You will be able to describe the general concepts and advantages of geosynchronous orbits, polar orbits, walking orbits, sun-synchronous orbits, and some requirements for achieving them. The terms orbit period, periapsis, and apoapsis were introduced in Chapter 3. The direction a spacecraft or other body travels in orbit can be direct, or prograde , in which the spacecraft moves in the same direction as the planet rotates, or retrograde , going in a direction opposite the planet's rotation. True anomaly is a term used to describe the locations of various points in an orbit. It is the angular distance of a point in an orbit past the point of periapsis, measured in degrees.
Nodes are points where an orbit crosses a plane.
To completely describe an orbit mathematically, six quantities must be calculated. These quantities are called orbital elements, or Keplerian elements. An inclination of 0 degreesmeans the spacecraft orbits the planet at its equator, and in the same direction as the planet rotates. An inclination of 90 degrees indicates a polar orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination of 180 degrees indicates a retrograde equatorial orbit.