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There must be because the Enterprise always meets the aliens with their ships both the same way up;-) –  Martin Beckett Jun 6 '12 at 3:26

4 Answers 4

North is determined by the geographic poles of our planet, i.e., by the axis on which the planet rotates. There is such a thing as "magnetic north", determined by the poles of the Earth's magnetic field, but "north" by itself almost always refers to geographic north. (For one thing, both sets of poles move over time, but the geographic poles are much more stable.)

We do use magnetic compasses for navigation, but typically only when (a) the few degrees difference between magnetic north and "true" north doesn't matter or (b) when we know what the offset ("magnetic declination") is.

There are other possible meanings of "north", determined by whatever rotation axis happens to be relevant:

  • The plane of the ecliptic, determined by the Earth's orbit around the Sun, defines a kind of "north", about 23.5° away from geographic north.
  • If you're on or near another planet, "north" is typically determined by the rotational axis of that planet. Most of the planets in the Solar System have axes that are close to parallel to Earth's axis; Uranus is a notable exception, and Venus rotates in the opposite direction.
  • The Galaxy itself rotates on an axis; if you're in deep interstellar space, you might use that as a frame of reference.

So far no astronauts have gone far enough out for most of this to be relevant. Unmanned spacecraft such as Voyager have gone to other planets, and their navigation is quite precise, but it doesn't necessarily depend on defining what "north" means; you just need a way of determining where you are and a consistent way of describing it.

Within our Galaxy, it should be possible to navigate by observing the positions of known stars.

It's not likely we'll be using magnetic fields as a primary source of navigation information in deep space. As long as you can see the stars, there are better sources of information.

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The galaxy is big, and we haven't mapped out enough stars that can be identified. You would be better off looking for known objects orbiting the black hole at the center, and figuring out your position from their plane. But the easiest way is CMB. –  Ron Maimon Jun 6 '12 at 6:05
@RonMaimon: I suspect that by the time we're able to travel thousands of light-years, we'll have a very good idea where all the stars in the neighborhood are. –  Keith Thompson Jun 6 '12 at 22:09

You could orient yourself using the CMB fluctuations as your compass--- given a detailed WMAP picture of the CMB flutuations, you could tell which way is which anywhere in the local galactic region, and how fast you are going relative to the CMB.

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On Earth the north and the south are defined by the south and north magnetic poles respectively of the Earth's Magnetic field. In space there is a thing called the Galactic Magnetic field which permeates galaxies, including the Milky Way (http://arxiv.org/abs/astro-ph/0207240). However, the strength of the Galactic magnetic field is much much lower than the Earth's magnetic field. Our compasses wont be able to detect this and be much more influenced by magnetic fields of nearby planets or stars, rather than the Galactic field. The Galactic magnetic field acts on much larger scales. In 1997 one scientist discovered that the supernova remnants (left overs of an exploded star) align them selves to Milky Way's magnetic field. Intergalactic magnetic fields have also been recently discovered but I believe little is known about their structure.

So in short, I think, theoretically there can be a "north" in space because there is a magnetic field present. Whether or not we can make use of it (due to its weak strength) is a different story. There are other coordinate systems that are used e.g the Galactic Coordinate system and Right ascension/declination.

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Another issue in using the galactic magnetic field as a compass is that it undergoes reversals, and seems to follow the gas in the spiral arms. You are not going to be able to use a compass to easily determine direction, since it will be very much location dependent. –  jdmcbr Feb 29 '12 at 6:14
I agree, there will be a "north" but that north will vary depending on your location. Although, would the intergalactic magnetic fields be any better? They appear to exist on super cluster scales and presumably will be less susceptible to variations from individual galaxies? –  Elegant_Cow Feb 29 '12 at 6:25
Actually, the cardinal directions are defined by the rotation of the Earth/the position of the Earth's axis. As the other answer pointed out, the geographical and the magnetic poles don't overlap. The magnetic field is used as only a convenient rough approximation. –  Daggerstab Mar 5 '12 at 9:26

there is no axis of rotation, and no plane of rotation, so there is neither longitude nor latitude; likewise, there is no North, south, east, or west. Navigation consists of knowing where Here is, in relation to There (as it does all forms of wayfinding). Thus, the Sun is a Prime $(x, y, z = 0)$, the plane of planetary rotation can be the equator; locus of Earth at time of departure can be the prime meridian. I posit these conditionally, for one may only need know $x^2+y^2+z^2=w^2$, being the Direction & Distance hence. This is merely triangulation in three dimensions.

Another method is similar to coastal navigation: the arc (angle) between any pair of stars, or a trio of stars, gives a fix; there are plenty of stars, seen from earth, that appear close together, yet are separated by a great distance; also using celestial coordinates in the same manner as Terrestrial astro-navigation, in order to reckon where Here is, in relation to Whence & Whither.

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