Once we achieved orbit around the sun, I first encountered orbital nodes. These nodes are a fairly simple notion, as relative to a plane of reference there are two points of intersection with that plane. At one, you are crossing from underneath and are ascending, hence the name ascending node. The descending node is where you cross below the plane. While attempting to encounter a planet's gravitational field, we attempted to make our orbital nodes as small as possible, getting them to 0.1 degrees away from the plane. These nodes make sense, but I never would have encountered them without KSP.