic pole wobbles around and its position on the earths surface changes.
As its position changes, its location relative to true north changes, hence the magnectic declination (angle between true north and meagnetic north) changes.
This change is pretty small but is of interest to surveyors and anyone making accurate magnetic measurements. Several programs exist in which one types ones lat and long, altitude and date and the program provides the predicted earths magnetic field strength, dip and declination.
Have a look here
(NOAA and British geological Survey and a couple of other gov organizations.)
http://www.ngdc.noaa.gov/seg/WMM/data/TRWMM_2005.pdf
nb 8 meg pdf
for a paper on how it the earths field is measured and modeled. The intro also discusses the mechanisms for the changes in the earths magnetic field, but to take a short extract
Scroll down to page 65 and you will see a contour map of the predicted declination changes over the year 2005 to 2010.
here for the single map
http://www.ngdc.noaa.gov/seg/WMM/data/wmm-Dsv05.pdf
nb 350k pdf
When very high accuracy magnetic survey measurements are required, a ground station can be set up where the exact field strength, dip and declination measurements can be made and, by knowing the time the magnetic survey measurements were made in the field, those measurements can be corrected to a very high accuracy, almost approaching gyro survey tools.
or look for the moss on tree trunks. 
[Doctor Who] Lots of planets have a north. [/Doctor Who]
In New England Moss is usually found in the end-zone.
In the southern hemisphere moss is found on the south side of the tree trunk on account of the south side being a cool and shady sort of place. Northern hemisperical lichen seeking compassless people beware.