A friend sent me this question:

Paul,

How's your celestial mechanics? I'm having trouble understanding
astronomerspeak. Epsilon Indi ("Home" in Niven's Known Space) is 11.83
ly from earth and has a radial velocity of -40.4 km/s (meaning, I think,
that the redshift indicates that it's headed toward us at that speed).
But I'm trying to figure out where it will be, distancewise, in 50,000
years, which should require all three relative motions. Wikipedia lists
these as "U=-77, V=-38, W=+4". I don't know what the three letters stand
for, and I don't understand why none of them correspond with the radial
velocity (unless the V is a slightly different value for the same
thing). Any ideas?

I posted this in rec.arts.sf.science, and there seems to be some
confusion as to whether the velocity is given in a co-ordinate system
in which the Sun is stationary or moving. So I thought I would start
from scratch in this newsgroup. I would be grateful if anyone could
tell me:

* What units go with the numbers?

* Is the co-ordinate system rectangular, cylindrical, or spherical?

* Is the Sun stationary in that co-ordinate system, or is it moving
as well?

* If the Sun is moving in that co-ordinate system, what is its velocity
vector?

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