Bearings in space

[Phantasm]

Originally Posted by Than

I agree a small scale campaign will probably use Earth or some other reference world as the origin, but the XY-plane need not necessarily be the orbital plane of the reference world. Having a central origin makes for easy math by keeping the numbers small; which way the plane is oriented doesn't really matter. Any one of the reference planet's equator, reference planet's orbital plane, or the galactic plane would be equally likely. Actually scratch that; the most likely is all being used to some extent, with potential confusion if maps aren't labelled accurately.

The biggest problems with using the planet's equator as a reference for the XY-plane are two-fold:

First, as the planet orbits the star, the actual location of the equator changes minutely; the equator is rarely on the same plane as the orbit, causing seasons. This doesn't mean much, but can lead to inaccurate astrogration.

Second, the planet wobbles over time, causing what folks on Earth call precession. Polaris hasn't always been our pole star; during the time the Great Pyramids are commonly believed to have been built, Sigma Draconis was the pole star. In another 16,000 years or so (I'm fuzzy on the actual time frame), Vega will be the pole star. Imagine setting a trip for 0,0,30, intending to hit the vicinity of Polaris and hitting Vega's neighborhood instead.

As far as I know, the planetary plane does not change in regards to the sun it orbits, which would make for a slightly better reference in the long run than the planetary equator.