On Earth, every point can be described as a relation to another point : X miles North/South/East/West of another, with North and South being based on fixed coordinates (i.e. “towards the pole”) and East/West as relations to the former (i.e. “orthogonally to North”).
However, as far as I’m aware, there are scant fixed points in space. Every star moves around its galaxy’s center, and galaxies move around as well. Not to mention, of course, the third dimension which while pretty much irrelevant on Earth, does matter in the black.
So, how would spaceship captains orient themselves ? Does NASA (or other space agencies) have a non-Earth based referential ready for when we finally leave this pebble ? How does it work ?
The only thing I could think of was something out of Discworld, at least for the Milky Way (since it has a definite center - not sure if the Universe has one) : Hubwards/Rimwards, Clockwise/Counterclockwise… any thoughts ?
As long as you are relatively close to the Solar System, the stars would be a good reference point, because (apart from the Sun) their relative positions won’t change much. So you could just identify Polaris, and know that’s the direction of the north celestial pole. Polaris is about 430 light years away, so you’d need to be a few light years away from Earth for its apparent position to change much. At those distances, you’d still be able to use the directions of galaxies as a reference point.
Hmm. I’d think we’d use galactic center as the origin point with a line to the center of a neighboring galaxy as the reference. Then you just have to know the arc from that line, distance from center, and offset up or down from the line. Up being referenced from the galaxy spinning clockwise.
Obviously, but the axis (axises ?) must have a reference point to be of any use. If you don’t know where the x, y and z axis point to, cartesian coordinates are about as useful as “just go forward”
I’m intrigued by Projammer’s suggestion. Would a line between galaxy centers be stable enough that any object’s coordinates remain more or less the same as both galaxies move around ? And for that matter, are there practical ways to “zero in” on a galaxy center (be it by radio telescope or some other mean ? I would assume we’d drop a big navigational beacon at the center of our galaxy, but we may not have the luxury of travelling from one galaxy to the next) ?
Within the solar system, the orbit of Earth around the Sun (visually represented as the paty of the Sun against the stars), describes a plane, the Ecliptic. Except for some comets, Pluto and Mercury, the planets, 90%+ of asteroids, and miscellaneous space junk are all within a few degrees of the Ecliptic. (And Mercury’s orbit is only a few more degrees inclination to the Ecliptic.) I believe that either this system or one based on the Equator that places Polaris at ~+90 declination is what is in use for Earth-based observations today.
The angle of inclination to the Ecliptic is declination, written in degrees from +90 to -90 (which would be directly above North and South ‘poles’ of the Sun respectively). Set some point (traditionally, where the Ecliptic crosses the imaginary line separating Pisces and Aries, “the first point of Aries”) as an arbitrary zero, and measure clockwise 360 degrees from this. This is right ascension. (Note: using the “first point of Aries” is useful for Earth, as it puts the point where the Sun is directly overhead on the Tropic of Cancer at exactly 90 and at the Tropic of Capricorn at exactly 270.
A similar system can be used for the galaxy. Owing to the shape of a spiral galaxy, there is a clear plane that is the median of orbits of stars in the arms and disk, 90 degrees off from “galactic north”, the direction pointed at by the (imaginary) axis on which the galaxy rotates. Declination on a galaxy-based system is obvious; I’m not aware of any arbitrary base for right ascension on such a setup.
To the best of my knowledge, nobody has tried to describe a coordinates system based on the Universe as a whole (observations of distant galaxies are described on the Earth-centric system mentioned above).
Well, there’s supergalactic coordinates, but that’s just a set of spherical coordinates on the sky. Still, you could adapt it quite readily to a three-dimensional coordinate system.
IIRC, the galactic year is about 250 million terrestrial years, so while there would be some drift incidental to that, it’s (a) a trivial offset in the affected time frame and (b) a trivial trig function for the Tom-Tom guidance system to keep itself updated.
The same arguments apply to the relative path of the reference galaxy to our own. High school math to figure out the adjustments to compensate for drift every millenia or so.
