Periapsis and apoapsis are two important terms used in orbital mechanics to describe the closest and farthest points in an object's orbit around a central body. These terms are commonly used to describe the orbit of a satellite or spacecraft around a planet or moon.

Periapsis:

Periapsis is the point in an orbit where an object is closest to the central body it is orbiting. It is the point of closest approach between the two objects. The term "periapsis" is derived from the Greek words "peri" (near) and "apsis" (arch or orbit). For an elliptical orbit, periapsis is the point where the radius vector from the central body to the orbiting object is at its minimum.

Apoapsis:

Apoapsis, on the other hand, is the point in an orbit where an object is farthest from the central body. It is the point of maximum distance between the two objects. The term "apoapsis" is derived from the Greek words "apo" (away from) and "apsis" (arch or orbit). For an elliptical orbit, apoapsis is the point where the radius vector from the central body to the orbiting object is at its maximum.

The distance from periapsis to apoapsis along the major axis of the orbit is called the major axis length or the orbital diameter. The midpoint between the periapsis and apoapsis is known as the semi-major axis, which is an important parameter used to define the size of an elliptical orbit.

Periapsis and apoapsis play a significant role in various aspects of orbital mechanics and space missions. They are crucial in determining the orbital period, velocity, and energy of an orbiting object. The knowledge of periapsis and apoapsis is essential for planning satellite maneuvers, trajectory corrections, and spacecraft rendezvous missions.