Look what I found here:
Lagrange

What are "Lagrange points", also known as "libration points" or "L-points"? These are all jargon for places where a light third body can sit "motionless" relative to two heavier bodies that are orbiting each other thanks to the force of gravity.





The really interesting thing is that the Lagrange points L4 and L5 are stable equilibria as long as the heavy body - the Sun in our example - is more than 24.96 times as massive as the intermediate-sized one. In other words, a small body put in orbit at the right speed at either of these points will tend to stay there! If you nudge it a little, it will drift back. On the other hand, L1, L2, and L3 are unstable equilibria: the slightest nudge will make satellite at one of these points drift away. To keep it there, you'll need to equip it with thrusters that keep correcting its orbit now and then.

Cool! Think of the implications of a space-colony or station at such a point! What a coo place to observe the universe from.