1.   A spacecraft passing through its perigee point is moving faster than an equivalent craft on a circular orbit of the same radius.

2.   Another burn at this point can accelerate the craft, placing it on the higher circular orbit.

3.   Consequently the force of gravity at perigee is not enough to hold it in a circular orbit.

4.   Similarly, radial burns can be used to convert circular orbits into elliptical ones.

5.   So far we have been discussing circular orbits.

6.   The planets follow almost circular orbits around the sun.

7.   This changes them from a circular orbit to an elliptical one.

8.   That planet has a more circular orbit than our planet does.

9.   A star passing too close could knock a planet out of its generally circular orbit.

10.   Both the inner and outer planets should have almost circular orbits in the same plane.