Re: How do we find direction in space

That's a very interesting and complex question! I hope I can explain it 
well enough: let's give it a try.

GPS satellites actually don't need gravity to work: all gravity does is 
keep them in orbit around the Earth. In principle, GPS satellites could 
work in orbit around the Sun, or even around a galaxy (of course, faster-
than-light radio would be needed to make a galactic GPS work because the 
distances are so huge, but in Star Trek they already have that!)

What GPS and all other navigation systems need are some sort of reference 
frame to base their positions upon. Everything else is then described in 
terms of where they are from that reference.

For example, in your house you might say the front door is your starting 
point, and facing directly in the door is your starting position. Every 
point in your house can be described by how far forward and how far to the 
left or right you would walk to get there. In my house, the bathroom is 
three feet to the left of the front door, then straight into the house for 
twenty feet.

We can generalize even more by creating a grid of the house, and designate 
anything to the left of the door as a negative direction and to the right 
of the door as a positive direction. Forward in to the house is also 
positive. The bathroom can then be described by a pair of numbers for how 
far to the left and how far into the house it is: since we went two feet 
left (the negative direction for left-right movemement) and twenty feet 
into the house (the positive direction for front-back movement) the 
location of the bathroom can be written as (-2,20). The first number is 
the left/right move, and the second is the forward-back.

If you need to show distance above the ground in your house, you will need 
to add a third direction to your grid - up/down. If you make up positive 
and down negative, you can then describe any point in the volume of your 
home (one foot right of my front door, three feet forward, and five feet 
up is the hook I hang my hat on: in our grid the hook is at point (1,3,5) 
indicating one foot, three feet, and five feet.

In space, everything can be described by selecting a distance unit (feet, 
parsecs, meters, or whatever is useful), setting a zero point, then 
setting a base line to define "forward" and "back", a direction as "left" 
or "right", and s line as "up" and "down". The position of any object is 
then given by distance up, down, left, right, forward or back from the 
starting point - and only groups of three numbers are needed for the 
entire universe!

If you'd like more detail on how coordinates and directions are set for 
outer space, check out the following sites:
 http://www.seds.org/~spider/spider/ScholarX/coord_bas.html
 http://www.seds.org/~spider/spider/ScholarX/coords.html

I hope this helps!